Replacing BMS 5s on cheap chinese tool batteries.

[Argentvs]

Helo, I registered after finding this forum. I was asking Grok AI but I don't like how it constantly changes what it recommends.

I searched a bit here but I am not sure to post in other topics already open.

I'm from Argentina. I have this Daewoo tools, this battery was new but it got damp and stopped charging. Once I replaced the BMS on my own and it worked a while, then dead.

I bought 2 new BMS 5S on aliexpress. After starting over, I found I most probably connected the previos one wrong.

This is one working, original.
This is what I did to the dead one.


I think this is how it is, Grok also says it. But it doesnt work, and it baffles me that it worked previously. It just don't charge, the charger stays on green light.



Is this right?. It doesn't have CH+ or P+. Allegedly this means the charging pin from the case goes connected in line with the B+ output in the battery pack.

I am unsure the B- on the top right goes to the B- in the battery pack soldered along the B- cable from the white connectors.

Thank you for your time.

 

[scianiac]

I think it looks correct but hard to see everything perfectly. Yes the B- in the white connector is connected to the same battery - that B- on the board. Two wires to the same place so the current in the power wire doesn't effect the voltage reading. You did make sure the batteries were still good though, as in at no point where the batteries every discharged below 2.5v like when it got wet?

 

[Argentvs]

I think it looks correct but hard to see everything perfectly. Yes the B- in the white connector is connected to the same battery - that B- on the board. Two wires to the same place so the current in the power wire doesn't effect the voltage reading. You did make sure the batteries were still good though, as in at no point where the batteries every discharged below 2.5v like when it got wet?

Sadly I measured them, and yes. I have 6 on 0.0, so it was a waste.

I have another 20v but 2 amp (just 5 cells) to repair. It just stopped charging out fo the blue. I checked it and it has 1.7v, tried to bump it but it died, now is 0.1. So I may try to replace that cell with one of the 4 of the above battery pack that have 3.5V.

 

[Nix Liou]

Sadly I measured them, and yes. I have 6 on 0.0, so it was a waste.

I have another 20v but 2 amp (just 5 cells) to repair. It just stopped charging out fo the blue. I checked it and it has 1.7v, tried to bump it but it died, now is 0.1. So I may try to replace that cell with one of the 4 of the above battery pack that have 3.5V.

Mixing and matching cells from different batteries, especially when both have had failures, is at best a temporary stop gap measure. Power tools are additionally not easy on batteries, especially cheaper cells. Swapping some cells may work temporarily, if you balance the "new" cell to match the others, but it will probably begin falling out of balance faster than a cheap passive balancing bms can account for at some point, and you will get the same failure mode again.

 

[Argentvs]

Mixing and matching cells from different batteries, especially when both have had failures, is at best a temporary stop gap measure. Power tools are additionally not easy on batteries, especially cheaper cells. Swapping some cells may work temporarily, if you balance the "new" cell to match the others, but it will probably begin falling out of balance faster than a cheap passive balancing bms can account for at some point, and you will get the same failure mode again.

Oh, so even if a buy a new cells it wont be good?. Ok. Thanks.

 

[Nix Liou]

Oh, so even if a buy a new cells it wont be good?. Ok. Thanks.

If you buy enough new cells to replace them all, that would be good. You could make it probably a lot better than it was originally. But mixing and matching cells is risky and doesn't work well especially for this use case.