Battery repair, how daft is this?

[KirstinS]

I've a one year old 17.5ah pack of mji cells. The pack I sourced welded but without BMS or a case I could use. So I added a BMS, kapton taped and shrink wrapped, then it went in a frame bag.

Water ingress has fried cell string 9. A neat take down as both 8 and 10 remain perfect. They test at 4350 per cell. I have very very carefully dremmeled out the dodgy cells and inserted brand new 30q as I happened to have a few spare. They test at 4150

Anyway I saved most of the plastic holders and resokdered the pack plus some hot glue. Then kapton tape and reshrinked

All charged up fine

Question is the cells are clearly mismatched and this is, let's face it, a bodge

However as long as never run the pack past say 50% depleted (which is easy on my commuter) I don't think I'll have too many issues

How wrong am I?

no photos load as too big other than this one

 

[anotherkiwi]

You now have a <15 Ah battery.

Watch the battery voltage and watch sag on your LCD if you can. That will tell you if something is wrong.

 

[Sturmey]

........How wrong am I?

The problem is that if you discharge the battery fully, you will reverse volt the 30q as they have smaller capacity and then, possibly??

 

[vfr400]

The problem is that if you discharge the battery fully, you will reverse volt the 30q as they have smaller capacity and then, possibly??

That's not very likely. The BMS will switch off when the first cell reaches 2.9v. I can't see any problem with that reapair other than the reduction to 15Ah on a good day. It's probably a good idea not to run the battery low because that cell will go lower than normal. It's normally the controller that switches off before the BMS, but that will be the other way round now.

 

[KirstinS]

That's not very likely. The BMS will switch off when the first cell reaches 2.9v. I can't see any problem with that reapair other than the reduction to 15Ah on a good day. It's probably a good idea not to run the battery low because that cell will go lower than normal. It's normally the controller that switches off before the BMS, but that will be the other way round now.

That is exactly what I thought but wasnt totally sure

Thanks

 

[KirstinS]

You now have a <15 Ah battery.

Watch the battery voltage and watch sag on your LCD if you can. That will tell you if something is wrong.

Yep. That's what I was thinking too.

Thanks

 

[KirstinS]

The problem is that if you discharge the battery fully, you will reverse volt the 30q as they have smaller capacity and then, possibly??

Well possibly but that's why I said never discharge low

 

[Sturmey]

Well possibly but that's why I said never discharge low

I am inclined to see the BMS as an additional item of safety but not as replacement for bad battery design. Anyhow, if it was me, I would consider using replacement mj1 possibly with solder tags which would probably cost you about £25 delivered.

LG INR18650-MJ1 3400mAh - 10A - 18650 - Li-ion - Rechargeable batteries | NKON

<p>LG INR18650 MJ1 3500mAh (10A)</p> <p><a href="/sk/k/Specification%20INR18650MJ1%2022.08.2014.pdf" target="_blank">Datasheet </a></p>

eu.nkon.nl

 

[wheeliepete]

I'd rate that well above bodged Kirstin . It's not ideal, but as you say, I think it will be safe as long as you don't run it too low. Just a thought, but I wonder if the higher max. discharge rate of the 30Q's will help maintain their voltage and narrow the difference in capacity in real world use?