Regenerative Braking on Battery in series

[kramzteog]

Hey everyone,



I’m running 2 × 48V 22Ah batteries in series, giving me about 107V fully charged. I went this route to power my Fardriver 96450 controller more efficiently — at higher voltage I can push the same power (14 kW+) with less current, which keeps heat losses down.



To bypass the 50A per-battery limit, I wired B- and B+ directly into the pack leads to “trick” the BMS, since otherwise it would choke the current draw.



Now my main concern:

When I use regenerative braking, at what voltage will the batteries actually be charged?

• Will each pack see ~54V, or

• Will the system push the full ~107V back into the series connection?



I don’t want to risk overcharging or damaging these packs since they weren’t cheap, and I haven’t found a clear explanation of how regen charging behaves in a series setup.



Any insights from people who’ve done this before would be hugely appreciated!



Thanks



Disclaimer: I know bypassing the BMS and running regen on series packs carries real risks (overvoltage, cell imbalance, thermal runaway, etc.). I’m sharing what I’ve tried so far, but I also understand this setup is not the safest. Please consider this before replicating anything similar.

 

[AdR]

Is it your batteries or your BMS that has the 50 amps limit?. The controller can be used to set the max battery charge voltage limit.
Your BMS is use for individual cell safety and battery current safety limits.
In my setup, I use the BMS to monitor the individual series cells and control a power contactor to disconnect the battery if there are any individual cell issues. I use the controller powered thru the contactor to limit my powering and regen currents

 

[AdR]

Your controller only see the total voltage, not two 48V batteries.
I am assuming you are using a single BMS for both batteries in series?

 

[kramzteog]

Your controller only see the total voltage, not two 48V batteries.
I am assuming you are using a single BMS for both batteries in series?

each battery has its own bms

 

[bengxe]

each battery has its own bms

Not if you bypassed them. Are your BMS boards designed to be run in series? Most can't handle double the voltage they're meant for. If they can handle the voltage then you just need to turn down the current limit in your controller programming to what the BMS is rated for. If they can't handle the voltage you could wire a single 96v bms to both batteries.

 

[kramzteog]

Not if you bypassed them. Are your BMS boards designed to be run in series? Most can't handle double the voltage they're meant for. If they can handle the voltage then you just need to turn down the current limit in your controller programming to what the BMS is rated for. If they can't handle the voltage you could wire a single 96v bms to both batteries.

Im not sure if y’all can’t properly read or you’re just blind.

I explicitly stated that i bypassed the BMS.

 

[bengxe]

And I thought you might be interested in fixing that. My mistake. Good luck.

 

[E-HP]

Hey everyone,



I’m running 2 × 48V 22Ah batteries in series, giving me about 107V fully charged. I went this route to power my Fardriver 96450 controller more efficiently — at higher voltage I can push the same power (14 kW+) with less current, which keeps heat losses down.



To bypass the 50A per-battery limit, I wired B- and B+ directly into the pack leads to “trick” the BMS, since otherwise it would choke the current draw.



Now my main concern:

When I use regenerative braking, at what voltage will the batteries actually be charged?

• Will each pack see ~54V, or

• Will the system push the full ~107V back into the series connection?



I don’t want to risk overcharging or damaging these packs since they weren’t cheap, and I haven’t found a clear explanation of how regen charging behaves in a series setup.



Any insights from people who’ve done this before would be hugely appreciated!



Thanks



Disclaimer: I know bypassing the BMS and running regen on series packs carries real risks (overvoltage, cell imbalance, thermal runaway, etc.). I’m sharing what I’ve tried so far, but I also understand this setup is not the safest. Please consider this before replicating anything similar.

I use a separate voltmeter for each pack, do the mental math, and act as a human BMS. I note the low voltage threshold of each pack that I’m comfortable with, and monitor that neither drops below the threshold. I use a combination of high quality lipos and lithium ion packs, and haven’t had any cell groups of any of the packs go out of balance in a couple of years and maybe 1000-1500 light charge cycles (I charge to 4.1v and never go below 3.7v). I charge at 8A-12A, which is good, since I don’t charge unattended. I fully charge once a year to check capacity, with no detectable degradation since the packs are rarely stressed.
If you’re doing what you’re doing, but still have the questions you posed, I’d pump the brakes and do more reading before charging, or at least never charge unattended, or charge outside at minimum.

PS. Not sure why you think bypassing the BMS is a good idea. There’s no benefit.

 

[ak136]

• Will each pack see ~54V, or

Yes