Resurrecting a Nikon S8100 from "Lens Error" death (that is, S8100 "Lens Error" fix)

Nikon S8100 "Lens Error" fix

A friend of mine came back from vacation and handed me his P&S (which supposedly stands for "Point and Shoot") camera - a Nikon S8100.

Ostensibly a nice, compact camera, it seems to have earned a bit of a reputation - namely that if dropped from a short distance (say, a kitchen table) onto the floor (a carpeted one, for example) while in its padded, protective case, it will henceforth turn on with a cryptic and useless "Lens Error" diagnostic  It will then refuse to turn off with the power button, but turn itself off after a while.

Unlike other point-and-shoot cameras that I have seen with problems with their retractable lenses, this one DID NOT make any noise at all - other than a musical "bling" - when it was powered up:  No clicking, whirring, or anything else to indicate that the lens was even trying to do something.  This was highly suspicious and I guessed that a connection might have dislodged preventing the motor from operating - but I wasn't sure.

A quick Google search revealed many hundreds of comments about other owners having suffered this issue - many within 3-6 months of buying the camera new and lots of useless information from a site called "FixYa".  Apparently, Nikon originally did offer in-warranty repair, but the anecdotal evidence would seem to indicate that they soon ceased doing so, citing what many posters referred to as "normal wear and tear" as "out-of-warranty" damage and socking them with a bill that was typically around $110 U.S.  - roughly half of the camera's original purchase price!

So it was with this camera, presented to me in June 2013 - well out of warranty.  Unfortunately, no-one that I could find on the web offered a clue as to what caused this problem, much less a solution to it.

Until now.

IMPORTANT:

Before you go any further, please read the following - and then read it again:

• NO, I will not fix your camera!  That's just not a business that I want to get in to at this time.
• NO, I will not fix your camera!  (I just thought that I'd say it twice!)
• The details on this page apply only to the S8100.  This same problem may occur on other, similar models, but I don't know that for certain.  Having said that, this is the fifth camera that I have "repaired" (all of them different makes and models) where the "failure" was just an internal cable coming loose, so I know that it's a common problem.
• I do not own an S8100 - I just fixed it for a friend and gave it back, so I don't have it around to look at!
• Unless you are familiar with the delicate work on miniature electronics, you probably can't fix it!
• DO NOT attempt this without at least some knowledge of electronics and mechanical systems.  Many of the parts are fragile and tiny and need to be handled appropriately.
• This camera contains a flash capacitor that may have hundreds of volts on it.  If you accidentally touch this, you will, at the very least, receive a very unpleasant shock.  At worst, this discharge through your skin can destroy other camera electronics.  It is also possible - although unlikely - that such a discharge can stop your heart and kill you!

This potential solution describes ONLY where the camera exhibits the following traits:

1. The lens is fully retracted (e.g. IN).  If your lens is stuck while extended, this probably doesn't apply!  Having said that, you may have nothing to lose by trying this.
2. When you turn the camera on, it does NOT make any mechanical noises such as motors whirring, clicking, whining.  The ONLY noise that it makes is a musical "bling" from the speaker.  If you hear ANY motors whirring and clicking, you may not be experiencing the same problem.
3. It says just "Lens Error" with an exclamation point on a mostly white screen.  Nothing else.
4. It will not turn off with the power button, but it will eventually turn off by itself.
5. With the camera, you cannot access the images stored on the memory card to review them.  (You could always remove the card and read it directly via a card reader, of course!)

Having said all of the above, let's get on with it.

Disassembly:

First, remove the battery and memory card!

What you will need:

• You will need to read - and re-read this entire procedure before starting.
• A clean work area that is well-lit.
• A magnifier and/or a set of strong reading glasses.
• A pen/pencil and paper to make notes and drawings of how things came apart - and how they go together.  Taking pictures at each step wouldn't be a bad idea, either!
• Small containers to hold the screws and various camera parts.  A container used for holding eggs with pieces of paper describing where the individual parts come may work for you.  (Just make sure there aren't holes in the bottom of the sections!)
• A thin, plastic blade to pry pieces apart.  If you are really careful not to cut/short anything, a thin metal blade can also work, but you are more likely to break something this way!  (You'll notice in the pictures that I used a metal blade - but I've done this before...)
• A very small "Philips" type screwdriver:  I used a size "000" of the sort that might be found in a set of Jeweler's tools that I bought some years ago from Harbor Freight.
• A pair of tweezers.
• Patience, a steady hand, and some experience with successfully having done a similar thing before.

Figure 1:
Bottom of the camera showing the 7 screws.
Click on the image for a larger version.

 
There are six different types of screws holding the portions of the camera together that need to be removed and I arbitrarily labeled them types "A" through "F".  All of these screws should be removed as noted.

• Bottom cover:  Four black, flat-head screws of "medium" length around the tripod mount - I called these type "A".
• Bottom cover:  Three black, flat-head screws that are very short - two to the left of the battery door (bottom-up, lens facing you) and one by the upper-right corner of the battery door.  I called these screws type"B".
• Left side:  (The side without the HDMI connector/wrist strap).  Two "B" type (short, black) screws.
• Right side:  (The side WITH the HDMI connector/wrist strap).  One "B" type (short black) screw near the bottom of the camera, and a long black screw (what I call type "C") UNDER the HDMI cover.

Remove the wrist strap (if attached) and very carefully remove the panel on that side while carefully noting how the HDMI cover attaches in its slot!  Under this panel you'll notice 5 screws of two different types:

• Two short silver-colored flat-head screws that hold on the wrist strap.  Note carefully how this mounts and draw a picture if you have to!  I called these type "D" screws.
•  Two more "D" type screws holding edge of the black plastic of the camera's rear panel, one on each side of where the wrist strap was connected.
• With the camera on end and the lens facing you, there is another screw in the lower-right corner under the HDMI connector.  It is a long-ish silver screw with a machine head.  I called these type "E" screws.

At this point, you can very carefully snap off the rear panel.  The back of the camera which includes the LCD (display) bezel (but not the display itself!) and the knobs stay with the camera and not the bezel while the four buttons on the back (the one with the red arrow, the "play" button, menu and "trash can") stay with the bezel.

You can also carefully pop off the panel on the other (left) side of the camera if you wish - but it is not necessary although it may make it easier to put the back panel on again, later.

Note:

There are a number of plastic catches/tabs that hold the rear panel in place and removing it is a bit tricky.  I would suggest the use of a thin plastic blade to press between the gaps of the connectors to try to release the catches.  Do not force the panel to come off!

This is a bit of a pain and other than be patient and observant, I don't really have any other advice on how to remove it except to assure you that all of the screws holding it in place have been removed by this point!

Once you have removed the rear cover/bezel:

Removing the panel with the buttons and rear-panel wheel:

Once you have successfully removed the camera's back, place it lens-down, preferably on a clean, soft, lint-free cloth, with the bottom facing you.  In the upper-right corner, tucked almost underneath the metal panel with the rear-panel knob and buttons, you'll see a screw (type "E") that needs to be removed - and you'll probably need tweezers to remove it when it is loose.  See Figure 2, below.

Figure 2:
The screwdriver (upper-right) points at the screw that holds the wheel/button assembly in place.
This panel slides out to the right.  You may need to push in on the panel slightly to release it.
Click on the image for a larger version.

Now, the panel with the four buttons and the rear-panel knob needs to come out.  Pressing down on it slightly, slide it to the right (away from the LCD) to allow it to clear the one tab at the top and the two at the bottom, noticing carefully how it went in.

Be careful to not pull on the flat cable attached to this panel.

Figure 3:
The screwdriver is pointing at the "upper" tab holding the panel in place and two other tabs hold the
bottom edge.  Once the screw is removed, this panel slides out of position - but be careful to with
the cable attached to it!
Click on the image for a larger version.

Now, to remove the cable from the connector on the circuit board, using your fingernail to flip up the black plastic cover on the connector.  To do this, you slide your fingernail under the same side that the cable enters and when it flips up on its hinge.  It is only after flipping up this door that you will be able to easily remove the cable, noting carefully how far it went into the connector!

Note:  The on-board connectors to which the cable attaches are a bit tricky.  These particular connectors have a black (or very dark brown) "door" that flips up, the hinge being on the side opposite where the cable comes in or, in other words, it is the side where the cable comes in that flips up.

You will need to remove two cables with this type of connector:  The fairly small one coming from the control panel, above, and the much larger one connecting the LCD.  You are cautioned to lift up from the center of the flip-up portion with only a fairly soft item - such as your fingernail.  If you break this door off you will not be able to secure this cable into position and your camera will probably not be usable! 

Removing the main LCD panel:

Comment:  You may be able to complete the repair without having to disconnect the LCD panel from its connector, but rather carefully laying it over the side of the camera with the HDMI connector.  Remember that this cable is extremely fragile and can be easily torn or partially pulled out of the connector.

Removing the LCD panel from its connector - and getting it back in again - is probably the trickiest part of this procedure, so pay very close attention.
 

Figure 4:
LCD cables.  The black flip-up portion (which is hard to see) locks the large cable into place.
The smaller cable for the LCD's backlight may be seen to the left:  It just slides in/out of place.
Click on the image for a larger version.

At this point the LCD is only held in by snap-in points around the edges.  Using a thin plastic blade, work your way around the LCD and work it free.  At this point all I can suggest is that you be observant and carefully note where it is hanging up if it doesn't come free.

Once you have freed the LCD, take a close look at its large cable and you'll see that it, too, has a "flip-up" connector - a sort of black "door".  Using your fingernail, flip this up and you should be able to flip it up and free it.

Note:  Pay very close attention to how far the large LCD cable went into the connector.  Re-inserting this cable is rather tricky and is very critical!

For the smaller cable, it is held in place only by friction so carefully pull it straight out of the connector - but note how far in it slides.

