LIFEPO4 Battery discussion thread

cavedog said:
Okay the imbalance is worse that I thought. I might need to remove it from the pack and see if I can balance it separately. It disconnected due to low cell cutoff but the highest cell is still 3.278

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You can’t bottom balance, let’s see what happens at 56V. It could just be a weak cell and it is what it is. This is where a diy battery comes in and you just replace the cell.
 
wingnut771 said:
You can’t bottom balance, let’s see what happens at 56V. It could just be a weak cell and it is what it is. This is where a diy battery comes in and you just replace the cell.
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Yes definitely not I got the battery at 85% soc from him and just added it to my others to test and the voltage were pretty much the same so there wasn't a massive current shift into or out of it when I linked it up.

Its just an observation about the imbalance because the bms cutoff due to cell08 UVP so that cell does look weak or well behind the others.

Will see later today at 56v
 
wingnut771 said:
Of course they will be welded. I predict this cell will peak first too.
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That means that cell is kaput right? This battery is a problem then because it will probably have welded terminals which I can't do so I can't even replace it.

At 355 cycles it seems odd that it will fail though? So quick?
 
cavedog said:
Yes definitely not I got the battery at 85% soc from him and just added it to my others to test and the voltage were pretty much the same so there wasn't a massive current shift into or out of it when I linked it up.

Its just an observation about the imbalance because the bms cutoff due to cell08 UVP so that cell does look weak or well behind the others.

Will see later today at 56v
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Remember I said to do capacity test before purchasing? Let’s hope that 93Ah is accurate.

Yeah because lfp voltage curve is very flat, you have a large range of soc you can have before paralleling and still not get large inrush current.
 
wingnut771 said:
Remember I said to do capacity test before purchasing? Let’s hope that 93Ah is accurate.

Yeah because lfp voltage curve is very flat, you have a large range of soc you can have before paralleling and still not get large inrush current.
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What is fire/failure risk if that cell is like weak? The BMS will just keep shutting it down. Is it worth keeping or should I look at getting it repaired rather?
 
cavedog said:
That means that cell is kaput right? This battery is a problem then because it will probably have welded terminals which I can't do so I can't even replace it.

At 355 cycles it seems odd that it will fail though? So quick?
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Another reason for diy. Some cells are just faulty from manufacturing defects. Let’s see what happens at 56V. How many Ah did it do yesterday?
 
wingnut771 said:
You can’t bottom balance, let’s see what happens at 56V. It could just be a weak cell and it is what it is. This is where a diy battery comes in and you just replace the cell.
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I am putting my money on just low cell and it will lag behind the others
 
cavedog said:
What is fire/failure risk if that cell is like weak? The BMS will just keep shutting it down. Is it worth keeping or should I look at getting it repaired rather?
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No risk, only if it’s swollen I guess. Doubt you can repair it. I would just live with it.
 
cavedog said:
What is fire/failure risk if that cell is like weak? The BMS will just keep shutting it down. Is it worth keeping or should I look at getting it repaired rather?
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If they are welded like like wingnut mentioned its ass to replace
 
DTBA said:
I am putting my money on just low cell and it will lag behind the others
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Let’s hope it doesn’t peak before the others which would indicate lower capacity than the others and it’s just a balance thing.
 
DTBA said:
If they are welded like like wingnut mentioned its ass to replace
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Maybe you can cut the welded busbar in half and drill holes into remaining busbar and use cable and lugs to connect new cell?
 
wingnut771 said:
Maybe you can cut the welded busbar in half and drill holes into remaining busbar and use cable and lugs to connect new cell?
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That was what I was thinking if it has to come to that. Hacksaw the busbar in the middle. Take the problem cell out. Install new one drill 2 x new holes carefully and put a bolt with a new busbar. At least I would have the entire pack again if the rest of the cells are good that is.
 
wingnut771 said:
Maybe you can cut the welded busbar in half and drill holes into remaining busbar and use cable and lugs to connect new cell?
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Could work yea would be interesting to try at-least.

Like you said if its a deadish cell maybe run it till the battery start to give like 50ah then replace it
 
cavedog said:
That was what I was thinking if it has to come to that. Hacksaw the busbar in the middle. Take the problem cell out. Install new one drill 2 x new holes carefully and put a bolt with a new busbar. At least I would have the entire pack again if the rest of the cells are good that is.
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You can use flex cable with but and bolt as a busbar, probably multiple thinner ones is easier than one thick one. That’s easier than trying to put another solid busbar in.
 
DTBA said:
Could work yea would be interesting to try at-least.

Like you said if its a deadish cell maybe run it till the battery start to give like 50ah then replace it
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Problem is then the 300Ah bank becomes a 150Ah bank.
 
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