Friday, August 31, 2012

Problems with Lithium Iron Phosphate (LiFePO4) Batteries

Update:
For an update about what turned out to be happening with these batteries - and one possible solution - see the May 18, 2013 post, "Lithium Iron Phosphate batteries revisited - Equalization of cells" - link


About 2 5 years ago over the period of several months I got three 13 volt, 6+ amp-hour Lithium Iron Phosphate (LiFePO4) packs from Batteryspace.com for about $95 each.  These packs seemed to be a reasonable alternative to my old standby of portable battery power - the ubiquitous 12 volt, 7 amp-hour sealed lead-acid (SLA) battery, often (mistakenly) called "Gel Cells."

Why switch from SLAs?
The three LiFePO4 battery packs in question.

LiFePO4 packs seemed to be attractive for the following reasons:
  • LiFePO4's were lighter than the same-capacity SLAs - roughly 1/2-2/3 as much weight.
  • Claimed 1000-2000 charge durability for LiFePO4's versus 100-200 or so for SLAs.
  • Claimed 10 year lifetime for the LiFePO4's versus 3-5 years for SLAs and conventional Lithium-Ion packs (even the polymer types.)
  • With all of the above, the relatively high initial cost ($95) of the LiFePO4 batteries that would last 10 years seemed to be reasonably comparable to $15-$30 (when new) on a "per-year" basis with typical 7 amp-hour Lead-Acid packs - and the lighter weight was a plus!
As it turns out the LiFePO4 packs aren't quite as "energy dense" as  "normal" Lithium Ion cells - that is, when cylindrical LiFePO4 cells are assembled in a battery pack it takes about the same amount of space as a lead acid battery of the same capacity - but they weigh much less.  It's also worth remembering that conventional LiIon packs will typically last 3-5 years from the date of manufacture and thus didn't have much longevity advantage in that respect over SLAs.

So, over the period of several months, I ordered three of these 6.2 amp-hour LiFePO4 packs that put out about 13-ish volts over their discharge cycle - slightly higher than SLAs, but still well within the realm of what typical "12 volt" gear will accommodate.

As I typically do with newly-acquired batteries I checked the amp-hour capacity of each of the three battery packs shortly after arrival using my West Mountain Radio Computerized Battery Analyzer at 700 milliamps and found that they were reasonably close to the advertised capacity - that is, around 5.8 amp hours:  Typically such batteries are rated at the "20 hour" rate which would have been about 310 milliamps and the higher rate that I used would reduce the measurement by 10-20% so I was pleased with the results.

At about the same time I acquired some 2 year-old 12 volt, 7 amp-hour lead-acid batteries that had been pulled from UPS service on a routine basis and these were found to have about 6.2-6.5 amp-hour capacity at the same 700 milliamp rate.

In the intervening years I used these batteries (both LiFePO4 and SLA) about equally, running radio equipment and the like and earlier this year I suddenly realized that something was amiss:  The LiFePO4 packs were dropping out far earlier than they should have.

A bit of explanation here:

All rechargeable lithium-ion packs have built-in circuitry to protect against excess over-discharge, the reason being that if you run a lithium battery down too far an irreversible chemical change occurs and they cannot be safely recharged ever again.  For this reason when a lithium pack runs down too far it will suddenly drop off, the internal circuit disconnecting the battery to protect it.

Lead Acid packs, on the other hand, do not do this:  Their voltage slowly drops down and their effective internal resistance goes up and one eventually realizes that the equipment being powered is no longer working correctly.

As it turns out both Lithium-Ion and Lead-Acid packs are charged in similar ways.  One simply connects a power supply of voltage appropriate for the type of battery pack and let it charge.  Both types of batteries, when discharged, will pull more charge current but this will gradually drop off as the battery approaches full charge and for this reason it's typical for these power supplies to be current-limited as well as be fixed voltage.

A major difference between how one treats Lithium-Ion (including LiFePO4) and Lead-Acid (SLA) batteries appears at the point of full charge:
  • For SLAs one obtains the best lifetime by continuously maintaining them at a constant voltage - typically 13.5-13.8 volts for a "12 volt" lead acid battery
  • Lithium types should not be maintained at the "full charge" voltage after full charge has been achieved.

What happens with Lithium-Ion batteries (including LiFePO4) is that if you maintain the "full charge" voltage its internal chemistry degrades much more rapidly than if you were to fully-charge the battery and then immediately disconnect the source, allowing the voltage to sink down a bit on its own.

What this means is that you will get much better longevity out of a Lithium pack if you do not keep a high-level float charge on it.  In fact, the best longevity of Lithium-type rechargeable batteries can be obtained if you store them in a half-discharged state - provided that you check once in a while to verify that their self-discharge hasn't caused their voltage to go so low that they become damaged from that!

* * *

That is how I treated the LiFePO4 battieries:  I would attach the pack to a 1-amp, regulated 14.2 volt power supply for 12-18 hours and then disconnect it and then place it on the shelf, possibly topping it off briefly just before using it.  The Lead-Acid batteries, on the other hand, are left connected to a 13.6 volt power supply and allowed to sit there all of the time when not being used.

