Repair LiFePO4

[Jepp]

I'm fairly new to LiFePO4 batteries and have a pack that I think it has at least one shorted cell. It's 20 AH 36V, which gave me more than 40 V when charged. Now it only shows 36.6 after a charge, and the charger cuts out almost immediately - it seems to think it's fully charged. I opened it up and it looks like the metal plate connecting one of the cells has melted (there was burned material around it). Does anyone have any experience replacing cylindrical cells? They seem to be really glued together well and I'd like to try to take them apart and see how many cells were damaged. I attached a few photos - not too hard to see where the problem is!

I'd like to try fixing it myself, or if that doesn't work, send it off somewhere to get it fixed (I'm in the Vancouver Canada area). Does anyone know of anywhere I can send it?
- Thanks

 

[ambroseliao]

It would probably be best to contact the seller to get a replacement cell for this pack.

It's not too difficult to swap it out. It would also be a good time to check all the other cells to make sure they are at least up to the proper voltage.

Ambrose

 

[olaf-lampe]

Please don't send it around before you have found and eliminated the cause for the shortcut.
LiFePO4 doesn't burn or explode easily but we don't need negative headlines in media ( again )

Thanks
Olaf

 

[dnmun]

add a second on that. in fact i have sent people emails recommending they actually discharge the pack to some minimal level before shipping. if there is less power stored in the pack , then if any short happens, there will be less energy released and less chance of fire or smoke.

i think this is important enuff people should use it as a way of explaining the risk to others too. as a way of crossing the divide between those who have experienced a battery getting too hot and melting in thermal runaway and starting a fire, and those who think it will be like a detonation.

i know john also has had a fire, not sure who else.

i just took apart two packs with one dead cell in each. i was lucky that the dead cell was on an outer corner and i only had to cut one link. but i ended up deciding to combine the two packs into one 16S and keeping the other 6 for spares in case another pack goes bad and needs a swap out.

so i have two 5 cell groups, and one 6 cell group. total 16S.

it looks like that cell in the corner had some metal fatigue which caused it to crack and then it overheated under load, but we would have to see how those cells look and the voltages too.

 

[Jepp]

Thanks for the warnings about finding the cause of the short before attempting any repairs. Finding the cause was easy, it was me. I was doing some trouble shooting on the motor controller and didn't see the end of a brake cable sticking out from the bike. I now have a very big respect for the power in the battery packs!

I tested voltages at the BMS and found a couple that were out. Most were 3.3V, but one was 0 (the top corner one in the photo) and one was 2.5 (from the row below it), so I'll be replacing the cells. The place I got the battery wasn't much help. They said they could send the battery pack back to the factory to have it fixed, but it would be $500 EACH WAY for shipping! They said they could maybe get some replacement cells, so that's what I'm going to try.

Any advice on how to reconnect the new cells? I have heard that soldering them may heat up and damage the cells.

Thanks.

 

[dnmun]

2.5 is ok, but you tested at the BMS and the sense wire may be broken on the 0V. so that broken tab is from shorting to a brake cable?

i still can't figure out what the other parts of that tab connect to either. not enuff info. just can't tell.

 

[olaf-lampe]

They said they could send the battery pack back to the factory to have it fixed, but it would be $500 EACH WAY for shipping!

That's a nice way to give lifetime warranty on batteries, too. No one will ever claim it this way
-Olaf

 

[robb]

I replaced and resoldered about 20 cells in my cammy pack and that seemed to work fine. I don't think the soldering reduced their capacity though it is hard to say accurately. Some of the cells had discharged to below 2 volts and they seem to have recovered. After soldering I immediately cooled the joint with a damp sponge to reduce the heat as quickly as possible. Cammy was helpful and replaced for free the damaged cells and a damaged bms including the postage, so I would try and at least get replacement cells from the seller of the battery..
Rob

I tested voltages at the BMS and found a couple that were out. Most were 3.3V, but one was 0 (the top corner one in the photo) and one was 2.5 (from the row below it), so I'll be replacing the cells. The place I got the battery wasn't much help. They said they could send the battery pack back to the factory to have it fixed, but it would be $500 EACH WAY for shipping! They said they could maybe get some replacement cells, so that's what I'm going to try.

Any advice on how to reconnect the new cells? I have heard that soldering them may heat up and damage the cells.

Thanks.