# Reviving a dead LiFePo4 cell (Hi-Power, 40Ah)



## hydrochloric (Oct 23, 2011)

Well, the BMS issue was finally sorted (a different thread, turned out the main controller was dead). Have been charging for a while now, but we've got some cells that REFUSE to come up to voltage, and force a long balance. Without cooling, the BMS's cells report high temp from the resistors and cut the current, so the balance is abysmally slow (this'll be improved when we make the new box with integrated fan and heater).

I think, as a result of the slow balance, the BMS was repeating the balance cycle over and over. I've heard that it can be bad to hold the LiFePo4s at 3.65, but most sit at ~3.35, and none higher than 3.41 when not charging, so that sound good at least.

However, back when the BMS was being troublesome, all the modules ended up trying to balance the cells to 2V, and managed to drag three cells (each "cell" is actually three 40Ah cells in parallel) to 1.4, 1.7, and 1.6-ish volts. I removed these cells and individually charged them with an RC charger, unfortunately without a LiFePo setting, so using a LiPo setting. They got to about 3.3, and held it. However, while two of them might be alright, we're currently troubleshooting a super-low full throttle current (125A out of 500A), and while monitoring the cells, we've discovered the #3 cell (on the bottom layer, almost farthest up, of course) has decided to be a jerk. This leads the car to having a top speed of, oh, about 25MPH.

Basically, as soon as the draw from the pack exceeds 100A, the #3 cell drops so low, the cell module turns off and kills communication for the rest of the pack. This means it drops below 2 volts.

So, does a cell that holds 3.3v static, then falls when a load is applied mean it's truly, completely dead? Or is there another way it can be recovered? We can get three new identical cells, at a cost of ~$160, but if these can be revived, that's a much better choice.


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## hydrochloric (Oct 23, 2011)

I should mention I've looked this up, but can't really get agreed upon info. Additionally, since I'll have to take 90% of the pack apart to get at the bad cells, I'll be able to do individual charging if needed.


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## icec0o1 (Sep 3, 2009)

Sounds like they're dead. There isn't any way to reduce their internal resistance.


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## hydrochloric (Oct 23, 2011)

That's what I feared. Ah well, it's a good excuse to switch to the properly designed battery box.


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## Batteredneck (Sep 16, 2015)

OK, honestly, this will piss a few off as I don't even know how to execute a proper search for what I'm experiencing, but I hope for some help. I bought 300Ah LiFePO4 cells x4, but not for EVehicles...actually not sure why I did to be honest, but I felt they were a good deal. I've been using them to drive a kegorator or two while travelling. Well, I may have found out why they were a steal: I cannot get all four cells to balance whether top or bottom balancing...
I have the smallest of smart chargers intended for RC users as I only intended this to be a fun small hobby before I ran across these. I've been trying to use them to power family camping outings where I'd rather camp (i.e. they don't have "campers"). Turns out they can power two small refrigerators on inverters. Bring the shun, I'm ok...
Here's what happens on the latest attempt at a bottom balance: each cell individually brought to 2.8v (should they all be charged at the top first?), then charged up as a group to the default 4S setting, when charging cell 4 varies up to 3.5v+ (I do not "balance charge"), but drops as soon as load is applied and they all are in harmony until ~3.0 or so when cell1 falls to dangerous levels quickly (as in hours on a 1A draw) and is currently sitting at 2.3v while the rest are at 2.9 on another to 3.1 on the other two.
Any other thoughts or things I could do? I don't know their history for sure, only what I was told was that they were "new" when I bought them...


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## dougingraham (Jul 26, 2011)

Batteredneck: If you have a smart charger you should cycle each cell. Number each cell 1 through 4 and Charge them one at a time using the LiFe setting. Discharge them to 2.5 volts and write down the discharge capacity. Then you will know what you really have. The smallest capacity cell is what defines the capacity of the pack. Smart chargers usually can also measure internal resistance but if a 300 AH cell is tested on any smart charger it is going to tell you 0 or 1 milliohm because it is not capable of giving a reasonable answer on such a large cell.

If it turns out all four cells have similar capacity I would charge them up again individually and then connect them in series. At this point they are top balanced. You cannot top and bottom balance at the same time without carefully selecting cells of the same capacity from a large number of cells. Only cells of exactly the same capacity can be both top and bottom balanced at the same time. If you are using the smart charger to charge the pack then make yourself a balancing connector and just use the smart charger to top the thing up after use always using the balance connector on the LiFe setting. After charging you should be able to disconnect the charger and any load and let the cells sit for half a day and measure the voltage of each cell and they should still be close. If not then you have a leaky cell or there is something attached to that cell causing a drain. Properly operating LiFe cells don't display measurable self discharge.

A 300 AH cell is going to weigh around 20-25 lbs so your pack will weigh 80-100 lbs. If it doesn't then you didn't get what you were sold.

Do a capacity test on each cell individually and then you will know what you have. It seems like cell 1 is the weakest cell. Any cell with a resting voltage below 3 volts is essentially discharged.

Best Wishes!


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