I was, therefore, chagrined when after just two years the now 4 year-old SLAs were outlasting my LiFePO4 packs.

This observation spawned some further testing, so I put the LiFePO4 packs back on my battery tester I was further distressed to note that those that had originally tested out as having 5.8-6 amp hour capacity were now, at the very most, in the 1.5-2 amp-hour range while the much older SLAs were still in the 5.0+ amp-hour range.

Comment:
 In the time since I did the testing for this entry, the LiFePO4 packs have continued to degrade at about the same, alarming rate while the old Lead-Acid cells are still holding in, degrading much more slowly.

Hmmm...

So, what's the deal?  Why are the 4+ year old SLAs still in better shape than the 2 year old LiFePO4 packs?

I really don't know.  I've attempted to correspond with the sellers of the LiFePO4 batteries (batteryspace.com) to find out their "take" on this observation, but I've not heard back from them - too bad since I've had reasonable luck with their customer service in the past...

Perhaps they got a batch of "bad" cells - but since the three LiFePO4 packs were actually purchased several months apart it would seem to me that it's more a problem with manufacture/chemistry of the cells themselves. 

What to do?

At the moment I'm sticking with the old, heavy SLAs since I'm now understandably "gun shy" when it comes to LiFePO4s since the former do seem to be fairly predictable in their longevity and performance - at least when treated properly!

Update:


For an update about what turned out to be happening with these batteries - and one possible solution - see the May 18, 2013 post, "Lithium Iron Phosphate batteries revisited - Equalization of cells" - link

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!

[End]

This page stolen from ka7oei.blogspot.com

16 comments:

  1. How cold was it at your place at the time of measure. When LiFePO4 batteries drop below +/- 2 degrees celsius they will have more internal resistance and last less long, this problem will cease to exist when temperatures rise again.

    ReplyDelete
    Replies
    1. The temperature in all of the tests was approximately the same - in the area of 20C-25C so I do not believe that this could explain the >75% loss of capacity.

      As part of a long-term experiment, I have allowed the three batteries to sit on a basement shelf in a partial state of discharge as recommended by various manufacturers and every 3-6 months or so I will charge them to the recommended voltage and retest: The alarming loss of capacity has continued.

      In examining the batteries, it would appear that they all use the same make and model of cells. By this time I'm sure that the seller (batteryspace.com) is well aware of the problem as all of the stock using this particular batch of cells must be exhibiting the same problem.

      Delete
    2. I've had great luck with ping batteries going on 4 yrs and I use them everyday. A 48 -20ah and a 36volt 20ah . still act like new. There being used on electric bikes. That's probably why pingbattery.com has nothing lower than a 5 star rating and customer service is A+ and he is in china. Just a thought.

      Delete
  2. Hi friend,
    your blog about problem with litium ion battery is really informative. The energy density of lithium-ion is typically twice that of the standard nickel-cadmium. There is potential for higher energy densities. The load characteristics are reasonably good and behave similarly to nickel-cadmium in terms of discharge. The high cell voltage of 3.6 volts allows battery pack designs with only one cell. Most of today's mobile phones run on a single cell. A nickel-based pack would require three 1.2-volt cells connected in series.

    Thanks,
    Rechargeable Lithium Batteries

    ReplyDelete
  3. LiFePO4 cells, only charge to 3.4 per cell. sounds like you overcharged them.
    they operate at a nominal 3.2v, 3.6 is WAY too high.
    Most EV guys only charge to 3.4 or less.

    ReplyDelete
    Replies
    1. According to the manufacturer of this - and other - cells, the charging voltage of 14.2 volts (3.55 volts/cell) is well within the specifications for the LiFePO4 chemistry.

      Soon after charging was complete, the voltage was removed which allowed the cell voltage to drop - an important point since in contrast to lead-acid cells, the longevity of rechargeable lithium cells is negatively impacted with the constant presence of its charging voltage over a long period of time.

      Now it is possible that in the 2-3 years since I bought these batteries, experience by users has shown that the lower voltage is more appropriate for maximum longevity, but hindsight is, as they say, 20-20!

      Delete
  4. Hi !

    The problem is most probably the circuit inside the pack.
    They tend to bleed down one or more cells in the pack to a level where the imbalance between the cells makes the pack almost unuasble.

    I have had this experience with BMS on LiFePo4 packs.
    Try to open the pack and measure the cell voltages of each cell.

    What I have done is to remove the BMS and use the pack without protecting circuits. That way the pack does not get a low constant current draw.

    I would think that your packs would work just as good if you:
    1. remove the protect circuits.
    2. bottom balance the cells in the pack.
    3. use that charging methods that you did before.

    You can do some extra voltage check of the cells when your in the later stage of charging to see that the cell voltages dont get to high.

    REgards
    /Per

    ReplyDelete
    Replies
    1. Thank you for the comments!

