Can you draw 40 amp if your bms says 50 amp?

[Glens carzy ebike]

Can you draw 40 amp if your battery has 50 amp hour bms?

 

[john61ct]

Of course, the BMS is a max rating, like a fuse or power source.

And by using the BMS to control external contactors even that can be ignored.

 

[MadRhino]

Not sure what you mean...
Let me guess

If your display does show a max of 40A even with your 50A BMS, it is because your controller is set to 40A.

If you want to limit your current draw to 40A, set/buy a controller to draw 40A

 

[john61ct]

Note also Ah units have no place in this context.

50 amp hour bms?

But if you mean a BMS that is rated for 50A discharge, yes pulling 40A is no problem, even continuously if the BMS rating was for that.

If the cheap chinese vendor slapped a 50A rating on it for marketing purposes, but the fine print says 20A continuous, 50A for just 1 second, then no, you need to buy a stronger BMS.

Or bypass it as mentioned.

 

[flat tire]

Only with a carzy ebike. So you could, yes.

 

[John in CR]

Running that close to the max rating don't be surprised if the over-current protection in the BMS gets tripped, especially once warmed up. For batteries, bms's, fuses, controllers, etc, I like to run at or below half of the max rating for the sake of durability and life. While yes it should work in your example, I wouldn't. The same goes for voltage limits, though a 20% cushion for max voltage rating is typically fine. Those who run at or close to limits are the guys who don't have 100% reliable ebikes, and I'd rather build a new and better ebike than spend time frequently fixing a broken one.