Bridging (-) BMS with (-) Battery terminals?

[2Big2B]

I have a application that uses 24vdc /100ah as motive power: My power wheelchairs. They are designed for 2x12v series lead acid batteries.

I have successfully been running with DYI LifeP04 battery packs each using an 100ah, 24v 8s Overkill BMS. The issue is that I lose power suddenly and intermittently. It seems related to power draw. I have no issues on straight battery though. I believe the BMS is seeing it as a short circuit and shuts down.

Assuming that, one solution, which Overkill Customer Support seems to agree with me on, is to install an A/B switch with one position for charging through the BMS and the other position for running direct from battery. It seems safe enough as long as I flip the switch as appropriate.

But what if I just bridge the negative output from both together instead of isolating them? Would that work without frying something the BMS, and would the LifeP04 battery cells remain balanced by the BMS?

 

[sunshine_eggo]

Motors have surge about 5X run current, so I wouldn't be surprised if you're exceeding the surge limit of the BMS.

If you join the two as shown here:

You have bypassed all BMS protections - charge and discharge. The BMS will never protect the battery for any reason.

 

[hwy17]

I have done this as described. It works. You will have no over discharge protection while the bypass is engaged and coulomb counting (soc tracking) will be wrong because it's only seeing half or less of the current, and you should especially never charge the batteries through the bypass.

Edit: My bypass was not an A/B switch it was just a disconnect on the bypass wire, so when the disconnect was changed to on, the bypass negative wire was paralleled with the BMS negative.

 

[hwy17]

Now I use an Orion BMS. If you are interested in a very intense DIY project, an Orion with contactors could give you adequate amperage with protections still working.

 

[hwy17]

Oh and if you don't have a fuse or breaker that is already sketchy, but even more sketchy with the bypass. That would be zero overcurrent protection.

 

[robbob2112]

Maybe I am crazy but seems like a larger BMS might take care of the problem. Or fiddle with contactors to bypass the BMS when starting forward for a few seconds then take it out. A little RC circuit and a relay to control the contactor. Or just the RC circuit depending on the current/voltage to engage the contactor.

 

[sunshine_eggo]

Assuming this is an over-current issue, wire gauge and interconnects could also be a factor. More voltage drop = higher current for a given power.

Ensure all interconnects/busbars are properly torqued.

I would also confirm that there are no issues with individual cell voltage drop, i.e., if min cell voltage is exceeding limits, that would trigger discharge protection. If a cell LVD is greater than 2.5V to be conservative, I would lower them to 2.5V.

Hopefully, watching what's happening on the app will give some clue(s).

 

[wpns]

Can you program the BMS for a higher surge current or something? Bypassing the BMS protections sounds like a bad idea...

 

[sunshine_eggo]

Can you program the BMS for a higher surge current or something? Bypassing the BMS protections sounds like a bad idea...

Yes, but the problem is the BMS ratings. The FETs handling the current may burn out.