      For testing this - and other - batteries and cells, I use a "West Mountain Radio" CBA-II battery tester. This is an (older) USB device that provides a constant-current load and allows graphing and logging of the discharge curve while calculating the effective capacity in amp-hours. Regardless of whether or not it is a true and accurate representation of the useful energy stored in a battery, it has proven to be consistent in its measurements and it is in general agreement with the amp-hour ratings of new batteries.

      In the past when I have had cells go bad - particularly LiIon of various sorts - one sees the voltage drop slowly, and then abruptly disappear as one of the cells in the pack drops below the threshold set by the pack's internal circuitry and is disconnected.

      These packs, on the other hand, exhibit the behavior of a properly-working pack in the sense that the voltage slowly drops until it reaches the lower limit set by the battery load tester itself. As it turns out, this voltage is slightly above the voltage at which the pack itself would cut off.

      Now, were one or more of the paralleled cells to drop more quickly than the other over the course of the discharge, I would expect that at some point, the voltage would cut off as those cell(s) went below the circuit's threshold voltage, but the other "good" cells were still at a reasonable voltage.

      It doesn't do this, however: The fact that the discharge curve decreases gradually throughout the entire test indicates that the cells themselves are fairly equal in terms of capacity.

      Having said this, when I get a chance I will do more investigation of the state of the cells within the packs, measuring them individually to see how "equal" they are to each other and if there is, in fact, some sort of odd behavior that might be causing uneven charge/discharge of the cells within.

      Delete
  5. This comment has been removed by the author.

    ReplyDelete
  6. Hi, Glad I found this. I was just about to click complete order from the same company on 3 packs using the 26650 cells 3.3Ah packs

    So - what's the work around? And would you recommend a different path? Any battery I get needs to be light - I have no intention of packing an SLA around.

    I'm a little concerned about pulling the control board which I believe (not positive) keeps the cells from both over charging, and from dropping below 10V which is supposed to be BAD.

    Any new information would be most welcome.

    ReplyDelete
    Replies
    1. I've recently had time to analyze the battery packs and have determined that they cells within had become grossly "un-equalized". Refer to the

      May 18, 2013 post for more information about this issue.

      Thanks for the comments!

      Delete
  7. LifePO4 batteries require each cell to be 'ballance charged' for best performance. The 'off the shelf' 6.6 or 13.2 volt packs with built in charger/protectors do not do this. They attempt to balance after one cell reaches full charge. Actually over full charge, I've tested two different built in controllers and they will charge a single cell to 3.9v instead of the manufactures recommended 3.6. To truly get the life out of an liFe you must buy a pack that has the balanced charger connector and charge with a balance charger. The RC (radio control) guys have this pretty much worked out. Perhaps someday the Chinese will build a good built in lifepo4 charging circuit.

    ReplyDelete
    Replies
    1. Thank you for the comments. As noted in the follow-up of this post (links are within the original post) I devised a means of equalizing the cells within each pack.

      More recently, another commenter to the follow-up article noted the availability of some new protection circuits that do include the ability to force equalization of each cell. Unfortunately, this doesn't appear to be a standard feature and is likely to be of ultimate disservice to both the owner and seller of those packs that lack it!

      Delete
  8. I (and some others in our flying club) use lithium iron phosphate batteries in our airplanes. My experience is limited to install it and forget it. My engine starts briskly in all weathers. The battery voltage stays up at all times. I have had the battery four years. The first two years (while the plane was being completed) it was stored in the refrigerator. It fits the same holder that was designed for lead acid, but is much (much) lighter. If anything it is too light (!) because that lead (no longer there, of course) was initially intended to provide part of the weight that balances the plane in flight!

    ReplyDelete
    Replies
    1. The LiFePO4 lead-acid "replacements" have proliferated since this article was originally written.

      In perusing the various vendors of SLA (Sealed Lead Acid) replacements using LiFePO4 batteries very few of them make any mention of the battery itself having any sort of BMS (Battery Management System) that would be responsible for equalizing the charge across cells and disconnecting it on overcharge/discharge. I suspect that reputable branded batteries must have such a thing or else they would fail after only a few accidental over-discharge events - but then again, the cynical side of me suspects that even if not, such an induced failure would likely occur outside a year's warranty for the vast majority users.

      Thanks for the comment!

      Delete
    2. I've had a Shorai LiFePO for at least three years now. The aircraft build was a bit slower than anticipated, so the unit spent its first year in the refrigerator. Soon after being put into service I left the lights on over a weekend, leading to a dead flat, but the plane was started okay with jumper cables, and the battery still seems fine, giving instant starts in all weathers with no signs of deterioration (yet). Maybe I'm wrong to do so, but I happily recommend them whenever I'm asked.

      Delete





PLEASE NOTE:


While I DO appreciate comments, those comments that are just vehicles to other web sites without substantial content in their own right WILL NOT be posted!

If you include a link in your comment that simply points to advertisements or a commercial web page, it WILL be rejected as SPAM!