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Charge won't get to 'full' (float) without tripping overvoltage in the BMS

g__0

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Have 4 200ah lifepo4 cells with the overkill BMS.

I top balanced the cells, then discharged down to 0%. Got approx 205ah out of the pack, so that looks good, but I had noticed that when I charged back up to 100% it never got to 14.4v,, and a cell would hit overvoltage and shut down the charger before it got there.

I left it for about a week and it settled down to a variance of approx 0.014.

Hit it with a quick charge today and it did the same thing. The variance spikes as soon as I put the charger on it, then one cell gets to 3.65 and shuts down the charger before the entire pack gets to 14.4.

This isn't ideal as I was hoping to leave the pack on the charger when I'm not using it and not be constantly plugging/unplugging the charger, or have it constantly stuck in the cycle of tripping the overvoltage and shutting off.

Am I doing something wrong?
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I connected the cells in parallel and charged them to 3.65. After I’d let them settle for the week, cell #2 was slightly low but the other 3 were very close. Now it looks like #2 and #4 are too high - finding this a bit confusing

They've already settled out considerably since I took that screenshot a half hour ago.
 
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Yes. You probably did something wrong.

What was your termination criterion?
What was the minimum cell reading?
What did the other cells read?
Did you have balancing active on discharge as well, or do you only have it active on charge?
What charger to you have?
What is it set to?
Leaving it on charge?
What's your float voltage?


A deep discharge can effectively undo a top balance, particularly if you have balancing on at all times.
 
I think you need to bring each cell up to 3.65 individually.
You can probably do that without breaking the pack.
For the time being disable bms balancing.
The when you get the pack balanced set the low cell cutoff to 3.0 volts and the low pack cutoff 11.8 volts and then do a discharge/recharge cycle.
See how they hang together without bms intervention.
 
As @snoobler says you should only balance during charging and only above float voltage.
What is your float voltage?
 
Is it safe to charge the individual cells with my 20a charger? It isn’t configurable so will just charge at 14.4v
 
Is it safe to charge the individual cells with my 20a charger? It isn’t configurable so will just charge at 14.4v
The max voltage for a cell is 3.65 volts, so no.
How did you top balance them initially?
I suspect user error.
 
definitely potential for user error. Didn’t even know this forum existed until I’d finished. I had them all connected in parallel and used a multimeter to see when they got to 3.65
 
definitely potential for user error. Didn’t even know this forum existed until I’d finished. I had them all connected in parallel and used a multimeter to see when they got to 3.65
Yes but what charger did you use?
 
Wow you charged your cells with a 12 volt charger.
That is very far from optimal.
Typically folks top balance with something like this.
 
aha. The missing piece of the puzzle...

I always go into these projects knowing I may be making money evaporate the first time around. Part of the fun. Although I did get 205ah out of them, so I can't have messed them up too badly.
 
OK, so I will grab a cheap bench top charger, then individually bring each cell to 3.65, and adjust the bms settings as you mentioned. Then see where I'm at. Thanks!
 
For the longevity of your LiFePO4 pack , do not leave it on trickle charge when not in use , unlike SLA and other flooded cells.
 
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