What Electrical Components on BMS limit Continuous Discharge Current?

[YoshiMoshi]

So if I was looking at the BMS PCB from a power tool battery, and I'm not told what the maximum continuous discharge current of the BMS, is there anyway for me to examine the board to determine this information. If not, generally speaking which components on the board limit this value?

 

[amberwolf]

The only reliable way to find out is to test it until it fails, then don't go that high on an identical replacement BMS.

You can look at the FETs, find their spec sheets for the max drain current, and guesstimate that the max BMS current would be close to that limit x the number of parallel FETs, but that doesn't take into account derating for heat build up in the system, possible counterfeit or low-grade parts that should be rated lower than their specs suggest, PCB design, etc. that will affect continuous current ability. (short peaks of max FET capability might be ok even if heat dissipation is poor, if there is time between them to get rid of it).


But a good bet is that the battery is only barely capable of the max current the biggest tool it is for requires. (they're not going to spend money making it better than that, if they can help it).

 

[fechter]

Generally speaking, there is a shunt resistor that passes the current and a control chip that looks at the voltage across the shunt and will trigger shutoff when a certain value is reached. The MOSFETs are the switch that turns off the load. The control circuit is supposed to trip before the current gets high enough to damage anything. The control chips tend to be custom, but you may find a name brand one you can get a datasheet for.

Post a picture.

 

[batteryGOLD]

Take an example of a 30A rated BMS, it will handle twice Amps means 60A continous with no problems for many seconds, and will detect overcurrent at 120A (4 times rated current)
But it will detect short circuit at 200A (it will cut power at this point, similar to active fuse)
This is example for a good trustable BMS (according datasheet specs)

The best way is too measure max peak current using amperemeter max function to know the real max amps peak draw by powertool.
if peak power amps is less than 120A is safe to use a 30A rated BMS (120A is a lot, I doubt any power tool could peak this value..)
use x4 times rule, a 30A BMS will handle max 120A for few time (peak Amps is at start of motor, than will go lower)

Any one disagree this sentence?
Happy new year

 

[amberwolf]

Take an example of a 30A rated BMS, it will handle twice Amps means 60A continous with no problems for many seconds,

Continuous is continuous (no time limit), not "for many seconds".

If something can't handle something for an unlimited time, it's not a continuous rating.