Low battery voltage

General Discussion about large electric scooters and motorcycles and other things with no pedals.
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MrKaos   1 µW

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Low battery voltage

Post by MrKaos » May 09 2018 10:56am

Hello, I just recently started working with electric scooters. Now here in my shop I have one Govecs 1.5 which user has left battery stand by for too long voltage dropped to 31.4V (should be 72V). I also have one battery that is working fine fully charged. I know that slow charger that I received from dealer needs at least 39V to start charging battery so I would need to boost this battery a little bit.

My idea was to connect both batteries with not very thick wire so it would hopefully boost the one with low voltage just to charging point. What do you more experienced members think about that. I think it should be enough to boost a voltage a little bit... And hopefully with low amperage since thick wire will probably break on high amps.

I can't open battery pack that is main reason why I want to start with this idea.

And advice is welcome. Thanks

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amberwolf   100 GW

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Re: Low battery voltage

Post by amberwolf » May 09 2018 1:49pm

Thin wire will probably just catch fire or melt, if the BMS of either pack didnt shutdown the overload before that point, and if the overcurrent thru the already-overdischarged cells didnt cause them to burst into flame first.

Most likely the BMS in the battery (if it is a lithium type and not SLA) has shutdown to prevent cell damage and fire, so it wouldnt try to charge anyway. If you force it to recharge, without checking the cells first to see why its not charging, you could potentially have a pack with fire risk now or in the future. Thats why the BMS are made to prevent charging if it drops below a certain level in the first place. (otherwise itd just let you charge it up even with a totally dead pack, if it didnt matter).

If it is an SLA or BMSless lithium pack, youd want to use a DC-DC converter to limit the charge current to a very tiny amount, a few hundred mA at most, until its back up to the point it will charge normally. But again, if its a lithium type pack, it could be a fire risk from this point on. And if its an SLA pack, the cells are probably sulfated and wont fully take the load the scooter will place on them even if they will reach and hold full voltage again, and/or they wont last much longer after this and will all need replacement.

Voltron   1 MW

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Re: Low battery voltage

Post by Voltron » May 09 2018 2:57pm

Even if the jumper wire can handle the amps, maybe the cells in the battery could get damaged by the inrush. Maybe if you hook it up with a rheostat or something inline, but even if it's difficult, better to get into the pack enough to check the individual groups to charge them, as you should be checking the voltage balance between them anyway to see if the BMS is working. Could be a giant fire hazard to charge if groups have been sitting at zero voltage long enough.

MrKaos   1 µW

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Re: Low battery voltage

Post by MrKaos » May 09 2018 3:02pm

Thank you for your answer that is exactly what I was thinking why I shouldn't do it... It is li-ion based pack and I am 100% sure that it has some bit more complicated BMS. Won't touch it then.

It would be much cleaner to open battery pack replace foulthy cells and get it working again... But they don't want to do it since it would void warranty for rest of scooter.

Thank you all for answering you have been really helpful. I am really interested in this niche and will learn more about it in feature.

Voltron   1 MW

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Re: Low battery voltage

Post by Voltron » May 09 2018 3:28pm

Yw... It's part of the endless sphere payback cycle :)

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