Removing the LCD mount:

Now that you have removed the LCD (or not - see the comment above) you are left with the LCD mount and metal cover/RFI shielding for the imager.  Under where the LCD was sitting you'll see four screws.  With the lens facing down and the bottom of the camera facing you:

• The two upper screws and the lower-left are what I called type "F" - longer that "E" and silver with a machine head.
• The lower-right screw is a type "E".

Figure 5:
This LED, which is indicates that the flash is fully charged and ready, must be "un-stuck" from the
metal frame and moved out of the way.  Don't forget to put it back later!
You can see from the picture that I used a metal blade, but I'd recommend that you use a plastic
blade unless you are really careful.
Click on the image for a larger version.

Now locate where the upper-right corner of the LCD would have been and you'll notice an LED on a small, flexible cable that is stuck to the metal with double-sided sticky tape.  Carefully pull this off by sliding a small, plastic blade under it and move it aside, noting its exact placement.  See Figure 5.

At this point, the metal frame that held the LCD is fairly loose, but note carefully that parts of it wrap around each side of the camera, held in by some of the side-panel screws.  Remember how these tabs are arranged and where they go!

Now, while pulling up on the metal mount that held the LCD you'll see that it is now held in place with two pieces of copper foil tape.  Using a thin, plastic blade, reach between the metal and the rest of the camera and use that to pull and release the tape.  Once you have managed that, the metal frame will come out easily.

Figure 6:
The LCD mounting frame/shield with the 4 screws already removed.  The knife is pointing at one of the
two pieces of self-adhesive copper foil.  Just above it is another piece that must be carefully detached
as the frame/shield is removed.  Again, you see that I used a metal blade, but I'd recommend a plastic
blade unless you are really careful!
Click on the image for a larger version.

At this point you are looking at the backside of the lens and the imager.  With the camera facing down and the bottom facing you, look at the connector coming off the lens assembly at about the 2-o'clock position, being pointed at by the red pen in Figure 7,  and you'll observe that it is probably loose, if not now floating in space.

This is the problem!
 

Figure 7:
The pen is pointing at the connector that comes loose and causes the "Lens Error" problem.
The connector just below it carries the image (picture) signals and if this is loose, one can
experience image quality issues.  Both connectors should be properly re-seated!
Click on the image for a larger version.

At this point I would recommend popping off this connector and the one next to ("below") it (at the 3-o'clock position) - which also comes from the lens body and contains the imager's signals - and firmly, but carefully, re-seating it.

Note:

It may be that reports of degrading image quality are the result of the other connector - the one below the one pointed to in Figure 7 - coming loose.
 

What seems to be the problem is that while there is some foam on the backside of this connector to hold it in place between the LCD support frame, there seems to be a slight "bias" and the connector appears to be being compressed with this foam at a slight angle.  When the camera is jarred - say, by falling a short distance onto a padded surface while in its protective case (or something worse!) this connector tends to pop off!

It may be that one could put a piece of thin plastic in there to hold it in position, or perhaps, a dab of tacky adhesive or small bit of RTV ("silicone" - the sort that does not smell like vinegar!) - and if you do, that's up to you - just make sure that it doesn't get anywhere it shouldn't (such as inside the connector!) or on anything else and that it has cured/dried before reassembling the camera.

Reassembly:

I hate it when I read it elsewhere, but I'll say it now:  Reassembly is the reverse of disassembly!

In a nutshell:

• Make sure that the connectors from the lens assembly are firmly and properly seated - and check other connectors while you are at it!
• Slide the metal LCD mount back into place, noting the proper placement of the "fingers" that extend along the sides of the camera.
• While installing the metal LCD mount, make sure the LED that you moved out of the way (the one picture in Figure 5) is clear and isn't being sandwiched under the metal mount.
• Reinstall the 3 "F" type screws (top, lower-left) and the 1 "E" type screw (lower-right).
• Put the adhesive-mounted LED (the one in Figure 5) back where it had been (just beyond the upper-right corner of the LCD.)
• Make sure that the pieces of copper foil are again pressed down on to where they were originally stuck.

Reinstalling the LCD and its cable:

If you haven't dealt with this type connector before it can be really tricky to get right!  You'll recall that to remove the cable, you had to flip up the plastic lid on the wide LCD cable and simply pull the small cable out of the compression-fitting connector.

With the LCD facing down, toward the table and its cables going to the left of the display (e.g. flipped over to the right from its normal mounting position) you'll see that you can arrange the large cable so that it more-easily fits in the connector.  You must have the lid/door on this connector flipped open at this point!

You can now align the cable so that it slides into the connector - but this is quite tricky.  One thing that is not obvious to the casual observer is that the very end of the cable must slide partway underneath the hinge of the flip-up door in order for it to properly make contact - but it only moves about 1-1.5 millimeters when it does this!

Once you have inserted the cable, use your fingernail or a piece of soft plastic (or a pencil eraser) to carefully close the door.  If you have done it properly, the cable should look perfectly straight and as you shut the door, you should see/feel it compress against the flat cable as it locks it into place!

Assuming that you have done this, use a pair of plastic or bamboo tweezers and carefully slide the small connector - used for the LCD's backlight - into place.  If you are dextrous - or have very small fingers - you may be able to do it that way as well.

Once the connectors are installed, you can now snap the LCD back into position in its metal frame.

Testing the LCD before final assembly:

At this point - before you go much farther - it would be a very good idea to verify that you have, in fact, installed the LCD connector properly, even if you did not disconnect the LCD's cable(s).  Insert the battery (but do not install a memory card) and power up the camera while holding it in your hands, off the workbench so that the lens can extend.

If all goes well, the lens will extend and your LCD will display an image or menu.

Whether the lens does/doesn't extend and/or you do or do not see an image on the LCD, turn the camera back off (to retract the lens) and remove the battery.


If the lens doesn't extend:

All I can suggest is that you go back and make sure that the connectors depicted in Figure 7 are firmly seated!

If you don't see an image on the LCD:

If you see the LCD light up (e.g. the backlight turns on) but there is no image or menu displayed, you probably didn't get the LCD's large cable properly seated in the connector.  You don't have to remove the LCD from its mount to attempt to re-seat the connector, and having it improperly seated probably won't damage anything - but it just won't work!  If the LCD's backlight did not turn on, make sure that the smaller cable is properly inserted.

If you experience a problem you'll have to try to re-seat the connector.  You may want to use small pieces of plastic or wood (toothpicks) to manipulate the large LCD cable so that it properly seats within the connector.

Remember:  Make sure that the "door" on the connector is flipped open and that the edge of the cable slides just a little bit under the door's hinge!  The cable must be perfectly straight in order for it to fit and mate properly!

If you can't get it to work, you might want to set it aside and try again later - or have someone else try it.

In my opinion, this can be the trickiest part for someone who doesn't know exactly how these connectors go together!


Final reassembly:

Remember:  Remove any fingerprints from the surface of the LCD using a soft piece of cloth or lens tissue (but NOT a paper towel!) before reinstalling the rear cover panel/bezel.

Note:  At this point, the lens and the LCD are assumed to be working and you should have turned the camera off to retract the lens and also removed the battery!

Now, continue with the "reverse-disassembly."  The only tricky parts that I noted were:

• The HDMI connector cover.  The tab of the HDMI cover goes in the slot on the plastic side trim piece.
• The rear edge of the top cover.  I had a bit of difficulty getting the edges of the rear panel and the top cover to mate and snap together, but a bit of pushing on the joints successfully re-seated them.

If all goes well, your S8100 should be working again!


Note:

Please read, and then re-read the warnings and comments at the top of this page!  Again:

• I won't/can't fix your camera!   If I did, I'd probably have to charge you as much as one would cost on the used market!
• This is for the S8100 only!  Some of this advice may apply to other cameras, but I don't know!
• Unless you are skilled at working on small electronics, you will probably not be able to do this repair!
• This camera contains dangerous voltages that can result in damage, injury or even death!
• If you do this, consider your camera to already be lost and that you aren't going to be successful.  That way, if it does work, you'll be happy but if not, you won't!

 You have been warned!
  
Best of luck!

As of July, 2014 when I write this addendum, my friend is still using this camera, regularly!

 Tags: 

• Nikon S8100, Lens Error, Lens won't extend, Nikon S8100 Lens Error, Nikon S8100 Lens won't extend, lens won't move, Nikon Lens Error, lens doesn't make a noise, camera turns on with "lens error", camera chimes and says "lens error!"

 [End]

This page stolen from ka7oei.blogspot.com

Long-term observations of NiCd versus NiMH cells and how to make them last longer.

Nickle-Cadmium (NiCd) and Nickle-Metal Hydride (NiMH) cells are ubiquitous, but their behavior in typical consumer items is not at all well understood by most people.  Much of this is because one never thinks about what is powering that portable device until it stops working, but a lot of it has to do with confusing advice and misinformation about them and how they behave.

Another problem is that in long term, NiCd cells can have longer life spans than NiMH cells, but why is it that in so many applications people find that the NiMH cells outlast the NiCd cells that they used to use?

We'll answer that question along the way.

The economics and convenience of rechargeable cells:

Even if they don't last very long (in terms of years) rechargeable cells are almost always much less expensive to own and operate than their non-rechargeable Alkaline cousins, but there is a convenience factor involved:

• You can probably get more run time (from many - but not all - devices) from a set of alkalines than you can from a single set of rechargeables.

• When you put in a fresh set of alkalines, you have a pretty good idea how long that device will run.  Unless you pull rechargeables off the charger - and you know that they are good, you don't know before-hand how long the device will run.

• A set of alkalines can sit around in the package, unopened, for several years and still be good.  Again, with rechargables you don't know their charge state for certain - particularly if you haven't used them for a while!

 Based on the above, one might be understandably wary about using rechargeable cells, but for many devices - such as portable power tools - there's really no option!

