Using a PIR to reduce wear and tear on a Nixie clock

"Does a lit-up Nixie tube in a forest wear out even if there's no-one to see it?"

Figure 1:
The "Black'n'wood" Nixie Clock (blue back-light turned on)
with the PIR (Passive InfraRed) sensor to the right.
With no detected movement in the room, the high voltage
supply turns off, reducing wear on the tubes.
Click on the image for a larger version.

This millenia-old riddle has a simple answer:  Yes.  Yes, it does.

This article has not so much to do with this specific model of Nixie clock, but rather adding a PIR (Passive InfraRed) sensor to turn off the display when there is no-one in the room to look at it.

They wear out!

Nixie clocks and other neon-glow displays (e.g. Panaplex), along with VFD (Vacuum Florescent), Numitron and CRTs have a "wear out" mechanism when they are operating:  In other words, when they are on, they are slowly degrading.

By limiting the "on" time of such displays only to when someone is likely able to see it one can prolong its overall useful life in many cases.  As many of these devices are no longer made, the supply of "new, old stock" tubes is very limited and what there is still available is becoming more expensive year upon year.

* * *

A few years ago - at a swapmeet - I picked up a "Nixie" ¹ clock - the "Black'n'wood" by Nocrotec.  It was in a plastic bag with loose parts, but for only $20 I couldn't resist!

Getting it home I found the problem:  One of the elements of the "10s of hours" tube was shorted internally and very visual close inspection revealed that two internal wires were touching each other.  A bit of "percussive repair" (banging it on the table) moved the two wires away from each other and the tube was once again usable.  I suspect that the problem was originally caused by the tube experiencing mechanical shock.

The current limiting resistor associated with this same element was burned, so replacing it returned the clock to full operation.

"Could I use one of those microwave motion sensor boards instead of a PIR module?"   Motion sensor boards that use low-level microwave energy are cheap and available.  These work in a manner similar to the old "Proximity Fuse" - an electronic sensor used to detonate bombs a certain height above ground.  By detecting interference to its own oscillator by the disturbance of standing waves in a room caused by movement, they can also be used to turn on lights, open doors, etc. While the use of radio waves instead of (infrared) light means that they can sense movement through walls or behind many non-conductive materials like plastic or wood - albeit with some diminution in sensitivity - this property may make them less desirable in this application:  If you want to turn off the clock when no-one is there to see it, you likely don't want it to turn on when it detects movement at a farther distant - even in the next room. One advantage of a PIR sensor is that it may be placed to limit its range of sensitivity to reduce false triggering - including from pets:  If you can't see it, it probably can't see you!

Over the next year or two the clock has continued to work fine - although the display got "glitchy" and began to dim - but the biggest clue was that the flashing colon neon lights were flickering but this was quickly traced to the failure of the main high voltage filter capacitor on the 180 volt supply:  These problems went away with the replacement of that capacitor - but I digress.

* * *

All of this brings me to the main topic of this article:  Reducing the wear and tear of the neon tubes.  

Sitting unused, in a box, many "vacuum" devices (I'll include neon indicators and other cold-cathode tubes in this category even though they are not strictly "vacuum" devices) have the property that laying on the shelf, they (usually) have little/no degradation over time.  There are many (now) century-old devices that have been sitting around that work just as well as they did when they were made - the caveat that they haven't been compromised in some manner (e.g. broken, corrosion, failure of a seal, etc.)

Like most "vacuum bulb" devices - which include thermionic tubes/valves (with a filament) and those without a filament - like neon indicators - there is a definite lifetime related to acceptable performance when they are operating.  For normal tubes/valves, the emission from the filament/cathode will inevitably drop over time - often due to gradual degradation of emissivity and the "work function" of the cathode.  "Cold cathode" devices (e.g. those without a filament) like neon indicators also suffer degradation - and the causes are broadly similar:  Degradation of the materials and subsequent contamination.

In the case of the neon indicators, one major cause of degradation is the inevitable "blasting" of atoms from the electrodes' surfaces (called "sputtering") where the metal gets liberated - only to redeposit elsewhere.  The most obvious result of this is that the inside of the of the glass envelope darkens, reducing the brightness of the display - but even if the glass were to remain clear, this and other effects conspire to reduce the brightness overall.

