How to troubleshoot battery pack ?

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hary
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How to troubleshoot battery pack ?

Post by hary »

Hi.

I have a DIY ebike with a 16S18P 18650 LiFePO4 chinese cells battery pack that is supposed to be 48V an 20Ah capacity.

The ebike would quickly stop working because of Low Voltage protection acting because one of the raw is getting below 2.5V when the others are still at 3.2V
The cell would recover to 2.82V for the bad cells and 3.21V for the others after 1 night resting.

Can I deduct the 2.82V row cell is bad ?
I opened the battery pack because I was guessing some part of that row would have disconnected, but that's not the case. All the cells in that bad row are well connected together and all cells shows the exact same voltage !

My quetsion are :
How a all bunch of 18P cells can all turn bad together ?
I must say, before my test/troubleshooting, I charged/balanced the battery, it took about 1 week to balance it properly as it had sat doing nothing for a year or so.
Then it rested for 5 days before starting the road trip on the ebike. Before starting, that same row cells had already strangely lowered to 3.15V when all others rows where around 3.3V.

Would somebody give be any direction to deeply diagnose and maybe save my pack or at least a part of it ?

I'm ok to diagnose a lead acid battery cell by cell, but feel hopeless with LiFePO4 cell or lithium in general. Having multiple (I mean a lot of) cells in parallel makes it even more dificult !
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jonescg
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Re: How to troubleshoot battery pack ?

Post by jonescg »

If it can't be brought back to some kind of state of balance, then yes I suspect those cells are toast. All cells in parallel should be at the same voltage, yes. As one cell goes low, the other cells push current into it. Until they are low, and so on. Eventually they will all reach the same voltage as the lowest cell.
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hary
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Re: How to troubleshoot battery pack ?

Post by hary »

I think I understand what you're saying but :
If there is 1 bad cell drawing current from other cells in //, isn't there a way to isolate that bad cell and save the rest of the battery pack ?

Is it possible that this bad cell draw current from others cell only to a limit voltage, then stop drawing current ? Let say that this bad cell has depleted all its neightbours cells to 3.15V and then stopped ? It should have heated up to disspate all that energy, isn't it ?

Quite scary ! :shock:
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brendon_m
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Re: How to troubleshoot battery pack ?

Post by brendon_m »

Can you split the parallel cells up? Then you could test each cell in the suspect row individually
hary
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Re: How to troubleshoot battery pack ?

Post by hary »

Well, I was afraid that was the only solution !
Of course I can do it despite they are all spot welded together !

I do think these large number // pack are not a good solution.

I'll do it as I think otherwise the pack is really dead !

What tools do I need to properly work with batteries ? I mean how to capacity test large amount of cells, and is that the only point to test ?

Would some of you have good link to state of the art tutorial about working with Lithium batteries ?
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brendon_m
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Re: How to troubleshoot battery pack ?

Post by brendon_m »

hary wrote: Sun, 24 May 2020, 18:06 I do think these large number // pack are not a good solution.
It does mean that if only one cell is dragging it down (and it hasn't damaged it's comrades yet) then you can replace just the one bad cell.
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brendon_m
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Re: How to troubleshoot battery pack ?

Post by brendon_m »

hary wrote: Sun, 24 May 2020, 18:06
What tools do I need to properly work with batteries ? I mean how to capacity test large amount of cells, and is that the only point to test ?
You can get by with a multimeter, constant load(light globe, resistor etc), a stop watch and some time. Oh, and some way to charge the cells back up.
hary
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Re: How to troubleshoot battery pack ?

Post by hary »

It is a 16S18P pack, so 288 cells suposely 1100mAh each.
So about 288 hours at 1A /1C ! :cry: I feel just discouraged at that point !
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brendon_m
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Re: How to troubleshoot battery pack ?

Post by brendon_m »

Well theoretically you could just do the suspect row, that brings it down to 18 hours. And you could do them in groups to narrow the field.
Ie 9 and 9, then if only one set of 9 are weak then split it again etc.
If they are both weak then you are probably going to have to replace the whole row anyway but you could try to keep narrowing down the field
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