Alkaline Cells:  A comparison to NiCd and NiMH:

Other than the fact that they are rechargeable, what are the main differences between alkalines and NiCd/NiMH cells?  As it turns out, voltage isn't really an issue since modern devices will happily run at 1.2 volts per cell - the same as NiCd/NiMH and a half-discharged alkaline.

What about capacity?

A good-quality alkaline AA cell has a capacity of about 2.5-2.8 amp-hours.  Comparing an AA-size NiCd, its capacity will be in the area of 0.6-1.1 amp-hours and a NiMH will have a capacity of between 1.8 and 2.8 amp-hours, depending on the brand and specific type.  In general, the rechargeable cell will have less capacity than the equivalent sized alkaline, but why is it often the case that it runs an electronic device longer?

Internal resistance is the answer.  When fresh, the internal resistance of a good-quality AA alkaline cell is on the order of 0.15 ohms per cell, increasing to 0.3 ohms per cell when the it is 50% discharged and over an ohm when 80% discharged!  If your camera uses a battery of 4 cells in series that means that the total resistance of new cells (excluding resistance of battery contacts and wiring) is about 0.6 ohms, rising to 1.2 ohms when the battery is just 50% discharged - and it only gets worse (much worse!) as the it is further-depleted!

If the digital camera consumes, say, 800 milliamps (a reasonable amount when a flash is charging, a backlit display is operating, etc.) then cell resistance alone will dictate a voltage drop of 0.48 volts for a battery with new cells, and 0.96 volts or so for cells that are 50% discharged.

Again, this does not take into account other resistive losses - such as contacts and internal wiring - some of which can be significant!
 
For new cells in a 4 cell battery, this voltage will (optimistically - assuming a nominal 1.5 volt unloaded output) amount to about 5.5 volts under these conditions, dropping to about 4 volts when the cells are 50% discharged - a voltage that may be inadequate for operation of the camera.

There is yet another problem.  Often, cameras contain switching-type voltage converters.  While these are efficient in their energy conversion, they attempt, by their nature, to maintain a constant power output over a varying input voltage.  What this means is that, as the battery voltage drops, the current consumption will increase as the voltage converter attempts to maintain the constant voltage output - exacerbating the problem of already-low voltage and resistance.  This problem can get worse when the camera's load changes because of a charging flash, a backlit display being illuminated, or the camera's CPU pulling more current when processing the image and saving it to memory.

In other words, the cells may be, say, only 50% discharged, but the equipment (the digital camera, in our example) may simply be unable to use the energy that is still available.  If this is the case you'll probably get plenty of life out of those same batteries if you put them in a small flashlight or portable FM radio, or TV remote control.

In other words - don't throw them away just yet!

NiMH, NiCd cells and internal resistance:

NiCd and NiMh cells, on the other hand, typically have a much lower internal resistance over their charge life and under typical conditions, this resistance is typically lower than that of an alkaline cell - even when the NiCd or NiMH cell is significantly discharged.

According to info from several well-known manufacturers, a relatively new AA NiMH cell typically has about 0.17 ohms per cell when fully charged (as opposed to 0.15 ohms for a "fresh" Alkaline AA cell of good quality) but this rises to just 0.18 ohms at the "100% discharge" point.

As we demonstrated above, a typical AA Alkaline cell can be expected have over an ohm of internal resistance at 80% discharge - and this value skyrockets as the battery is discharged further!  From what information that I have been able to find, a typical NiCd seems to have about half the internal resistance of the same-sized NiMH cell and is one of the factors that explains its suitability in very high current situations.

What this means is that while an alkaline cell may be able to run the digital camera (our example from above) only until the cell is at its 50%-70% charge level, a NiCd or NiMH battery can probably output the required current and voltage until it is at or below its 15% charge level.  The lower intrinsic resistance also means that they are more likely to be able to tolerate impulse loads (i.e. additional current drawn by the flash charging, for example) without causing the camera to shut down due to low voltage.

Comment: 

At the current level of technology, NiCd cells are often preferred over NiMH cells for certain applications, most notably those requiring very high current consumption such as in battery-powered tools, etc.  In these applications, the high current drawn by the tool would over stress a typical NiMH cell and likely result in shorter operational and useful life than a NiCd cell.

There are NiMH cells that are specially designed for "high drain" applications, but these are special purpose cells that often trade this high current capability for capacity, putting their amp-hour ratings below those of other types of NiMH cells.

When cells go wrong: "Memory"
 
One of the best-known properties of NiCd cells is this thing that people refer to as "Memory" - that annoying property of cells seeming to go dead much sooner than expected.

It is unfortunate that this effect. while called "memory" by many people (and some manufacturers of electronic devices) is almost never that phenomenon that is really the "Memory Effect."  Instead, this phenomenon is usually due to cell damage caused by reversal - more on this later.
 
One of the first places that the so-called "memory effect" was first noticed and quantified was when NiCd cells were first used in communications satellites.  These satellites rely on solar panels for their power, but the Sun is eclipsed by the Earth at times and it is during these periods that the satellite must operate from battery power alone.  For many satellites these eclipses were typically of very similar duration which means that during the "eclipse season" the battery was run down by about the same amount, time after time.

The "memory" was noticed when, after several eclipses, the battery voltage would relatively quickly drop to the voltage approximately that attained during the latter part of the eclipse - and typically stay there.  It was also noted that this "memory" effect could be reversed simply by charging the battery and then discharging it to a different point for several cycles and was done by clever management using multiple battery strings onboard the satellite and preventing a battery string from being discharged to the same point repeatedly.

It is important to know that little (or no) permanent damage was actually done to the cells by this "memory" effect - the result was (more or less) a temporary reduction in the cells' capacity until they were conditioned appropriately.

In typical use by consumers who use NiCd-operated devices, it is unusual to discharge the battery to precisely the same point time-after-time.  Typically, the amount of discharge is somewhat random - and just one or two variations from a precise cycle will largely "erase" a weak memory effect.  Among the very few documented cases of "terrestrial memory effect" were been in pager service where, regular as clockwork, the batteries would be run down during the day and recharged overnight.  This was a long time ago - back in the days when pagers were those half-brick sized things that only VIPs and doctors wore - and batteries only lasted a day or two anyway!

It has been reported that NiMH cells can also exhibit this same "memory" effect - but remember that it is atypical to expose a cell to very precisely repeated discharges of equal depth time after time:  Most people just don't use their battery-operated devices that way!


"What is this thing (mis)called 'Memory' then?"
 
Abused NiCd cells will typically exhibit a loss of capacity and/or the inability to take or retain a charge, and it this property that is too-often misidentified as "memory."  But, this is not "memory."

What is going on, then?

Cell Reversal:

Good quality battery packs are made from individual cells that have been matched in terms of resistance and capacity.  This is important in terms of maximizing battery life.

Here's why:

A battery typically consists of cells wired in series for higher voltage. Ideally, all cells will run down at exactly the same time.  This is not usually the case, however, especially as the cells age and some get weaker faster than others.

Temperature also has a large impact on cell longevity.  A cell that is operating at a higher temperature will generally have a shorter overall lifetime than one that is cooler.  An effect of this can be noted in a large battery pack (such as that on a cordless drill) in which a large number of cells are grouped together.  Often, the cell(s) in the "middle" of the pack die first as these are surrounded by other cells.  Not only can these "inner" cells not get rid of their own heat as easily as those cells on the "outside" layer of the pack, but they are also exposed to heat from the cells that surround them!

How important is heat to the life of a cell?  One oft-quoted statistic (that I've not verified personally with NiCd cells) is that for every ten degrees F of temperature rise above 80 degrees F, the usable lifetime of the cell will be halved!  Even if these numbers aren't exactly correct, cells that are warmer will die sooner!

Inevitably, one or more cells will run down sooner than the rest and its voltage will drop.  Because the other cells still have some charge, current is still flowing through it and the now-dead cell's voltage will not only drop to zero, but it can go below zero and effectively start to "charge" backwards because at least some of the remaining cells are still outputting voltage.

The effect of this is a very quick death to a NiCd cell! 

Why?
 
It comes down to chemistry.  When a NiCd cell is reverse-charged, a strange thing happens:  Conductive metallic "hairs" (often called dendrites) begin to form - and they "grow" from one electrode to another.  Eventually, this dendrite forms a short across the cell - one that can have a range of resistance from high to low, depending on the severity of the damage and the size of this dendrite.

Once this dendrite has formed in the NiCd cell it is permanent and cannot be "dissolved" by charging the cell correctly or by doing any sort of "conditioning."  Furthermore, this dendrite can form a leakage path that can cause the cell to run down by itself - the rate at which can vary depending on the resistance and relative size of the dendrite.  The effect can range from a cell that just doesn't "hold a charge as long as it used to" to or, in extreme cases, the dendrite may be big enough that the cell won't even seem to take a charge at all (except, maybe, on a "quick charger.")

Perhaps the worst thing about the dendrites is that they represent an amount of electrolyte that can no longer be used to contribute to the charge capacity of the cell.  What this means is that not only is the cell likely to run itself down more quickly because of charge leakage due to the dendrite, but even if it is fully charged to begin with it will be the first in the battery pack to run down next time it is used and go into reversal - again - and will be prone to forming even more, bigger, and better dendrites!  (In other words:  A vicious little circle...)

Note:  NiMH cells do not seem to exhibit this "dendrite growth" problem, but cell reversal tends to cause gasses to be generated.  If these gasses are produced as too high a rate, they cannot be reabsorbed internally and pressure will build within the cell, causing outgassing when the safety vent releases and resulting in a permanent loss of cell capacity.
 
"ZAPPING" NiCds:

You may have heard about a technique for "restoring" NiCds often referred to as "Zapping."  As the name implies, one dumps a brief surge of energy into the cell and, almost as if by magic, the cell is "restored" to operating condition.

