What is the best practice to choose a BMS ?

[Louis68]

Hello everyone ,

I need to create a battery pack 10S2P , with 18650 cells:
And a motor nominal power is 250 W.

So , my question is how to set up the following parameter:
- Rated discharge current
- Charging voltage
- Rated charging current
- Equalization turn-on voltage
- Discharge over-current protection current
- Discharge over-current protection delay
- Charge over-current protection current
-Charge over-current protection delay
- Charging high temperature protection temperature


Thank you

 

[glennb]

The motor shouldn’t factor into it, because your controller should be protecting it from excessive current, as well as providing rudimentary low voltage protection for the battery, meaning the BMS’ sole function is protecting the cells.

However, some controllers are a little too rudimentary, so you might conceivably want to provide a degree of motor protection via the BMS by way of current limiting.

But, it’s not best practice, and involves some tricky balancing acts, so best forget I ever mentioned that, and presume you’re just seeking to protect the battery.

In this case, the parameters you’re interested in configuring in the BMS depend on the particular cells you intend to use. They serve to create safe operating bounds, which are specified in the manufacturer data sheet.

First thing to do, is post said data sheet here, or a link to it. Then people here can interpret that data for you to help you choose suitable parameters.

It’s important that you understand the reason for each parameter, rather than have someone prescribe a list of settings for you to input.

 

[docw009]

Very few parameters can be chosen ala carte when buying a BMS. All you can do is specify 10S (36V), that it's for Li-ion and not LiFepO4, and finally the rated current. You didn't mention balance.

If you want a balance BMS, also look in the specs for the balance current, around 40-60 milliamps. If it's not in there, they're lying when they say it balances.

For a 36V 250W bike, and a 10S2P battery, I would probably use a 10S 20A BMS. The 20A current rating is just the max current rating. Nothing stops the inexpensive BMS units from flowing more, but they get too hot if they have to sustain those high currents. What's the max current rating for your cells? I

Finally, a 10S-2P battery is pretty small. With the best cells, you can make a 36V7AH pack, which is a theoretical 250 watthours, but is probably more like 200 watt hour in real life. Your bike will proabbly average about 8A, so it's not too heavy of a load at 4A/cell, but if you run it much harder, it will be harder on the cells. Battery life would be easier running a 10S-4P so the load is only 2A per cell.

 

[glennb]

I presumed a smart programmable BMS, based on the scope of the parameters listed.

I didn’t pick up that it was a bike application. You’re right, the question’s posted in a “bike” forum, so very likely a low power application where range is important.

Still, I wouldn’t presume to know the specifics of the application.

I have a Currie “250w” bike motor with stall current of 90A. Steel gears don’t shred like plastic ones.

I pull 20A from Samsung 20Q cells. And 15A from Samsung 40T. I’d happily draw 30A from the 40T if I had the need for it.

 

[docw009]

Well, it may not be an ebike application. But I didn't expect someone was putting in a $100 BMS on a 20 cell battery.

 

[glennb]

I initially assumed scooter because of the tiny battery, but I think you’re right about it being an ebike, given the sub-forum it’s posted in.

There’s cheap ones to be had, 40A JB BMS for <$50. They’re awesome. I’d only ever use a dumb BMS if space was at such a premium that it’s all that’d fit.