Is it possible to overcharge a batterypack due to defective BMS?

Batteries, Chargers, and Battery Management Systems.
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sleepless   1 mW

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Is it possible to overcharge a batterypack due to defective BMS?

Post by sleepless » Dec 11 2017 6:40pm

Hi all,

today I drove through a lot of rain. Suddenly the display shut of and the motor stopped working.
I thought the battery might have got some water so I opened the hailong case and it was indeed wet. Well, I dried it, cleaned it and re-connected it but still the display won't work. The battery's charging indicator is showing that it's empty. But it was about 60% charged when it suddenly shut of.

So now I'm scared if I can charge it with the charger? Can something happen if the BMS is defective (I don't know if it is, I'm just scared it could be) or do the cells have some overcharging protection themselves? The cells are Panasonic NCR18650PF. I'm just scared that they could explode or something. I'm living in a flat so I can't just charge it somewhere outside.

What's strange is that my phone charges over the hailong USB-Port.

Edit: okay, with charger connected the display turns on and shows 56V (max. should be 58.8V). So it should be charged but as soon as I remove the charger it instantly turns off. Something isn't right.

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jonescg   1 GW

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Re: Is it possible to overcharge a batterypack due to defective BMS?

Post by jonescg » Dec 11 2017 8:09pm

Get yourself a multi meter and keep an eye on it independently. Some BMS units can fail with moisture ingress, but see how it goes once completely dry.

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tomjasz   1.21 GW

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Re: Is it possible to overcharge a batterypack due to defective BMS?

Post by tomjasz » Dec 11 2017 8:42pm

Good advice, everyone of us SHOULD have a multimeter for more accurate readings.
Thanks Justin_le we're here thanks to you. All the best to the mods for their tireless work keeping it on an even keel.

jonyjoe303   1 W

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Re: Is it possible to overcharge a batterypack due to defective BMS?

Post by jonyjoe303 » Dec 11 2017 10:26pm

yes very possible to overcharge especially if the pack is out of balance. If one row of cells reads higher than the rest and your monitoring for overall voltage, the overall voltage can be below the max, though one row of cells are already past there max voltage.
A multimeter will only tell you overall voltage unless you check each row of batteries.

It looks like the bms on you battery pack is no longer good. I would not try to charge it with it, it might not be monitoring the batteries balance status anymore.

If you have balance connectors use 1 or 2 of these to measure each row of cells.
3in1.jpg
3in1.jpg (17.73 KiB) Viewed 441 times

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amberwolf   100 GW

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Re: Is it possible to overcharge a batterypack due to defective BMS?

Post by amberwolf » Dec 12 2017 12:11am

sleepless wrote:
Dec 11 2017 6:40pm
What's strange is that my phone charges over the hailong USB-Port.
The last battery I opened up, the USB board is separate and wired directly to the battery + / -, so it doesn't go thru the BMS. (meaning, it's possible for that particular battery to completely discharge due to leakage currents / drain from the USB board, and the BMS can't do a thing about it).

Yours is probably wired the same way.

sleepless   1 mW

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Re: Is it possible to overcharge a batterypack due to defective BMS?

Post by sleepless » Dec 12 2017 3:38pm

Thank you all for your replies. I got me a multimeter today but I don't feel much smarter now :mrgreen:

The output voltage is 42V which is empty and it explains why the display won't turn on. But measuring the cells (or the rows?) shows me 3.9-4.0V which should be almost full. So how can it be that the output is just 42V? It's 14S so 4V should lead to 56V output. I don't get it. Even if the BMS would be defective I shouldn't get a 42V output voltage.

Maybe it's like jonyjoe303 said and some cells are empty and some full? But how can I fix it and how can I charge just the empty cells if that's the case? I have no spot soldering machine so that I could unsolder them, charge them one by one and solder them together again :(

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tomjasz   1.21 GW

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Re: Is it possible to overcharge a batterypack due to defective BMS?

Post by tomjasz » Dec 12 2017 3:59pm

jonyjoe303 wrote:
Dec 11 2017 10:26pm
yes very possible to overcharge especially if the pack is out of balance. If one row of cells reads higher than the rest and your monitoring for overall voltage, the overall voltage can be below the max, though one row of cells are already past there max voltage.
A multimeter will only tell you overall voltage unless you check each row of batteries.

It looks like the bms on you battery pack is no longer good. I would not try to charge it with it, it might not be monitoring the batteries balance status anymore.

If you have balance connectors use 1 or 2 of these to measure each row of cells.
3in1.jpg
The balance connector will be a string of 14. The device you suggest would mean rewiring.
Most BMS these days will not allow overcharge, but all bets are off with a bad BMS.

I'd look to a new BMS, especially if the problems started just after the drenching. Upgrade to a BMS capable balancing. Pay attention to new BMS size to ensure a good physical fit.

Learn to solder if you are going to repair. A skill every builder will eventually need.
Thanks Justin_le we're here thanks to you. All the best to the mods for their tireless work keeping it on an even keel.

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