Well, not quite!

The surge of energy should be limited - often, a "zapper" consists of a very large capacitor (50,000 to 200,000 microfarads) charged to 50-100 volts, the source voltage disconnected, and the energy of this capacitor is dumped into a cell via a very heavy switch or a beefy SCR.  This "one shot" burst of capacitor-stored power prevents too much energy from being dissipated by the cell and blowing it (and the person doing the "zapping") up.

Another method uses a lower voltage - but much higher current - say, from a large power supply:  The obvious disadvantage of this latter method is that it is not "self limiting" as is the one-shot nature of the capacitor discharge and one can easily "pop" a cell either by burning open internal conductors or cause the cell to rupture due to a sudden buildup of heat and gasses.  Needless to say, neither situation (especially the latter) is particularly desirable!

What is supposed to happen in this process is that enough energy applied to "fuse" (or blow away) the dendrite that is shorting (or "almost" shorting) the cell.  Once this low-resistance path is removed, the cell can be charged again.  This doesn't completely remove the dendrite, but "disconnects" it (hopefully) but it still represents a degradation of the cell.

It should be kept in mind that such a "repaired" cell, although it may be more able to take a charge than before, will still have reduced capacity and, when used in a battery, is still very prone to discharging early and going into reversal - again.

Remember:  The material that formed the dendrite no longer contributes to the charge capacity of the cell - even after you "zap" it.  Furthermore, the cell contains a separator material that will often be damaged by dendrite growth and "zapping"  - something that further contributes to self-discharge.

If you do this technique, make sure that you have completely disconnected the cell/battery from the appliance being operated to prevent the voltage surge from the "zapping" process from damaging it.

Finally, while you may get some additional use out of a battery as a result of "zapping" I consider that "zapping" a cell may simply be buying me enough time to get a replacement ordered and on its way!

Note: 

It should go without saying that this "zapping" procedure can be hazardous:  Not only are potentially dangerous voltages and currents involved, but there is a chance that the cell may explode and/or leak hazardous material.

Finally, this procedure should be done only on an individual cell and not the entire pack at once - That is, you must be able to access and test each cell you plan to "zap", individually.

Getting the most out of your NiCd/NiMH cells:
 
For reasons unknown to me, some manufacturers of battery-operated equipment recommend that you "condition" NiCd battery packs by running them completely down, and then charging them again.  I guess that the claim is to prevent a "memory" condition from occurring - but it is already known that to cause this "memory" the cell would have to be precisely depleted to exactly the same charge state repeatedly:  This just doesn't happen with most people's usage of equipment.

Why do they make this recommendation, then?  The cynical side of me says that they are just trying to sell more batteries or devices:  By recommending you go through some steps that are guaranteed to shorten battery life, they can increase sales!  The other side of me would guess that the person writing these instructions is just poorly informed or just doesn't know any better.


Here are a few things you can do to prevent premature failure of NiCd battery packs:

• NEVER, EVER run a NiCd battery pack completely down.  Inevitably, one or more cells will go into reversal before the others, immediately causing permanent damage to the cell(s).  The only safe way to run a NiCd battery pack completely down is to guarantee that no cell can possibly go into reversal.  This can only be done by monitoring each individual cell and preventing reversal by bypassing it - but almost no manufacturer of consumer goods does this due to cost and complexity.  NiMH cells aren't totally forgiving either:  While they may not be immediately damaged by reversal, such operation can result in loss of capacity due to outgassing.  (Note:  Rechargeable lithium-ion packs use exactly this sort of protection because Li-Ion cells are completely unforgiving of a complete discharge/reversal.)

• DO NOT try to drill that "last hole."  Have you ever been using a cordless drill when, just before the battery goes completely dead, it suddenly slows down and loses most (but not all) of its power?  At that moment, one or more cells have collapsed and are going into cell reversal.  Your battery pack will last much longer if you stop using it the instant that the motor slows due to the voltage drop.  Unlike alkaline cells, NiCd and NiMH cells will put out (more or less) the same voltage until they are almost totally dead - at which point their voltage will suddenly drop.  Again, if NiCd battery packs had the same sort of circuitry in them that Lithium-Ion battery packs did (e.g. a circuit that "disconnects" the pack when any one or more cells' voltage drops too low) they would, on average, last much longer.

• Do not overcharge the cells.  Nowadays, "smart chargers" are pretty good about preventing cell overcharge, but if a battery pack gets unusually hot, something may be wrong. So-called "trickle" chargers won't destroy a battery pack too quickly if they are left connected after the battery is fully charged, but it isn't a good idea to leave it connected forever.  If the battery is noticeably warm when connected to a trickle charger, it is already overcharged.  Overcharging NiCd or NiMH cells can cause gasses to form in the cell's electrolyte and if this pressure builds up, a safety vent in the cell can open (which is better than having the cell explode...) and the gas will be vented.  This venting represents a loss of material - which also means a loss of cell capacity.  Another phenomenon that can shorten life of a trickle-charged cell is the breakdown of the plastic separator due to its continuous exposure to oxygen at elevated temperatures.

• Don't leave the pack on the charger and walk away!  Related to the above, it is a terrible idea to leave a battery on a charger all of the time.  If you can detect any warmth from the battery when it is left on the trickle charger for a day or so, it is being trickle-charged too strongly!  Unfortunately, it's very easy to "charge and forget" many power tools and slowly kill the pack.  Of course, there's the desire to have the portable device always at the ready, so the temptation to leave it on the pack is almost irresistable!

Interestingly, I hear from many people about the battery pack of their (whatever it is) suddenly dying - that is, seeming to have abruptly lost "run time".  In general, it is the nature of NiCd and NiMH cells to maintain a more-or-less constant voltage until it is (pretty much) dead, at which point it will suddenly drop off.  It is often the case that even a cell in poor shape will behave this way and the device will work properly for a while, but it is only when the run time becomes annoyingly short that one is really aware that there is a problem - particularly if it's a device such as a portable drill or vacuum that isn't normally run for long periods at a time.  In other words, it's usually the case that the battery has been sub-par for a long time, but it just wasn't noticed!

The differences in using NiCd and NiMH cells:
 
At first glance, it would seem that NiMH cells are just "better" versions of NiCd cells as they have the following advantages:

• Their energy density is better:  A NiMH cell has more charge capacity than the same-sized NiCd.

• They do not contain Cadmium - a toxic heavy metal - and thus do not pose as much of a disposal problem.

• They are (apparently) not prone to forming dendrites when they go into reversal - something that can kill a NiCd by shorting it out internally and/or raising self-discharge current - not to mention loss of capacity.

NiMH cells have a few disadvantages as compared to NiCds:

• Their lifetime with respect to the number of charge/discharge cycles is lower (250-500 for NiMH versus 500-1000 for NiCd - but this is improving)

• They have a relatively high self-discharge rate:  Just sitting around they tend to run themselves down more quickly - especially as they age.

• They have a slightly higher internal resistance and a lower current-carrying capacity than an equal-sized NiCd:  This generally makes them inappropriate for use in high-current drain devices such as cordless power tools where the load may be several "C" (i.e. 2-3 amps of load per amp/hour of cell.)  Newer types of NiMH cells are beginning to appear that do not have quite this limitation.

• It is more difficult to tell when NiMH cells are fully charged than NiCds.  When NiCds are nearly fully charged, the voltage suddenly rises - and then starts to go back down again when an overcharge condition is approaching.  NiMH cells will do this, but the magnitude of this voltage rise is only a fraction of that of NiCd cells and, under many charge conditions, may go completely unnoticed, hence the need to monitor the temperature of NiMH cells as well as to limit the amount of time over which a charge is applied.
  

In practical terms, a NiMH cell may actually outlast a NiCd in terms of charge cycles even though they supposedly have fewer charge/recharge cycles.

Why?

Again, a lot of NiCd cells "die" due to cell reversal (see above) and the resultant effects while NiMH cells do not readily form dendrite shorts when they go into reversal.  Damage to a NiMH cell may still occur, though:  Cell reversal of a NiMH cell causes gases to form and it is possible that pressure will build up faster than its chemistry can reabsorb these gasses and the cell will vent.  The resultant loss of gas means a loss of electrolyte material and a subsequent loss of capacity.


Self Discharge:

Even without loads, all cells slowly lose their charge over time as the cell's chemistry slowly changes.  In all cases, the rate of self-discharge increases dramatically as temperature also increases.

Comparison of self-discharge rates of various types of cells The table below shows the approximate amount of time that it takes to lose 10% of the cell's capacity at different temperatures. Cell Type 0C (32F) 20C (68F) 40C (104F) 60C (140F) Alkaline >15 yrs. 4 yrs. 18 mo. 3 mo. NiCd 3 mo. 1 mo. 14 days 5 days (A) NiMH 1 mo. 10 days 5 days 1-2 days Zinc Chloride 6 yrs. 2 yrs. 10-12 mo. 2-3 mo. (A) Li-Ion (B) - 1-12 mo. - - Lithium-Fe-S >20 yrs. >20 yrs. 4 yrs. 1-2 yrs. (A) Lithium-Mg-O >15 yrs. 10 yrs. 3 yrs. 1-2 yrs (A) These are typical values for new cells, published by various manufacturers.  Note that aging/mistreated cells will probably exhibit much higher self-discharge rates. Notes: A - Storage or use of this type of cell at 60C violates the manufacturers recommendation for consumer-type cells and one may expect poor lifetime.  It is not recommended that any cell be exposed to such high temperatures for an extended period of time. B - The self-discharge rate of LiIon cells varies widely according to its chemistry and manufacturer.  Information on self-discharge rate at temperatures other than 20C was not available at the time of writing but, as in the case of other types of cells, it increases dramatically with increasing temperature and the age (and past use) of the cell.