Running neon indicators such as these "Nixies" at lower than maximum current will reduce these effects - but what about not running them at all?

Nixies are meant to be seen!

The entire point of a "Nixie" clock is that it is cool to look at - but what if no-one is there to see it?  As the conceit at the top of the page states, if we operate a Nixie tube in a forest and no-one is there to see it, it still wears out!

The goal, therefore, is to turn off the display when no-one is in the room.

Turning off the display

First, we need to figure out how to turn off the display, presuming that the clock or other device has no obvious means of doing so (e.g. there is no "turn of the display" switch or pin).  Two options came to mind:

Approach #1:  Removing the power

Figure 2:
Interface with HV converter.  PNP transistor "Qa", when
its base is pulled low, injects current into Pin 5 of the
HV converter chip, effectively turning it off.
Click on the image for a larger version.

As the "Black'n'Wood" clock has a battery back-up, I first tried the most obvious thing:  Interrupt the power to the clock if no motion was detected.  This worked - in theory - but I soon noted that the clock was losing almost a minute every day.

After I'd first repaired the clock, I applied correction factors via its menu and got it to stay within a fraction of a second per day - but I had assumed that this was done in the clock module itself (as some Dallas/Maxim devices are equipped) but this was not so:  While the timekeeping module continued to run on the battery, the firmware on the clock itself - being powered down - was obviously not and the calibration that I'd applied was missing, explaining why it was keeping time so badly.

To be sure, I could have likely done something to "fix" this (e.g. trim the oscillator with a tuning capacitor, added a GPS module to auto-set the clock, etc.) but since the clock - while it was running normally - was very stable, I decided to try another approach.

Approach #2:  Turning off the high voltage

Figure 3:
A top-down view of the components added to the high
voltage switching converter IC to produce the circuit depicted
in Figure 2, above.
Click on the image for a larger version.

The clock itself runs on 12 volts DC and to get the 150-180 volts needed to drive the neon displays, there is an onboard voltage converter.  While no schematic seems to be publicly available for this particular clock, the various sections of its circuitry are easily identified by visual inspection:  A large-ish inductor adjacent to a high voltage capacitor flagged the location of the voltage converter and the chip next to those components.  

This clock uses two switching supplies - the first one converts the nominal 12 volts down to 5 volts for the logic, but the second one - near the high-voltage capacitors - is the one that produces the (approximately) 180 volts for the Nixies.  Both of these use a common type of switching supply controller chip - the MC34063 - and since the implementation of these chips is spelled out in the data sheets, we have some insight as to how they work and a schematic isn't necessary to complete our task.

Fooling the voltage converter into shutting down

Like most any voltage regulator or converter, it monitors its own output voltage - typically through a pair of resistors ("Rdiv" is one of them, depicted in Figure 2 - the other, not shown, would go between the chip and ground) that are chosen to divide the desired output voltage down something close to the chip's on-board reference voltage - in the case of the MC34063, 1.25 volts, which is applied to its pin #5:  If the voltage on this pin is lower than 1.25 volts, the switching converter adjusts the voltage higher but if the voltage is higher, it reduces the voltage.

Figure 4:
A side view of the high voltage switching converter IC
showing the components added to it to allow the
180 volt supply feeding the Nixie tubes to be turned off.
Click on the image for a larger version.

As can be seen from the diagram in Figure 2, I tacked a PNP transistor ("Qa") - and three resistors - across several of the pins of the MC34063 high voltage converter.  "Pre-forming" the shape of the components to match the locations of the needed IC pins along with using a hot soldering iron to pre-tin the leads of these components and the IC itself it's possible to attach this simple circuit directly to pins 5 and 6 of the high voltage switching converter IC without risk of damage to the chip or other, nearby components.

If the base of Qa, the PNP transistor (I used a 2N3906) is pulled to ground (via the 10k resistor, Rc), it will turn on - and with its emitter connected to Pin 6 of the MC34063 - its power supply pin - it will apply current, via the 3.3k resistor (Ra), to Pin 5 of the MC34063, dragging the voltage on this pin up.  When this happens, the MC34063 will "think" that the voltage is too high and effectively turn off.  If the base of Qa is allowed to float (nothing connected to it), this transistor is biased off by the 100k resistor (Rb) between the emitter and base and the high voltage converter will run normally (e.g. the display will be on).

