Jumpstart 48v BSL battery in protection mode

I severely discharged my 6.4 48v BSL battery and it now reflects 8.4v and won’t charge from my Kodak 7.2 OG. . I read online that it is probably in protected mode.
Some have suggested it is possible to jumpstart it by disconnecting all loads and connecting it in parallel to another 48v battery.

I’d value advice before I blow something up.

Alternatively a recommendation for who I can take it to for an opinion? I bought it from the powerfoum shop a few years ago and live in JHB

Yes, the BMS probably disconnected the cells in order to protect them. The 8.4V you measure is most likely some residual sitting in a capacitor somewhere, and not the real voltage of the battery (which should probably still be around 42V).

For most batteries, all you have to do is put a voltage on the DC terminals (enough so that the BMS can power up) and then turn the battery on (or reset it, sometimes there is a procedure for that, holding a button for 5 seconds or whatever), and it should start charging again.

I’m sure the BSL distributor in SA will also be able to tell you.

Thanks @plonkster
I’ll report back

Well that worked without a hitch and the battery is back up and running again. I disconnected both batteries from everything and used jumper leads to connect them in parallel. As soon as I started the full battery the ‘dead’ one sprang back to life.

Now to see if it charges correctly.

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Charge it to 100% on its own and then the parent. Then combine them…

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Are you able to check the cell voltages? It’s a good idea because it might be one cell is out of tolerance. (you then need to get them all balanced and start again…)

Afraid not. It’s attached to a Kodak OG 7.2 and I haven’t been able to find a way to set up comms. I have a SolarAssistant but that also doesn’t show any detailed info about the cells.

I don’t know if all BSLs work the same, but at least for the ones I’ve worked with/tested, they have a fairly low charge voltage (of either 54.5V or 55V), and they don’t freak out if you push to 58V (just keep below 3.8V on the highest cell). That means they are fairly forgiving about an imbalance.

Also, to be clear, the deep discharge doesn’t cause the imbalance. The imbalance is always there, because the cells are never exactly the same. There is always one that has a lower capacity than the rest, and they also age slightly differently. Under normal use, where the battery never bottoms out, you wouldn’t even know about it as all the cells are “top balanced”.

But if you do take it right to the bottom, sometimes that slight imbalance makes itself known again, for a few days, until it goes away again.

It also depends somewhat whether the BMS only top-balances. Batteries that only top-balance rarely suffer from a deep discharge. The battery is empty when the lowest cell is empty, which means all the other cells are only ever discharged as deeply as the worst cell will allow. That means that on recharge, they should all more or less reach the full point again at the same time. I have no idea what BSL does, but I doubt it is all fancy to balance at lower voltages… it probably just top-balances like most batteries out there.

Long story short: You probably don’t have to worry that a cell balance problem will arise.

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Many thanks @plonkster for your help and insight. Makes me feel a lot better.