The chart above compares various cell chemistries and their approximate rates of self-discharge showing how quickly one can expect to lose 10% of the cell's capacity.  Please note that these rates are typical published specifications by various manufacturers and, in the case of rechargeable cells, represent the sort of performance that may be expected from new cells.  In general, independent testing has shown that the manufacturers' specifications concerning self-discharge are more-or-less in line with what is actually observed.  Also, reduction of self-discharge is one of those parameters on which the manufacturers are continually improving.

As can be seen the clear winners are the non-rechargeables and the two cheapest type are the alkaline and Zinc Chloride (the so-called  "heavy duty" batteries) which do a respectable job of retaining their capacity over time.  Ahead of the pack are the non-rechargeable Lithium types (The Lithium-Iron Disulfite and the Lithium Manganese Oxide) and these two chemistries also perform better than the others even when they are very cold.

The worst of the bunch is clearly the NiMH cell which could easily be found dead after having been left in a vehicle for a month during the summer (if you have hot summers, that is...)  It is certainly worth repeating that NiMH cells are NOT the proper choice for your car flashlight, for example, or even for any item that is left idle for months at a time and is then expected to work (such as an emergency radio.)

What about putting cells in the freezer to "keep" them?  For the non-rechargeable types, it can be seen that freezing them will certainly slow the self-discharge rates, but if you plan to use them within a year or two you'll probably not see any real difference in longevity of those stored in your freezer and those simply kept at room temperature.

What is clear from this chart is that you should not be storing them in your attic or garage - or anywhere else that may tend to get warm:  It is preferred that they be stored simply in a cool location (such as a basement) as compared to a warmer room or a vehicle.


Replacing NiCds with NiMH cells:

Can you simply drop NiMH cells in place of NiCds?

It depends.

For optimal cell lifetime and performance under ideal conditions, the answer is probably no.

For "good" performance (that is, where overall lifetime and charge capacity will probably exceed that of NiCd cells) the answer is likely yes - as long as a few rules are observed:

• You (probably) can't/shouldn't use NiMH cells in very high-current devices such as power tools.  These sorts of demands on the cells will result in a very short lifetime and could be hazardous due to cell overheating and venting.  Again, there are certain types of NiMH cells designed especially for this type of service but you will have to do research on where to find them and if they will, in fact, be suitable for your intended application!

• A NiCd-only "smart charger" or "quick charger" may not be able to detect when a NiMH cell is fully charged.  This could result in undercharging (the cell isn't charged completely) or (more likely) overcharging and "cooking" the cell if the charger cannot detect a full-charge condition.  Make sure your quick charger is specifically designed for charging NiMH cells before you use it.

• Charging a NiMH cell from the original NiCd "slow" charger should work, but it will probably take more than twice as long as it did with the NiCd charger.  Typically, "slow" chargers will charge a NiCd pack in 12-16 hours, but this means that the same charger will probably take 30-36 hours to charge the NiMH pack.  This extra charge time is required because the storage capacity of the NiMH cells are likely to have at least twice the capacity of same-sized NiCd cells that they replace.

If you replace NiCd cells with NiMH cells there are a few things that you should keep in mind:

• Dispose of the dead NiCd cells properly - do not just throw them in the trash.  Do a bit of research and find out where to dispose of the dead cells.  (Your local recycling or trash-collection agency can probably tell you where to go... so to speak...)

• Take note of the guidelines in the above section when you charge NiMH cells:  They may not charge properly in a "smart" or "quick" charger and a slow charger will take much longer to charge NiMH cells.

• They have a much higher self-discharge current (see the chart above.)  If you charge the battery pack and forget about it, do not expect it to still be charged months later!

It should be remembered that one of the main reasons why NiMH cells seem to last longer than their NiCd counterparts is just that they can better-tolerate the abuse typically inflicted upon them.  At the risk of repeating myself, here are some examples:

• In a power tool:  Continuing to use it after one or more of the cells in the battery have gone dead and it slows down.  This is guaranteed to very quickly kill ANY NiCd cell!

• Putting it on a charger and walking away:  For a number of reasons, NiMH cells seem to be better-able to tolerate this sort of abuse than NiCd's, but it is still a bad idea!  If any rechargable battery pack is noticeably warm after being fully charged and is left in the charger, it is already overcharged and is likely being (slowly) damaged!

What could manufacturers (and you!) do to prolong NiCd/NiMH cell life?
 
Again, it somewhat irks me that the appliance manufacturer's recommendation (i.e. to completely discharge a NiCd pack) is precisely the thing that can kill NiCd cells prematurely due to the inevitable reversal that will occur in a series-wired battery pack.  What is so terrible about this is the cost of replacement and inconvenience that results:  Often, the user will simply throw away the entire appliance and effectively wasting money!

There are several things that could be done to greatly lengthen cell life of both NiCds and NiMHs:

• Don't recommend that the batteries be completely discharged.  Ever!  There is absolutely no need for this in most cases.  Most often, one cannot completely discharge a pack without causing permanent damage from cell reversal!  Again, NiMH cells are more resistant to damage due to reversal.

• Build into the packs (or the appliance) a device that will cause current consumption to cease if any cell drops below, say, somewhere between 1.0 and 0.6 volts.  This will prevent cell reversal from ever happening in the first place.  An example of this sort of protection is found in all Lithium-Ion rechargeable battery packs because allowing them to be run completely down and then recharged constitutes a very real safety hazard!

Ironically, NiCd cells are, by their nature, some of the most reliable, long-lived rechargeable cells around and will far outlast NiMH and standard (rechargeable) LiIon cells in terms of longevity and the number of charge/discharge cycles - but only if they are treated properly!  It is through abuse due to allowing them to be "reversed" and being grossly overcharged that they have developed an undeserved reputation for being unreliable!

For a longer version of this article with links to related pages, go to the "About NiCd and NiMH rechargeable batteries" - link web page.

[End]

This page stolen from ka7oei.blogspot.com

Lithium Iron Phosphate (LiFEPO4) batteries revisited - Equalization of cells

On August 31, 2012 I posted about problems that I'd been experiencing with three "12 Volt" LiFePO4 batteries that I'd bought in 2010 - see:

"Problems with Lithium Iron Phosphate (LiFePO4) Batteries" - link

I was alarmed by the fact that after just 2 years, all three of these packs had appeared to degrade badly:  Of the three, two of them had seen their original 6+ amp-hour capacity diminished to 2 amp hours or less (the third was at about half capacity) and their capacity was decreasing every time I checked them.

In response to this post I had several comments/suggestions (you may find them at the end of the original post, linked above) about what might explain the problems that I was having, including:

• Temperature variations.  Very cold temperatures will dramatically reduce the available cell capacity, but these batteries were all tested at similar (room-like) temperatures.

• Perhaps I'd charged the cells to too-high a voltage and damaged them?  I'd checked and found that the internal circuits prevented the cells from "seeing" voltages in excess to those noted in the manufacturer's recommendations.  Considering the rather light usage (only a few dozen cycles) it seemed unlikely that they would suffer such damage so quickly even if they were abused a bit.

• Self discharge?  Perhaps the protection circuits inside the battery were causing the packs to bleed down?  Maybe the cells themselves were losing charge on their own?

The first two of these points - while valid - just didn't seem to apply in this case.  Related to the third point, however, I'd checked out and found that while the built-in protection circuits of the battery pack did continuously draw current, it was very small in comparison with the normal self-discharge rate of the packs, but the poster did bring up some interesting points about the possibility of imbalance within the battery pack itself.  In my reply, I somewhat dismissed this as a possibility but in retrospect, probably a bit too quickly.

More recently I finally got the time and inclination to disassemble one of my LiFePO4 packs and do a bit of looking.

Analysis:

First, I charged the pack by connecting it to a 14.6 volt, current-limited (500 mA) source.  After the current dropped to nearly zero (overnight) I disconnected it from the charger and let it sit for a week or so to let the cells "settle out" and attain a steady-state voltage.  Upon dis-assembly to the point that I could measure the voltage across each individual cell I found that they were fairly close - within 0.1-0.15 volts or so - not radically different, but still a bit more than I would have expected.

I then put the battery pack on my West Mountain Radio CBA-II battery tester and did a capacity test which subjected it to a constant 2 amp load while graphing and integrating the results.  As expected, this pack, which had measured at about 6 amp-hours when it was new, now measured less than 2 amp-hours.

Upon measuring the voltages across the cells in this discharged state the precise nature of the problem became clearer:  Several of the cells were in the 2.8-3.1 volt area but one cell was at about 2.4 volts and it had no doubt been lower (probably down to 2.0 volts) when it was under the load of the tester!

Did I have a bad cell?

This was my first thought, but before I was about to condemn anything, I needed to do a bit more testing.

I charged the pack again and when complete - but still while on the charging voltage - I checked the cell voltages and saw that at least one of them was at about 3.9 volts - just at the voltage where the protection circuit would disconnect the charging current for the entire string - and this meant that charging would no longer proceed.  I then set an adjustable voltage supply to precisely 3.65 volts and connected it across each cell, in turn, for several hours (until the charging current dropped off) to assure that every cell was fully charged.

Interestingly, the cell that I had thought might be bad (the one at about 2.4 volts) took a charge - and then kept taking a charge for several hours before the current tapered off.  In contrast, the cell that had been at the highest voltage immediately reached full charge as indicated by the current dropping off quite soon after I'd applied it.

Cell imbalance and how it can cause apparent loss of capacity:

This behavior implied a problem that was different from what I'd expected:  Gross cell imbalance.

One vexing problem with any system of series-connected cells (e.g. a battery) is that since not all cells are exactly the same, one will inevitably discharge sooner than the others.

On "primary" batteries, this isn't too much of a problem since being non-rechargeable, a few percent of difference between cells won't have too much effect over the pack as a whole.  On a "secondary" battery - the types that can be recharged - this can be an issue, especially if the problem accumulates over a number of charges.

