Identifying that BAD 18650!

Ill try to explain it briefly, basically i have a 12s10p 18650 set up, board keeps cutting at 44volts (should be 50V full charged and can hit around 38V before needing recharge), this is a self build, there are 4 of the packs (pictured below ) in the board, each with 3 rows of 10 batteries ( 4 x 3 = 12s / 10 = series)

when taking the voltage across all the packs this is what it looks like

1st pack | | | X 3.53 3.28 3.53 10.4 2nd pack | | X | 3.8 3.8 3.8 11.68 3rd pack | X | | 3.8 3.8 3.8 11.67 4th pack X | | | 3.8 3.8 3.8 11.67

clearly the 1st pack is the problem which is what is in the pic - the middle row has the lowest voltage across it at 3.28. I have guessed two of the cells and cut them out the series and tested them and they both show 3.28. so my question is - how can identify the bad cell - do i need to cut them all out and test - i have a feeling they have balance charged since the board cut out so none will be “dead”

IMG_2510 BatterySkateboard

currently i think they are all now sitting at 3.29v individually in that “bank”, guess i need to solder them back into the series and run the board dead again , then cut them out quickly before other good ones surrounding it in that bank it balance charge it and revive the bad cell /cells and see if i can see what one/s are dead unless anyone else knows a way to test them?

Are you using a bms?

Easiest way to test for a bad cell is internal resistance. There’s plenty of chargers out there that do a quick easy test. Or you could fairly simply design your own.

Or you could attach a load thermal camera it and bad cells should be hotter.

Could also be an issue of pack design but I don’t know what kind of currents your pushing nor what your connecting wire is made from. But that kind of design is asking for uneven current draw Imo.

Yes it has a BMS IMG_2512

double check its working because it’s unusual to take out 3 series groups, unless it’s more than one bad cell

If you have one bad cell in a parallel pack, you now have X amount of bad cells in a parallel pack… All linked cells will be dead and need replacing.


How could i check if its working? I have looked up all other series in all other groups are balanced fine, Would you agree that it could be the center row of that group as it its showing as 3.29 - the others are at 3.58 (the board has been sat there for 2-3 days since it cut out last so they could have ended up charging the bad series)

Debating if its easy just to swap out the entire row.

it’s no wonder you’re having issues.

you’ve probably pumped A LOT of heat into your cells soldering it together.

just immediately you’ve got a cell isolated, no longer a part of a P group:


you’ve probably got a bunch of shorted out cells on the positive end of one or many of your cells.

frankly, this is one of the most frightening battery packs i’ve seen in a long time.

it’s probably not realistic to start over but…i’d start over.


As @darkkevind said the row is shot now and needs replacing. But it’s possible the whole group could need replacing aswell. Although oddly, (3.8+3.28)/2= 3.54. Could be a coincidence or maybe the bms found the middle ground



wow … this looks like a time bomb


I didn’t even know where to start on the build quality so stayed quiet. I can feel my blood pressure rising the more I look


i have sweaty palms looking at this.


Im out of words, this looks dangerous.

But the issue is most likely the one in the pics.



On a second thought maybe my pic has been rude. Well… I am not a soldered packs fan. I think they are dangerous and easy to fail. I hope you fix It.

Quite a horror show :slight_smile:

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Cold joint here in blue. It looks like the pack received some high G force and has changed shape a bit which has made some cells move and (possibly) break the connection. The hot glue has come off some of the cells. Or OP did that trying to disconnect the two cells.


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Maybe we missed this guys, the two broken connections where cut as part of the diagnostic.

You should carefully disassemble the pack, clean the cells with solder wick, test them, discard tha bad ones, balance the rest of them, spot weld them, use fish paper between the cells and rings between the positive terminals.

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