Any converter will do

While this article shows the example using the MC34063, this sort of technique could be applied to about any switching-type of voltage converter.  Determining a bit about the circuit itself could be done simply by referring to the data sheet of the chip that was used - as was done here - but it could also be done with a bit of reverse-engineering.

It would have also been possible to find the power supply lead feeding the voltage converter - in this case, about 12 volts from the external power supply - and interrupt it, perhaps with a relay, a PNP transistor or a P-channel FET.

If you are using an "old-school" power supply that does NOT have some sort of switching converter, perhaps consisting of a high-voltage winding, rectifier and capacitor to develop the high voltage for the tubes, your best option may be to use a relay to open the supply - preferably interrupting the pre-rectified AC side, directly.   At such voltages switching DC is best avoided due to the possibility of contact-damaging arcs:  Switching on the AC side (or between the rectifier and the first filter capacitor) is better in that the voltage falls to zero twice per cycle of the AC waveform and any arcing that does occur will extinguish at that time.

Getting the connection outside the clock

In perusing the manual for this clock I noticed that the 6 pin mini-DIN connector - intended for connection to an external GPS or DCF77 radio receiver - not only had ground (Pin 1) and power (Pin 2 for 5 volts), but also an unused pin (#4) that I verified to be floating - and to this I connected the end of the 10k resistor (Rc) to this pin with a flying lead inside the clock.  With the three needed signals (power, ground and the "disable" line) on the mini-DIN connector, I was ready to connect it to a sensor.

A PIR sensor to turn it off and on

A PIR (Passive InfraRed) sensor fits the bill for this task quite well - and they are inexpensive.  These devices use pyroelectric detectors to detect heat from warm, moving objects - which includes us humans - by focusing deep infrared energy onto a pair of sensing surfaces from an array of Fresnel lenses.  A moving object in the field of view will cause a difference in the pair that can reliably indicate that an object in view is in motion.

Figure 5:
This circuit was added to the output of the PIR to present
an open-collector to allow transistor Qa in Figure 2
to properly turn off when the HV was to be turned on.
Click on the image for a larger version.

The PIR sensor that I chose was found on Amazon - three of them for under US$10 - and it has exactly three connections:

• Power.  This particular PIR sensor was happy to operate from between 5 and 12 volts, having an onboard 3.3 volt regulator.
• Ground.  This is the negative supply and the reference to the output signal.
• Output.  This output pin goes "high" (to 3.3 volts) when motion is detected.

This sensor also has two potentiometer adjustments:

• Delay - Which is the amount of time the output will go "high" when motion is detected.
• Sensitivity - As the name implies, this sets the degree to which the device reacts to movement.

There's also a jumper:  The piece of paper that came with the PIR sensor implies that this determines if the output is "re-triggerable" (the default setting) or not.  Being re-triggerable means that motion will reset the delay timer whenever it's detected:  If it were not re-triggerable, the delay time would be reset only after the delay had expired and the output had turned off.  Clearly, we want to use the "re-triggerable" setting so that any movement simply extends the timer.

It turns out that moving the jumper on this board from its factory position stopped the unit from working at all and a quick bit of reverse engineering revealed that whoever designed this board simply connected it to the wrong place - probably due to poor reverse-engineering on the part of the "designer" of this (likely cloned) circuit board.  Fortunately, the wiring of the circuit is such that it is already wired as being re-triggerable, so we can leave it alone.

Figure 6:
Perhaps a bit messy, but this is the two-transistor circuit
depicted in Figure 5, tacked to the pins of the PIR module's
circuit board.  The three-conductor cable that connects to the
clock via the mini-DIN connector can just be seen.
Click on the image for a larger version.

One problem with the 3.3 volt output is that it is a logic output that is limited to 3.3 volts because it has both pull-down and pull-up transistors, internally.  As we discussed in the previous section, we need to ground the base of transistor "Qa" through the resistor to disable the high voltage and let it float to an unknown voltage to allow it to turn on.  