For good, old-fashioned lead-acid cells, the penalty of slightly overcharging isn't great:  Gasses are produced and one can trap them (as in a VRLA - Valve-Regulated Lead Acid) so that they can be reabsorbed if not generated in excess, you can convert them back to water (e.g. catalytic recombination as is done with "Hydro Caps") or simply add water as needed.  In a normal "float" service where the cells are held at a "full charge" state they constantly pull a small amount of current and if this is maintained for a fairly long period of time, cells/batteries that weren't fully charged will gradually equalize... for the most part.  In other words, we can - to a reasonable extent - prevent cell inequality from occurring by purposely allowing lead-acid batteries to be "gently" overcharged and the chemistry allows this without severe deleterious effects.

Perhaps the most notable of deleterious side-effect of cell imbalance is that experienced in NiCd packs where the premature discharge of one cell results in its effectively being charged backwards and a chemical process occurs in which dendrites (metallic hairs) grow, reducing its capacity and in severe cases, shorts the cell and/or causes it to self-discharge very quickly.  In this case, this cell - already weakened due to dendritic damage - will be the first to discharge the next time it is used and this will happen again, causing even more degradation!

In the case of a Lithium pack, this unintentional "reverse charge" doesn't occur as there are (supposed to be!) circuits that prevent any of the cells in the pack from being discharged too deeply by disconnecting the entire battery from the load - something that (in Lithium chemistry) can cause permanent damage.  This same circuit also does something else:  If, while charging, the voltage on one or more cells exceeds a particular threshold (typically 4.2 volts for most conventional Lithium-Ion cells) the entire battery is disconnected.

For the LiFePO4 battery, the chemistry is a bit different.  While the "low voltage disconnect" of these cells is comparable to that of some conventional Li-Ion chemistries (2.0 volts) the high-voltage disconnect on the particular LiFePO4 cells that these packs contain is 3.95 volts maximum.

Comments:

• The nominal "full charge" voltage for these particular LiFePO4 cells is normally 3.65 volts.  As is typical for any rechargeable cell, the voltage on a cell immediately decreases when the charging source is disconnected.
• Unlike lead-acid cells, if we attempt to even "gently" overcharge any lithium-ion type of cell to force an equalization, the chemistry of the cell is such that we will damage it in attempting to do so!  At the very least this can greatly reduce the lifetime of the cell and in the worst case, cause a catastrophic failure such as leakage, cell rupture or even fire.

What seemed to be happening to these LiFePO4 battery packs was this:

• When the battery pack runs down:  One cell reaches 2.0 volts, the internal circuit disconnects the load.  Other cells have probably not reached 2.0 volts and still have remaining charge.
• When the battery pack is charged:  One cell (but not the same one as above) reaches 3.95 volts and the internal circuit disconnects the charging current.  Inevitably, the other cells that did not reach this voltage and were thus not as fully-charged.
• When the battery is discharged the next time.  The cell(s) that didn't get fully charged the last time will likely be the one(s) that discharge first:  When that happens, the load is disconnected, but the other cells will still have charge on them.
• When the battery is charged again.  Those cells that had the most charge will reach the maximum voltage first, leaving the more-discharged cells with a less-then-full charge. 
• This process repeats.   The disparity between those cells that are fully charged and those that were not fully charged will increase.  Because the overall capacity of the pack is dictated by the weakest cell - in this case, the one that has the least amount of charge - the apparent capacity of the battery pack will gradually decrease.

Figure 1:
One of the LiFePO4 battery packs when nearly new (black) and after 2 years (red)
there is very clearly a loss of available capacity!
Click on the graph for a larger version.

Figure 1 shows the degradation of the capacity of one of these battery packs.  The black trace shows the capacity of the battery when it was fairly new while the red trace shows this same battery pack's capacity just 2 years later.

It should be noted that the load test current in the graphs on this page were 2 amps, much higher than the "20 hour rate" at which battery manufacturers typically rate batteries.  Because of this, the tested capacity was somewhat lower than the pack's official 6+ amp-hour rating.

In each case you can see that near the end of the discharge curve, the battery voltage starts to droop - and then is abruptly cut off by the protection circuit when one or more cells drops below 2.0 volts.  What I should have noticed was that the overall voltage before the curve started to get steep was actually lower when the pack was "good" than it was when its capacity had diminished.  What this indicated was that when the battery pack was newer and performing better (black trace) all of the cells' voltage were dropping more nearly in unison while on the red trace, we don't see that gradual drop to nearly the same extent implying that, perhaps, just one cell was dropping out sooner.

Equalizing the cells - the result:

As mentioned above, I individually charged the cells using a regulated, current-limited power supply that was set to 3.65 volts and then individually tested each cell's amp-hour capacity.  I was pleased to note that every cell seemed to be fairly close to its rated specifications and, as I now suspected, the cells that had been noted to have discharged first when the pack as a whole was tested were, in fact, those that had slightly lower capacity than the others by 5% or so.

As described above, I recharged the entire pack and then topped each cell off, individually, to 3.65 volts, and then re-tested the entire pack again:

Figure 2:
Same graph as Figure 1, but with the green trace showing the capacity after equalizing.
The black trace is that of the battery when it was new.
The red trace was that after 2 years, showing the degradation.
The green trace is the capacity after equalizing the cells in the battery.
Click on the graph for a larger version.

Figure 2 shows the result of that test.  As can be seen, the capacity of the battery pack after having equalized the cells (the green trace) is slightly better than it was when it was brand new!

Clearly, there was a problem to be solved:  How do I keep this - and the other - LiFePO4 battery packs "equalized" and still have them be practical and safe to use.  Here are the points to consider:

• I did not want to remove the battery's built-in protection circuit as it takes very few cycles of gross over/under voltage before one permanently wrecks Lithium-Ion cells.  While LiFePO4 cells are considered to be (more or less) "intrinsically safe" when compared with conventional Lithium-Ion cells, that just means that they are less like to catch fire/explode if mistreated, not that they won't degrade quickly!

• A practical method of charging the battery pack and maintaining equilibrium of the cells within.  What this meant was that I was not going to disassemble the pack every time I wanted to charge it so that I could get at each cell!

• Charging/equalizing should be relatively simple and convenient.  This goes with the above and it would imply a simple box that I could connect to the battery while charging, when I wanted to equalize the cells.

One thought was to attempting building a circuit into each LiFePO4 battery pack.  The problem with this idea was that there was not very much room in any of the batteries in which to fit a circuit - much less a way to dissipate the heat that was likely to be produced when the action of the circuity prevented overcharge of specific cells!  Additionally, any circuit that was left connected full-time would have to be designed to minimally load the cells and run them down when not being charged.  Needless to say, it would also be time-consuming to build a circuit for each battery and with the lack of room, there would be the likely need to repackage each battery to accommodate the circuitry!

Instead, I decided to modify each pack, bringing out connections from each individual cell so that the voltages could be monitored and controlled during "normal" charging of the pack.  To do this, I added a short cable with a 5-pin DIN connector which provided the points needed to which I could connect an external box with which the cells could be equalized when charging. What this also meant was that I needed to construct only one external device to monitor and control the voltage on three LiFePO4 battery packs that I had!

A charge monitoring and control circuit:

Figure 3:
Example of the charge monitor/control circuit.  This example shows a 2-cell circuit for simplicity, but one
could scale this design up to accommodate practically any number of cells.
The "X" in the leads marks where the connector used to attach the charge equalizer to the battery would go.
With the values shown (R103/R104) the shunt voltage is about 3.65 volts/cell which is appropriate for these
LiFePO4 cells:  This voltage would need to be adjusted if other types of cells were to be used. For more
than 2 cells, R1/LED1 would be connected across the entire pack (all cells)  to provide a reminder/ indication
that the device was still connected.
Click on the diagram for a larger version.

Figure 3 shows the circuit that was constructed to aid in properly charging and equalizing the voltages of the cells within the battery.  Based on the readily-available TL431, this is a simple shunt regulator that, if the voltage across any cell gets to be too high, starts to conduct the current around that cell by clamping the voltage to the maximum level.  In this way, if a cell reaches full charge before the others the shunt regulator will allow the other cells to continue to charge and, eventually, all cells will reach voltage equilibrium:  For our purposes, this is the best we can do in assuring that all cells are charged equally!

Please note that the diagram in Figure 3 represents what would be required for just a 2-cell (nominally "6 volt") LiFePO4 battery:  One would simply replicate the circuits as needed for as many cells as the battery pack contained, connecting them in series as shown in the diagram.  For the discussion below, we'll generally refer to the parts in the circuit connected to Cell 1 which are numbered with parts designations in the "100's".

How it works:

As noted above, wires are connected to the cells inside the battery pack and brought out through a cable that is separate from the main power connector.  I used a 5-pin female DIN connector as it had precisely the number of connections needed:  Battery - (minus) and the + (positive) terminals of each of the four individual cells.

You will note that in the diagram there are devices marked F101 and F201 which are 1 amp, self-resetting thermal fuses located at the point where the wires connect to the cells (or protection circuit) inside the pack.  These are extremely important as an accidental short-circuit of one of the wires emerging from the pack could result in burns or a fire without its presence as the cells will easily supply enough current to burn the wires open!  Adding these devices is a cheap way to make the modification safe and should a short occur, simply fixing the problem (removing the short) will cause the fuse to automatically reset when it cools down - more on these devices later.

The heart of the circuit is U101, a TL431 shunt regulator - a part that may be found in nearly every PC power supply.  This device is essentially a programmable Zener diode:  If the "adjust" terminal is more than 2.495 volts (nominal) above the anode terminal, it will conduct via the cathode.  The values of R103 and R104 are chosen so that if more than 3.65 volts appears between the "top" of R103 and the bottom of R104 (which is also connected to the anode of U101) more than 2.495 volts will appear on its adjust terminal and the device will turn on.

