Discussions about LiFePO4 cells and Battery University

Hi Dan,

Good questions, and it occurred to me a couple days ago that I should clarify what I've been saying here. 3.60V should be considered max for rested OCV. Charging Voltages up to 3.8 are generally considered permissible, but you don't want to stay there long enough that your OCV will relax to a point above 3.6, or you will suffer some level of Li plating. It may not be severe, but it is happening. If you are dealing with very high charge currents, pushing this envelope becomes more and more risky. Exceeding terminal Voltages of 3.8 is never advised, IMO, and our starter batteries are set up to never allow cell Voltages to exceed 3.8 under any circumstances. Li plating becomes more severe, and as you approach 4 Volts the electrolyte will begin to break down (again, the exact Voltage at which this begins and how severe it is depends largely on charge current). Above 4 Volts, electrolyte breakdown should be considered a sure thing. If charging current is high enough, gas pressure can build fairly quickly and cell rupture may occur. Add an ignition source at the time of rupture, and you could have some real trouble, but in most cases the event is pretty benign--often not even becoming apparent until many hours or even days later. The gas that builds up in the cells is mostly CO2, so it actually can have a fire retardant effect as it vents, but don't count on that. In some cases, the cell may remain intact and usable, though with noticeably decreased capacity--this is more common with cylindrical cells which typically take a little more internal pressure to crack open.

Because I'm a hands-on guy who knows how things go in a lab, I always recommend float charging at 3.6 and just waiting for the cells to stabilize at that Voltage. You can save some time with a higher charging Voltage, but if you aren't baby-sitting your setup you risk damaging the cells while you're not looking--an expensive lesson you'd probably rather avoid. If you do experience a cell rupture, never touch the electrolyte with bare skin, allow it into your eyes, or breathe the vapors. It is a serious irritant, though it is not carcinogenic nor is it known to cause birth defects. If you must handle a vented cell, wear nitrile gloves. Do not let fumes build up in an enclosed space, use good ventilation of your workspace. And, as with any energy storage medium, have good fire suppression on hand. I like Halon or CO2, but any ABC extinguisher is OK. Do not use class D extinguishers for these cells.

Hope that clarifies some things.

 

Your observations are true, whether Li plating is present or not, and your strategy for dealing with it is spot on. Good work.

Everyone has always told me Li plating is 100% permanent. My understanding is that there is no way to reverse the process. Please bear in mind, I have zero hands-on time with the other cell makes you mention here and cannot speak to what their precise behaviors will be.

 

You are learning that balancing is not always as simple as it looks. What will be the best scheme depends both on what cells you are using, and the nature of the application they are used in. The system you describe above is pretty typical of many balancing schemes I am familiar with. For A123 cells, I would change the 4.0V alarm level to 3.8 and probably leave it at that. The cells should get to 3.6 Volts for each cell group with few or--ideally--none of the balancers ever turning on. You are on the right path.

 

We should expect that the balancers will be required to do their job sooner or later, and that as time goes by they will do it more and more often. Li ion batteries--like all other types of batteries produced thus far--degrade over time, even when there is no Li plating going on inside. There are many possible causes for cell degradation, ranging from manufacturing defects to environmental abuse, to poor pack management. The only way to have a real chance at knowing what did a cell in is to tear the cell down post-mortem. In A123 cells, Li plating looks like a whitening of the black active material on the plates. Other makers, I wouldn't know. Li plating can also happen during charging at low temperatures, but it would have to be pretty darn cold to be a problem at 1C--at least with A123 cells.

You definitely are having lifespan issues if you lose 30 to 50% of your capacity in 600 cycles. 10 to 15% wouldn't be shocking under certain operating conditions, but at 1C or less, even that seems severe. Are the cell capacities fading equally across the packs, or is it just a bad cell group here and there limiting the rest of the pack? Any cell teardowns? How far out of balance have these packs been allowed to get, and has this led to greater chance of cells being exposed to overcharging or overdischarging? I'd need quite a bit more detail before I could make anything like a confident statement about what may or may not be your problem.

 

I really appreciate your experienced input to this forum. Could you please provide a photo of what Li plating looks like compared to one cell which does not have Li plating.

Do you have access to a full specifications data sheet for the A123 20ah pouch cell, does one actually exist. The one provided by the A123 website is very defficient.

Yeah, I am probably asking for too much.

Thanks

 

Can't blame you for not wanting to cut cells open. It's not something that should be undertaken casually.

On to the data....

It's not clear to me whether or not all this data comes from the same charge/discharge cycle or not, but if the "end of charge" and "end of discharge" windows are typical (same cell hits top and bottom of charge first all the time), then you have indeed identified the weakest cell in the pack. Did you manually balance in the end-of-charge window where cell 11 is held flat around 3.495 Volts? You can do that, but it's easy to overdo it and overcompensate, and this can make your results harder to interpret properly. However, looking closely at the charts suggests to me you did OK here and you are interpreting properly and cell 11 is the weakest link. Do you know the capacity of the strongest and weakest cells in the pack?