Because the output is not an open collector or open drain, it cannot be pulled higher than approximately one diode drop above the 3.3 volt supply on the PIR chip:  This voltage is lower than the emitter voltage of the transistor that we added ("Qa") which means that Qa will always be turned on, always disabling the high voltage converter!

To fix this we need to provide an open-collector output - but preserving the polarity of the output - which is to say that we want it to be an open collector to allow the base of "Qa" to float high when movement is detected, but go to ground and turn on "Qa" when it is not.  To accomplish this, transistor "Qb" takes the "high-active" pulse from the PIR and inverts it - and then transistor "Qc" will invert it yet again, but this time with the needed open collector.  "Qb" and "Qc" can be practically any NPN transistor - I used 2N3904 types in this cicruit.

In experimenting with this PIR sensor module, I noted that when set to "maximum" the "on" time from the output was about 150 seconds - about 2.5 minutes.  I was able to iteratively adjust the "sensitivity" control incrementally upwards until I found a setting that reliably detected even slight motion in the room - but seemed not to randomly "false" trigger, the result being that even when I was in the room and not moving much - say watching TV - it would stay on most of the time, but be easily (re)triggered by even slight movements.

Figure 1 shows the clock with the PIR sensor next to it, sitting on the shelf below my TV.  I purposely set the PIR sensor back from the edge of the shelf - not just to line up with the front of the clock, but to obscure part of the view of the floor to reduce the probability that a cat would trigger it:  Since cats sleep most of the time, anyway, their occasionally triggering the PIR sensor isn't a big deal and the display remains off most of the tim.

Reducing "wear-out"

For a "cold cathode" tube like a Nixie, turning the high voltage on and off is not a stress on the tube:  After all, simply changing the segments to show the time is also turning on/off parts of the tube.  With no voltage present, there is no electron bombardment on the elements within the tube and thus, it will not experience wear.

Powering down other devices in the absence of "viewers"

There are other types of "antique" displays that may benefit from having some sort of "human presence detector".  For example, a VFD (Vacuum Fluorescent Display) has a wear-out mechanism similar to a Nixie in that electron bombardment will gradually degrade the phosphors - and the cathode (filament) may also lose emission.

Similarly, if one has a "Scope Clock" - a vector-graphics clock that uses an oscilloscope tube to show the time - it, too, will wear out over time, the emission of the from the cathode will drop over time - not to mention possible burning of the phosphor.

Figure 7:
An example of a "Scope Clock" - a vector-graphic clock
display shown on an oscilloscope using a cathode-ray tube
(CRT).  In this photo, the CRT in a Cushman CE-50A
communications monitor is being used to demonstrate, but
an old, analog oscilloscope would work as a "permanent"
fixture and blanking it when no-one is looking would
extend the life of increasingly-rare CRTs.
Click on the image for a larger version

For these two examples, a bit of care should be taken in that while removing the high voltage source may partially remove the wear-out mechanism (e.g. degradation of phosphors) other steps would be required to mitigate the diminution of filament emission over time.  This could include turning off the filament - or at the very least, reducing its voltage, perhaps in steps, in the absence of the anode voltage.  If grid voltages happen to be present, those, too, should be carefully considered to see if they should be removed when the high voltage is turned off - but since these often share the same power supply, this problem may take care of itself.

In so-doing - and depending on the nature of the display tube - other precautions may also be required (e.g. removing all other voltage prior to powering down the filament) to avoid damage - and frequent power-cycling of the filament itself may be an issue:  These are potential issues that should be considered - but are beyond the purview of this article.

Footnote:

1. The name "Nixie" is a trademark of Burroughs Corp. to describe certain types of neon-glow indicators.  Like many trademarks, it's become "genericized".  As done in this article, nowadays it's commonly used to denote all types of similar cold-cathode glow devices in which each digit is indicated by a separate element within the tube in the shape of the desired numeral or symbol - whether they were made by the original trademark holder or not.

 * * * * * * *

This article stolen from ka7oei.blogspot.com

[END]

 

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.

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This page stolen from ka7oei.blogspot.com