When U101 turns on it will start to "pull down" on the base of Q101, a PNP power transistor, which will conduct, amplifying the effects of U101 and start drawing current through R101  (R102 keeps Q101 turned off otherwise.)  Were it not for the presence of R101, a 0.25 ohm resistor, the "knee" of this conduction would be extremely steep, but this resistor gives the circuit a bit of elasticity (a bit of range over which the current will go from nearly zero to maximum) and also assure that the circuit will not oscillate.  With the parts in the diagram a current of about 200 milliamps through the shunt regulator - plus the finite gain of U101 - will result in a clamping voltage on the cell of around 3.75 volts, not including the effects of the resistance of the wire/connector that runs to each cell within the battery.

As a convenient indicator that the U101 circuit is actively shunting current from the cell, Q102 detects when U101 is "pulling down" on the base of Q101 and turns Q102 on, causing LED101 to illuminate.  Q102 and associated components (LED101, R105, R106) are optional if you don't need an indication of a particular cell reaching full charge.

Finally, there is also a simple circuit consisting of R1 and LED1 that is across the entire battery.  The sole purpose of this is that of an indicator to show that this circuit is connected to a battery and serve as a reminder that it should be disconnected when charging is complete!

Comments on parts and construction:

As noted above, by cascading multiple copies of this circuit one may apply them to as many cells as needed - whether it is just a pair of cells, or many more than that.  Because my LiFePO4 packs were "12 volt" types with 4 cells in series, I cascaded 4 of the circuits together, building it on a small piece of perforated prototype board as shown in Figure 4.

The type of prototyping board that I used has a small copper ring around each hole making construction much easier, more rugged and more durable. 

I do not currently have plans to produce a circuit board for this project, but let me know if you do as it might be of interest to others.

Figure 4:
A 4-cell Cell equalizer circuit built on perforated
prototype board.
Click on the image for a larger version.

The TIP32A PNP power transistors are of the TO-220 type (with metal tabs) and were chosen simply because I had a lot of them on hand, but practically any PNP power transistor could have been used - but I would not recommend Darlington types due to possible emitter-base voltage limitations.  As can be seen in Figure 4, pieces of copper, bent to fit within the box and to increase surface area - were soldered to the tabs of these transistors to improve power dissipation.

With the heat sinks shown and with the box put together, the unit can easily withstand 200 milliamps of shunt current on each of the four sections without causing excessive thermal stress of the components inside from the 3-4 watts total heat generated.  If much more shunt current capacity is desired, larger heatsinks and/or better ventilation - or perhaps connecting the transistor's tabs to a metal box (making sure that their heat sinks were electrically insulated) - would be recommended along with the decrease of the value of R101.  Practically speaking, by proper choice of the transistor, the value of R101 and heat sinking one could design this sort of circuit to accommodate practically any amount of shunt current!

In the lid of the box I drilled some holes and used cyanoacrylate ("Super" tm) glue to to hold the LEDs in place temporarily (overnight) and then used clear epoxy to more-permanently secure them into position afterwards.  Because this shunt regulator was intended for 4 cells, there is a red LED for each of the shunt regulators to indicate its "full charge" status plus a fifth, bright blue LED as an indicator to show that the unit is, in fact, connected to a battery a serves as a reminder to disconnect it when charging is complete.

Figure 5:
An external view of the cell equalizer unit built to accommodate
up to 4 cells.  There are separate indicators that show which
cell(s) are at full charge - plus a "power" indicator.
Click on the image for a larger version.

Figure 5 gives a view of the completed shunt regulator showing the short cable and the male DIN connector that is connected to the mating connector on the battery.  As the label indicates, the five LEDs on the front show the full-charge status and whether or not it is connected to power.  It is worth noting that when they were installed, the LEDs weren't fully pushed to protrude through the front panel.  If they had pushed all of the way through the hole, they would have stuck out too far and would have risked being accidentally broken loose and pushed back into the box during handling.




Comments: 

• A "simpler" way to do this:  In theory, one could parallel a fairly low-value resistance across each cell - say, a 22 ohm, 1 watt device - and achieve equalization with this value of resistance equating to about 165 milliamps of current at 3.65 volts.  Eventually - after, perhaps 36-48 hours for a pack of this size - the cells would gradually equalize their charges - and the lower the resistance, the more quickly equalization will occur.  If this is done, resistors with the appropriate power rating should be used and they should all be of equal value and, of course, they should be disconnected immediately after charge has been completed, just before the charging voltage is disconnected.  Failure to remove these resistors after the charging voltage has been removed will surely run down (and likely ruin!) the battery!

• For other than LiFEPO4 cells:  For different types of cell chemistry one would choose the values of R103/R104 as appropriate (see the data sheet for the TL431 for more information.)  Note, however, that this circuit will simply not work for voltages much less than 3.0-3.2 volts!  If, for example, one wanted to regulate "2 volt" lead-acid cells you could use this sort of circuit directly, but multiple regulators could be placed across pairs of cells (and the shunt voltage set to approximately 4.55 volts) in an interleaved manner and achieve the same effect.

• Make sure that all circuits are equal:  Once constructed, the clamping voltage of the individual cells' circuits should be checked at the same current, say 200 mA and if necessary, adjust the value of R103 or R104 to make all of them equal:  All cells being within 50 millivolts (0.05 volts) of the target voltage should be more than adequate.  It's worth noting that normal variations in the resistor values (particularly if 5% tolerance parts are used) as well as variations among TL431's can cause voltages to differ slightly.

• Where to find the TL431:  As mentioned above, the TL431 can be found in nearly every PC-type power supply made.  If you have some scrap power supplies laying around, you may be able to find enough of these devices for your project!  The devices that I used are in TO-92 packages and look just like a transistor.  Different manufacturers have their own prefixes, but if you see a part that looks like a transistor and ends in "431", it's probably a TL431!  If you don't want to dig around in old power supplies, these chips are readily available from many distributors at well under US$1.00 each.

Modifying the battery packs:

Because of the high currents involved, there is some risk of burns/fire should connections be accidentally shorted.

Do not perform this sort of modification unless you know exactly what you are doing!

One of the more critical aspects of this projects was safely modifying the battery pack.  As noted above, the wires connecting to the outboard charge equalizer have current limiting on them in the form of (approximately) 1 amp, self-resetting thermal ("PPTC") fuses that are located inside the battery's case.  

For a Wikipedia article on these PPTC devices, click on this link.

Again, the fuses are necessary safety components to prevent burns/fire should the added connection(s) be accidentally shorted out!

Some builders may not be familiar with these self-resetting fuses so a few words of explanation are in order.

Typically resembling small, yellow disk-ceramic capacitors, these devices are actually specially-designed thermistors:  If the current through them exceeds the design threshold, they will get hot (but not too hot!) and their resistance will skyrocket.  When in this "tripped" state, enough current will still flow through them (typically 5%-15% of the rated trip current) to keep them "warm" and in the "tripped" condition with a high resistance.  Once the fault has been removed, they will cool down and reset themselves as their internal resistance returns to normal.

Figure 6:
This shows the modification to one of two identical packs.
This battery - housed in a water bottle for bicycle use -
was really two separated batteries, each with its own protection
circuit, the outputs of the two connected together.  These packs
were rewired in "2P4S" (series-parallel) format, still with the
protection circuits in parallel.
The four yellow self-resetting thermal fuses are visible.  Some
pieces of cardboard were added to protect the plastic cover
of the cells from the heat of these fuses (somewhat over
100 C) if they are tripped, which could damage their thermal
shrink wrap.
Click on the image for a larger version.

These devices are readily available from electronics suppliers like Digi-Key, Mouser and Jameco (to name but a few) and aren't all that much more expensive than normal "one-time" fuses.  Available in current ratings from below 100 mA to 10's of amps, the 1-amp rating noted is a suggestion:  The devices that I used were actually rated for 1.1 amps of "holding" current, but anything in the range of 0.6-1.5 amps would have been just fine in this application as that current range will keep the wire from burning up in the event of a short, yet be enough to handle the expected shunt current.

As mentioned, when "blown", these self-resetting fuses do not completely stop the current flow, but reduce it to a "safe" level.  What this means is that should the wire be shorted out indefinitely, the affected cell(s) will eventually be discharged (and the cell damaged/ruined!) but you aren't likely to set anything on fire - and this is particularly important since our connection to the cells must bypass the protection circuits within the battery pack!

Of course, if you wish you can substitute "normal" (one-time) fuses.  While these have the disadvantage of being usable only once, they do have the advantage that if a wire shorts and the fuse blows, current completely stops and you won't likely run down - and possibly damage/ruin - a cell.

Figure 7:
This shows the modification to a more normal "2P4S" battery
pack.  Some of the self-resetting thermal fuses are just
barely visible around the edges, connected directly to the
protection circuit board.  Using a voltmeter, the connections
on the protection board to the cells were identified and marked.
Note that the wire connections are insulated with
heat-shrinkable tubing.
Click on the image for a larger version.

Of the three LiFePO4 packs that I modified, two of them were built into plastic water bottles, the original intent being to provide power for bicycle lighting and being placed in the water bottle holder.  These packs actually contained two individual battery packs (a "4S+ 4S" configuration) - each with its own protection circuit.  Rather than connect two equalizer circuits to the pack (one for each "sub-battery") I simply connected the cells of the packs in series-parallel - as is the normal practice - leaving in place each of the protection circuits.  The end result is that there now one series-parallel sets of cells and two parallel protection circuits.

The other pack (Figure 7) was constructed in the traditional way:  Series-parallel connected cells (four pairs in series in a "2P4S" configuration) to get the desired voltage and capacity and as such, the modification was much easier to accomplish.

