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How to rescue lifepo4 battery pack with 1 dead cell

racmaster

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Jan 5, 2022
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situation:

- 4S lifepo battery 100ah each cell, connected in serie=13.6v battery

- 1 cell is semi/dead, holding voltage, but lost capacity to 20ah, probably overcharged before... it can be charged and discharged, but results in unbalanced battery pack and whole pack is therefore limited to 20ah...

possible solutions:

A:
adding another 20ah cells in parallel with the broken-one, which means 5x20ah cell= 100ah and connecting this parallel pack to the serie with 3 remaining still good 100ah cells. of course, before paralleling them they need to be charged same SOC, voltage and balanced top or bottom... then vice versa with remaining 3 old cells...

B: removing the wrong cell and replacing with new cells connected in parallel, but as near as possible to actual real capacity of remaining 3 old cells. which means, lets say the 3 good old cells are aged and they are no more 100ah, but lets say 60ah of real capacity. so the 4th cell to be added to the serie pack should be built from smaller cells connected in parallel to reach the real 60ah capacity. of course again, first charged, then balanced and then connected together...

C: replacing the wrong cell with the bigger one, for example 110ah

pls consider these 3 possibilities and lets discuss which solution will be best technically, which economically and/or which practically pros and cons welcomed
 
How old is the pack?
What type of pack is it?
What kind of cells?
I think a 100ah pack from battery source would make the most sense.
 
question is general. what should be the possible solutions for this chemistry in such a situation. doesnt matter how old and what producer... lets just explore and compare theoretical possibilities and formulate circumstances...
 
Personally...if this was my battery...I would go with...

d) Start with 4 new cells and give the other 3 cells away or sell them. Also...lessons learned about how to charge batteries.
 
Yeah, starting over would be the best. But to try and salvage what you have, I’d get an extra cell (always a good idea to have a spare) and see how it performs when you replace the defective one.
 
Get an extra cell. It doesn’t matter if it has greater capacity than the others, it will only discharge to the weakest cells capacity which in a series configuration won’t hurt it.

As the pack capacity becomes unacceptable, keep replacing the weakest cell until all original cells are gone.

In theory i believe that to be the best option. In practice it relies on a readily available single cell purchase which isn’t always possible.
 
The best and most economical / safe solution is to stop playing with trash, *find out why* that cell was overcharged (rather than assume with "probably"), and rebuild a new battery based on the techniques of how to do it right provided by this forum and elsewhere.

Otherwise, it is just an endless mental exercise playing in the trash dump. Um, recycling center. :) Waste of time.
 
My apologies for the harsh reply.

The intent was that quality LFP at affordable prices can be had today so one doesn't have to jury-rig stuff like this together, unless it was an extreme temporary emergency.

We also don't want to send a double-message to manufacturers that it's ok to cut corners because we're ok with it. Imagine if you opened your Sol-Ark, and found stuff like this inside. Imagine what Will would say with a teardown.

Yet on the other hand, we're promoting the use of mismatched components - even if you could jury-rig something together mad-scientist style. :)
 
If you are connecting to an inverter which can be programmed to accept parameters like hi or low-cutoff voltage,
Perhaps you could remove one dead cell from a bank of 16. I have seen some BMS instructions that say

connect cells in sequence 1 to 'the number of cells'. If there are leads for cell 17 to 24, for example, and you have 16 cells, then leave 17 to 24 without any connections.
It makes me think one could move the last cell (16) to the position of the failed cell and just use 15 balance leads for the 15 good cells. (if you prefer to rebuild the entire battery bank, you would get to the same end point, but with more work)
The important thing is to just have 15 goo cells in the bank.

Is this a reasonable idea?

This might not work for 1 dud cell in a bank of 4 cells. ;)
 
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