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BMS Not charging and cant figure out why (help needed)

So I recently finished building my third board (so I like to think I know what im doing at this point) and everything was going fine until my dad shoved the loop key into the charging port (there both xt 90 ports). Anyway it was lucky in that it only short for a mili second and seemed to have caused minimum damage as the board kept working so I resoldered a new xt 90 and plugged it in but it would not charge. So I checked the cells and one was at 1.5v, so Im assuming this cell took the current of the short circuit. I recharged the cell individually slowly to match that of the other cells and left it overnight to see if it would self discharge and in three days it only lost 0.01 volts so I think the internal resistance is still ok. So I assumed the BMS wouldn’t charge as the cells weren’t balanced properly and now that the cell matched the others I tried again but it still wouldn’t charge. So I though maybe the short circuit broke the bms and luckily I had a spare of the same type so I replaced it and again It wouldn’t charge. B- is the battery negative C- is charge port negative battery positive is positive charge port and balance wires are all correct so I have no idea what is wrong, any ideas? @Winfly @dareno

@Andy87 has a little bit of experience with a more severe form of this

What failed when it was shorted? Which wire melted? It’s not likely a human hand could have gotten it disconnected in a millisecond, so I’m confused.

Sorry I mean the xt90 melted and broke the connection almost instantly so I don’t think any wires melted

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Sorry not melted but the metal connection broke from the heat

Just a public service announcement for future readers: don’t use an XT90 for a charge port. But no matter what you use for a charge port, put a fuse on it. A 5A automotive fuse if you have to.

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Note taken

What every you plan for the future, First thing you should do, Change the damn charging port and loopkey. You was very lucky with it. Very very lucky. And believe me, it can always happen again that you plug the loopkey into the charging port. As min switch the xt90 one to male the other to female. Better make an xt60 for the charge port.

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Will do, but does anyone know what is happening with the bms?

This is very concerning. Is this lithium-ion? The cells will probably be damaged after dropping that low.

Are you certain all of the other P-packs are correct? Have you verified (CAREFULLY) that the battery voltage is present on the pins of the charge jack – i.e. there are no breaks between the BMS and charge port? (Warning: do not short it out again by accident)

Yes I used the multimeter and all looks good

Though for some reason the c- is 1v lower than the b-, is that normal?

That is an indication of a BMS safety shutdown

You’d be reading the battery voltage via the voltage drop of the MOSFET body diode and not through the actual activated FET junction itself. Meaning “it’s turned off” intentionally because there is a problem

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But I put a new bms in and still didn’t work

ill recheck voltage

Yes but this one senses a problem and is shut down. I would verifiy all the cell voltages again and make sure they are all connected to the BMS with no loose connection. Without accidentally shorting anything.

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Also check for an “off-by-one” problem where all the balance leads are in the wrong place

b- is 39.0 and c- is 38.6, charging wires are: 3.90 3.90 3.90 3.90 3.90 3.90 3.90 3.91 3.91 3.90

@b264 any ideas?

are you checking voltage from the balance wire?