To find the connections to the individual cells, the battery packs were (carefully) disassembled and the locations of the individual wires traced.  Using a voltmeter, find the "bottom" cell of the pack (usually marked "B-" and using it as a reference, locate and mark each subsequent cell by the successive increase in voltage.  Often, these other cells are marked "B1", "B2", etc. with the "top" cell marked with "B+".  Note that it is not necessary to put a fuse on one of the "end" leads (the B+ or the B-) - but I usually omit the fuse at the B- lead:  It is safe to do this as any possible combination of wires being shorted will always involve at least one fuse in the leads!

Note:  On the battery packs, female DIN connectors are used to prevent accidental contact with the voltages that are always present on these pins.
 

Using the Shunt Regulator/Equalizer:

Figure 8:
The three modified LiFePO4 packs, the extra wire/connector
being clearly visible.
Click on the image for a larger version.

When I charge my LiFePO4 packs I set a current-limited voltage-regulated supply at 14.60-14.70 volts which works out to 3.65-3.675 volts per cell - just enough to trigger the shunt regulator on each cell were the voltages to divide equally.  When I connect the battery to the supply, I also connect the cell equalizer/shunt regulator and then walk away for a while.  At a maximum charge current of 1 amp, I typically allow 12-16 hours for the pack to fully charge if it is already reasonably well-equalized.

Through experimentation, I determined the precise voltage at which all four shunt circuits would conduct at 150-200 milliamps - which turns out to be about 14.72 volts for the device that I'd built.  When the current from the power supply settles down this amount for a few hours I know that the voltages on the cells are equalized and that the battery should be removed from the charging voltage.  At a setting of 14.60 volts overall, a well-equalized pack is obviated by all four red LEDs equally glowing very dimly once full-charge has been achieved.

Using it for the first time or on a battery that is badly unequalized:

The first time I used the device on a battery pack that was known to be badly out of equilibrium for which I had NOT already charged each cell individually, it took 24-36 hours for all of the cells to come to equilibrium.  What happened was that once the "most charged" cell achieved its target voltage, the shunt regulator took over for that/those cell(s) and the others were charged by the "excess" current being passed around the fully charged cell(s).

Because this shunt current was only on the order of 100-200 milliamps, it took quite a while for these other cells - some of which were still at only 20-25% charge - to be fully "topped off" at this rather low "charge" current.

Subsequent charges:

One does not need to use this device every time a battery is charged.

It takes several cycles (and time!) for the cells within the battery to "drift" apart and fall out of equilibrium so one could probably get away with using it only every 6-12 charges (or 3-6 months, whichever is sooner) and still maintain good charge capacity on the battery pack.

When done:

Once the charging has been completed, the battery should be disconnected from the power supply used to charge it and the equalizer circuit should be disconnected immediately.

This equalizer circuit, when the voltage(s) on the cell(s) are below the clamping threshold, doesn't draw much current - somewhere in the 3-5 milliamp range - so it is not likely to discharge a pack of this size very quickly.  If this were were left on a battery connected across a small solar panel, for example, its contribution to the discharge of the battery would be negligible.

Final analysis of the problem:

One of the ways to most-quickly "kill" any Lithium-Ion based cell is to maintain it at a full charge voltage all of the time:  Best longevity is generally obtained if it is kept it in a relative state of discharge (30%-60%) and charging it fully only and just before you need it as keeping it in a state of "partial charge" reduces the rate of inevitable degradation that occurs over time due to the intrinsic chemical decomposition.  One must also be aware that allowing the voltage on any cell of a lithium-ion battery to drop below 2.0 volts can also damage  or make it unusable:  On some packs, if a cell voltage drops too low the protection circuit will actually prevent charging.

Remember that this discharge can occur "naturally", due to the current leakage in the cells themselves as well as the very slight, constant drain of the battery protection circuit so it is recommended that one check - and at least partially "top off" the cells/batteries every 6 months or so.  It should also be noted that this self-discharge rate accelerates rapidly with increasing temperature and can be alarmingly severe at higher temperatures such as those above 90F (32 C):  Under those conditions - should it be necessary to store them that way (avoid it if you can!) - they should be checked more often!

With this in mind, I generally keep my Lithium-Ion power packs in partially-charged state - or at least never leaving it on a charger for long periods - and have had good luck over the past 15 years or so that I have been applying that principal, typically getting at least 5 years out of the "new" pack before its capacity drops below about 60% of its design.

So was the intent when I got these new LiFePO4 packs:  I would top them off occasionally - usually just before using them - and when I was done, I'd typically leave them in their partially-charged state.

Again, I'm reluctant to put these batteries on a charger, set to 14.6 volts (3.65 volts/cell) and just walk away.  In theory, if I did do that they would eventually reach equilibrium as the fully-charged cells, having gotten above their maximum voltage threshold would occasionally self-discharge just below it and allow charge current to be applied across all the cells in the pack, if even occasionally, for a brief moment.  Again, doing this will inevitably reduce the overall lifetime of the battery, but perhaps this is an acceptable tradeoff for you.

What the "protection" circuits current found in batteries should really have is a more intelligent charge management capability in that if one cell was at a higher potential than one or more of the others, current would be shunted around it, allowing the other cells to equalize.  Alas, that's not the way they work, hence this need/purpose of this project!

Note:  In one of the comments below, the existence of such a circuit is discussed briefly, but I don't have direct experience with them.

Why don't other LiIon battery types experience this "inequality" problem more readily?  Part of this has to do with the fact that many of them (e.g. batteries in laptops) are typically "stored" in the device and kept at full-charge voltage all of the time and will eventually equalize.

Again, lead-acid (e.g. car) batteries along with NiCd and NiMH cells are less-often subject to this problem (inequality related to charge/discharge rather than cell degradation) mostly because they can be subjected to a degree of abuse (overcharging) without worrying too much about damage, and this allows all of the cells to equalize.  Since you can't do this to Lithium-ion cells and expect them to last a long time, this "purposeful overcharge" technique isn't really an option!

(Note:  There are equalization techniques applied to various types of batteries - namely lead-acid - but those will not be covered here.)

Results:

How do the batteries test now, after more than 2 years of ownership, before equalization, new and after equalization?  The graph in Figure 9 tells the story:

Figure 9:
Graph showing the capacities of the batteries over time, before and after equalization with the
dates of the "mid-point" readings shown in parentheses as well as indications of
readings taken when the battery was "new" and "equalized."
  Discharge current for all tests was 2 amps.  This is higher than the more typical "20 hour"
rate used for determining the capacity of batteries.  This higher discharge current reduces the
measured capacity of the batteries in comparison with the manufacturer's methods.  A graph point
for the "Metal (5/12)" data was not available, but drawn-in based on notes.
Click on the graph for a larger version.

As you can see in Figure 9 there was significant degradation of capacity as the cells drifted apart from each other, rapidly decaying in just a few months (5/12 to 9/12).  Packs #1 and #2 are identical, originally consisting of a pair of LiFePO4 packs connected in parallel at the outputs of the protection board (one of these was depicted in Figure 6) while the "Metal" pack had just one protection board with the cells originally connected in series-parallel format.

All three of these packs were used approximately equally, but it's interesting to note that the metal-cased pack seems to have degraded far less:  Was this due to differences in usage, or because of the way the cells were configured?  It could be that the "metal" pack with its "2P4S" configuration (rather than the original "4S+4S" format of the water bottle batteries) the rate-of-change of their respective states of charge from the pairs of cells in parallel caused a slower rate of degradation.

It's also interesting to note that in the case of the metal pack and water bottle #2, the capacity after equalization was actually higher than it was when new, indicating that when it was "new", the cells were already drifting apart and that the cells' capacities probably haven't degraded much in the 2+ years since they were made!

What do I think about LiFePO4 cells now?

I'm more than willing to concede that, perhaps, there is something to this longevity of LiFePO4 cells after all.  As can be seen above, once the packs were equalized the cells seem none the worse for wear after several years have passed, along with a degree of abuse!

What about this (apparent) propensity of the cells to drift apart?  It might be interesting to have another pack that is left "on charge" for long periods of time to see if it will equalize itself and/or if it degrades at an accelerated rate.  It could also be that newer protection circuits and cells are more likely to maintain a closer charge state than these (now) older packs.  Perhaps other brands of batteries would be less susceptible to this problem.

Would I get more LiFePO4 batteries in the future?

Yes, now that I know that I should be on the lookout for a problem like this!


Final comments:

Since I purchased these battery packs from Batteryspace.com, they have started offering "equalization boards" for LiFePO4 battery packs designed to address this very problem although there was no indication that these specific battery packs described on this page (which appeared to be still sold at the time of posting) contain such circuitry.  While I have not used these specific equalizer boards, comments that appear elsewhere on the web indicate that these types of boards do the job for which they are intended.

If you get a LiFePO4 battery (2 cells or more in series - that is, more than 3.6 volts or so output) I would strongly recommend that you make certain that it includes a charge equalizer - or be sure to add one yourself!

If you already have some LiFePO4 battery packs and are not absolutely certain if they have equalizers, you really should check it out - and if they don't, add them!


Update on battery longevity (June, 2016):

I recently re-tested the three batteries depicted above and found that their capacity ranged between 4.8 and 5.4 amps-hours - this for batteries that were at least six years old.  Based on their capacity when they were new, they have lost somewhere around 20% of their original capacity in that time.

While I'm a bit skeptical that they will make it to the 20 year mark it is worth noting that practically any lead-acid battery of this same age would have since been relegated to the recycler!

Update:

There is a related article:  Shunt Regulation of series-connected lead-acid batteries to equalize the voltage link.  That article describes a simple shunt regulator that can be used for both 6 and 12 volt lead-acid batteries, but it could be applied to LiFePO4 cells, as well.
[End]

This page stolen from ka7oei.blogspot.com