BMS current tripping

[E-HP]

Hello,
I'm trying to understand the conditions that cause a BMS to trip due to current, either due to a current spike vs sustained overcurrent.

Battery: Generic cell 52V 20Ah, 30A BMS; running some lipos in series, but the battery BMS is the main constraint
Controller: PowerVelocity 7kW
Display CA V3.1
Motor: MXUS 1000W DD rear

I noticed that throttle response with this controller feels snappier, allowing a lot more current to flow off the line. When I first hooked it up, it would wheelie and feel twitchy, especially when running higher voltages, so I use the CA throttle ramping to tame it. But, before I did, I noticed that rolling on the throttle too fast from a dead stop on a grade would immediately trip my battery BMS, causing my to cycle it.

Yesterday while doing some testing, I set the CA to bypass mode to look at this behavior more closely, since I will be upgrading my pack to something 40A or over, and I want to see how much more acceleration I can expect. Pegging the throttle from a dead stop (I never do this) tripped my BMS each time, but from a rolling start, pegging it results in a small wheelie and then strong acceleration, no trip.

My BMS has never tripped under load, and regularly see 40A+ peak for maybe a minute. My question is, what magnitude of current spike will cause a BMS to trip? I'd like to take advantage of the off the line acceleration as much as possible without tripping, once I upgrade to a more capable battery pack and BMS. I have all of my limits set via my CA (30A for battery current), so is it possible that there's a 7kW spike occurring for a millisecond? I've been using the CA throttle ramping to "solve" the issue, but is it also, in a way, serving to protect the battery from the current spike?

 

[john61ct]

Just as there are hundreds of different fuses with the same simplistic rating yet wildly varying profile curves (amps vs time)

the answers to your question will vary with the BMS in question

and since cheap-chinese makers swap supplied parts seemingly at random, probably vary by run at the factory too.

In both cases the only way to know for sure is hands-on testing with an accurate variable power supply.

But if you use a more reliable fuse for fire safety, then all that's left is preventing the BMS' components from frying, and that is most easily done simply by buying one rated for much higher current than you will ever use in practice.

Or just wire the BMS for voltage protection and use external contactors for the actual control switching, so current does not even pass through the BMS at all.

Might cost a bit more even with the cost savings of using a lower rated BMS

but IMO anything that replaces easily damaged cheap chinese circuits with robust gear from western engineered brands, is a good thing.

 

[E-HP]

Thanks. It's a UPP pack, but I'm not sure what the level of quality they use for their components. Fundamentally, I figured that for current protection, there'd be a slow trip circuit that shuts down when the current exceeds the continuous rating for too long. Then an instantaneous trip for short circuit protection. I'm not sure what other conditions the BMS is covering with respect to current, so maybe this throttle tripping is just being treated as a short.
Since it's looking like my next pack with also be from UPP, but with Sanyo cells and 40A BMS, I'm wondering if it will trip too. I have the CA to deal with it, and response is decent with the throttle ramp set to 10V/sec., but it would be fun to play with it in wheelie mode too.

 

[john61ct]

I would not accept a pack where the balance wires weren't accessible and the BMS easily removed / swapped out