A few comments--the mismatch here doesn't look that severe to me, but maybe a straight 1 or 2C steady discharge would look worse. I wouldn't call this a case of one radical outlier--many cells are showing degradation in varying degrees. The change in the shape of the curve for cell 11 is a strong indicator that there have been changes in that cell's characteristics--can you measure individual cell group impedance? Do the same cell positions seem to fade fastest, or is capacity loss completely random wrt pack position? Not sure what the minimum allowable Voltage for your cells is--I would have no qualms about taking A123 cells as low as 1.5 Volts under load. Your upper balance points look good, I see no reason to think Li plating from overcharging is much of a problem for you, if at all. Are the cells really 120Ah each (or are there, say, 4 30Ah cells in a parallel group)? Several smaller cells in parallel is preferable to one big cell, IMO, because any variation in cell quality is going to be exaggerated with one big cell. One bad cell will cause the pack to decline much more quickly if that bad cell makes up a whole series element all by itself.

How confident are you in the absolute accuracy of the Voltage measurements you're getting here? Is this the exact same data the BMS is using to make its decisions for balancing? Precision is very important with this, and erroneous readings can cause the balancers to mismanage cells.

 

I'm glad you realize the sensitive nature of your requests. I am not able to post any A123 documents here, and I am equally reluctant to post photos from lab reports. I tried to find a photo on the internet, but all I found were photos taken at very high magnification. There really isn't too much to describe here. A healthy electrode will have an even black sheen to it when first exposed to the air. The electrolyte evaporates quickly, right before your eyes, and the black then has a slightly lighter "dry" look as opposed to the fully black "wet" look it has when soaked with electrolyte. If Li plating is present, the black color will be uneven, with a whiteness that ranges from semi-transparent to fully opaque over the black surface of the electrode. The plating is never evenly distributed, IME. In more severe cases, the black active material coating may be flaking and peeling off the electrode as well.

This causes not only loss of capacity, but also impedance rise in the cell because now the Li that can still be cycled through the cell has to take a more and more tortuous path through the cell to do it's work. The plated areas are effectively dead, and the Li plated there is no longer available for cycling. That said, correlating impedance rise with capacity loss is, as far as I've been able to tell, an exercise in futility. The relationships between standard parameters as a cell degrades are wildly unpredictable. This can probably all be unraveled, but I believe doing so would be a major undertaking requiring teardown and detailed analysis of hundreds of cells that have been exposed to carefully controlled and documented abuse conditions.

 

OK, I kind of figured that, but wanted to be sure the data was of a single cycle (not just from a single day). I'm much more used to looking at data from HEV's that are not top-balanced....getting zeroed in on your data, I can see we have a top-balanced PHEV pack with SOH ranging from near 100% to ~50% (OK, I take one comment back here--that's a moderately severe spread, but am I correct in seeing that the worst cells are 9, 10, 11, and 12? If so, that could be significant--thermal issues come to mind as a possibility right away. Knowing the larger history of the pack would be helpful [what cells have been replaced and when, etc.], and while I'm asking for more info, I'll just note that I asked several questions in my last post that you've left unanswered. If I'm off course with my line of questioning I need to know so I can actually try to be helpful. Please respond one way or another to any questions I may ask.

Reading on into the next paragraph, things get more interesting:

OK, a picture is crystallizing here. The data is making more sense to me now...Are you sure there was no balancing at all? Several cells exceed 3.6V multiple times, albeit briefly. Just asking...the devil's always in the details with this stuff, hence all the questions. The phrase, "This is done with out any top balancing resistors," is a big red flag for me. Are you saying that this pack runs a cycled float charge (for lack of a better term) during which balance resistors never come on no matter what? Can the pack top Voltage float around? How much? I'm wondering what the data looked like a couple days ago prior to balancing.

Your description clarifies a lot. Is that cell 16 or cell 12? Either way, this behavior the two cells are exhibiting usually indicates a high self-discharge condition in the cell. It may be only intermittently "leaky" based on SOC or temperature, but it's just as likely constant and only plainly visible "at the tails" where Voltage changes fast enough for us to notice in a pack that gets used every day. Again, understanding of long term behavioral cycles (if you manually balance from time to time, there must be cycles) through your data is important.

It's great that you are taking all this data. Do you log every day in the car like this? What else are you collecting besides Voltage? I'm not familiar with "CellLogs", must be some kind of tool for taking pack data?

 

Thanks. I guess it was worth asking. I expect that maybe you have signed a NDA. However A123 has become bankrupt and since the cells have ended up on the hobby market and the general public I think that A123 are obliged to publish a full data sheet for these cells so people like myself have a better idea of what we are dealing with in terms of designing and building a large A123 battery pack. At least a MSDS.

 

If A123 was actually dissolved it would be one thing, but it's not--A123 has simply reorganized and is continuing on under new ownership. Yes, I've signed an NDA as has everyone who has ever come by anything from A123 through legitimate channels. Grey market cells are not the doing nor the responsibility of A123. There isn't much of value in the MSDS that I haven't already shared here. One tip I'll share is that if you get electrolyte on you, wipe off all you can with a rag BEFORE washing it off. The electrolyte can form hydroflouric acid (very nasty, very corrosive) when combined with water. That said, this stuff isn't nearly as scary as it sounds. The volume of electrolyte in the cells is very small--far below what I would consider "dangerous" from a chemical exposure standpoint unless someone was a real idiot about it.

 

delete....doubled post below.