How many amps for BMS?

I have a QS 273 5000W hub kit coming tomorrow. The controller is a sabotvon 72150 reportedly 50A continuous and 150A peak. I'm having a battery built at 72V 40ah with the LG MJ1 cells. (Unless they are crap - please feel free to comment). The builder 'Unit Pack Power' is open to changes but we're going to build pack with 60A BMS. Is that way to low? Are the LG cells a bad idea? Before I pay and order any help is greatly appreciated
Steve

The BMS needs to be rated to protect the cells. That's it's job.

So whatever the cells, when built into a pack, can handle safely, without heating up, that's what the BMS can be rated for.


However, the pack first has to be built so that it can handle the demands the controller will place on it. So if the controller can pull 150A, the pack has to be able to handle that, too, and thus so does the BMS.


If the pack can't handle what hte controller pulls, you'll age the pack faster, or even actually damage it, if the BMS is rated higher than the pack can handle just so the controller gets what it wants. At best, there will be a lot of voltage sag, and you'll get less power (watts) under higher loads, and less performance from the whole system.

If you can I'd go for a bigger capacity bms. 60a cont is only 10amps headroom and its likely to get hot and maybe die. Id say 80-100cont would be better. Max amp draw should be handled by the controller and the bms is there to cut out in case of overload like a short (you can't use your bms to limit Max amps).
I'm guessing you've got an 11s pack (40x3.5) which sounds fine for your continuous amp draw but I'd be concerned about the peak draw which will be ~13amps/cell. This is going to be pretty hard on them as it is over the mfr specs. Even 10 amps on these is pushing it. Consider switching to samsung 30q for a higher power cell. Mj1s are good, but you're going to get reduced life out of them if you hit 150amps often.

Also at these draws, pack design is pretty important. You standard 'bit o' nickel' Chinese battery is going to have further issues. Maybe get some assurances on build quality.. Series nickel thickness, current path/sharing, heat shedding, vibration protection are all questions I'd be investigating before paying good money.

A cheap BMS needs more "overhead factor" than a pricey one.

Personally I'd size the BMS at least 50% higher current than the maximum I anticipate will ever get pulled by the controller.

Now, the other side of the coin, a very large battery relative to what you need the controller to pull - to lower the C-rate and extend range - is a wonderful design goal if you can do it.

So that "maximum current I anticipate will ever get pulled by the controller" number may well be much less than what the setup could do in theory.

But, the more usual and safer scenario, is, size the BMS for the maximum current the setup **can** produce times 150%.

They have come back and suggest now a 100A BMS for a 5kw motor and a 50/150 controller.

so using 50% more than the highest "rated pull" of 150A would be a BMS of 225A or more, am I reading that right?

If you were intending on using that many amps often then yes, but otherwise it's probably overkill. A 225amp bms is going to be large and expensive. Bms's have a Max and continuous rating so get one that gives you ~double your continuous draw, with a peak over 150. 100 amps sounds good- peak is likely in the 200a ballpark (but as with all cheap bmss, it's likely to pop at that!)

And whatever you end up with for a BMS rating, make sure the pack itself is capable of easily handling the current draw without heating up, so you don't end up with cell problems because of it.

COntinuous current is usually what matters there, but if you draw peak currents frequently it'll still heat the pack, aging them faster.