What is going on with these lithium cells?

[WaldPinkler]

I put new cells in a power tool battery with a BMS. I didn't connect the sensing wires yet because of the weird voltages.

I'm scratching my head. One of these cells measures -15V. The are all just in series and I can't see a short or anything.

From the negative I get these cell measurements 3.3V, -15V, 4V, (the rest are 3-4V)

The cells all measure 3-4v when I remove the nickel strip.

It hasn't caught fire at least.


Simple BMS with cells in series


First cell is 4V


Second is -15V


Third is 3.3V

[Whales]

That -15V might be the full voltage of all the other cells?  Has that one cell gone open circuit?  If you remove the nickel from one end of that cell do you still get -15V between it's nickel strips?

[mikerj]

That suggests you have an open circuit between two cells.  Hopefully you haven't soldered those tabs to the cells (hard to see in pic) as that can easily ruin them.

[WaldPinkler]

They are spot welded.

I just tore off the open one yet again. It measures 3.7V. They are all measuring 3-4V again.

I'm measuring across the left over nickel strip. I don't get it. It's not a dodgy weld.

[jdragoset]

Test the voltage of each cell with a 10 ohm, 2 watt load resistor across the meter leads.

[tunk]

Don't know about the -15V.
You may want to fully charge all cells individually; even if the BMS has balancing,
it may not be able to balance when some cells are 3.3V and some 4.1V.
And if you don't already know, most 18650s cannot deliver the 20-30A that a power
tools may use.

[thm_w]

Don't know about the -15V.
You may want to fully charge all cells individually; even if the BMS has balancing,
it may not be able to balance when some cells are 3.3V and some 4.1V.
And if you don't already know, most 18650s cannot deliver the 20-30A that a power
tools may use.

Very good points. This is not a balancing BMS, it doesn't even have sense taps.

OP, stop what you are doing, charging this could cause an explosion.

[Whales]

Hmm, maybe it's a BMS for LiFePO4 cells (as opposed to the common LiCo/LiMn cells)?  I believe those can handle overcharging by a bit, hence lazy balancing?  If so then it might short the cells or do other weird stuff when it senses too high of a voltage (due to the wrong cells being used).

WaldPinkler: be wary of cell chemistries.  LiFePO4 and LiCoMn are very different types of cell that both come in 18650 form.  LiFePO4's can take more abuse, but don't charge to as high of a voltage, don't have as much overall capacity and tend to cost more.  I believe lots of power tools use them.  LiCo/LiMn/LiCoMn are the "common" 18650 & lipo chemistry family that charges to about 4.2V per cell and is famous for occasionally flaming out.  Not to be confused with Lithium primaries (CR123) which are famous for causing big health issues from HF acid.