How can I check for how much current my BMS is rated for?


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c_phil

New member
Joined
Feb 16, 2022
Messages
3
Hey guys,
this is my first post so bare with me if I don't follow the etiquette here :)
So I wanted to try and dive into DIY home energy storage and found a guy who gave me 900ish cells for free to tinker with.
Included was a BMS without labels whatsoever...
After digging around I found on the PCB HP14SK and googling that reveals its a 14S BMS but the ratings go from 20A to 100A depending where I look.
The most "trusting" link I could find was an AliExpress listing...
Is there a way to actually check for what current this thing is?
Thanks in advance
 

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OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,066
Well ok then I guess I have to test it... I mean more then the BMS shutting off can't happen right?
If a BMS doesn't shut off - then you don't want use it. Use appropriate wire for 100a+ etc and maybe set the BMS on something that won't burn for the test :)

FYI - I'm not pushing you to test them. Personally, I'd buy something else. But if you stick with these and cannot find info - then I don't see any alternative.
 

Oleksii

Member
Joined
Mar 18, 2020
Messages
60
Your 6 shunt resistors (4mR each) make 4/6=0.666 mR shunt. They can dissipate 6*2=12 watts of heat.
To get the heat level, you should pass = SQRT(12/0.000666) = ~134A of current. So, at least the shunts are thermally limited by 134A current at max. Max here means - they will be hot and should not be hot even more.

The same can be calculated for mosfets, if we would have a spec.
6 mosfets for discharge (only) - I'd evaluate this BMS as safe for the stated 30A continuous load.
2 mosfets are used for charge only, so charge current on a separate connection is 3 times less rated than discharge.

One of ICs (6 legs, set in a row near balance connector), to manage 1st cell in series, is also used to control overcurrent as well, usually, those are HY2113-CB1A or similar.
They have 150mV threshold for discharge overcurrent protection. to get such voltage drop on your shunt, the current should be 0.15/0.000666 = 255A. It's more like a short circuit detection threshold.
 

c_phil

New member
Joined
Feb 16, 2022
Messages
3
Your 6 shunt resistors (4mR each) make 4/6=0.666 mR shunt. They can dissipate 6*2=12 watts of heat.
To get the heat level, you should pass = SQRT(12/0.000666) = ~134A of current. So, at least the shunts are thermally limited by 134A current at max. Max here means - they will be hot and should not be hot even more.

The same can be calculated for mosfets, if we would have a spec.
6 mosfets for discharge (only) - I'd evaluate this BMS as safe for the stated 30A continuous load.
2 mosfets are used for charge only, so charge current on a separate connection is 3 times less rated than discharge.

One of ICs (6 legs, set in a row near balance connector), to manage 1st cell in series, is also used to control overcurrent as well, usually, those are HY2113-CB1A or similar.
They have 150mV threshold for discharge overcurrent protection. to get such voltage drop on your shunt, the current should be 0.15/0.000666 = 255A. It's more like a short circuit detection threshold.
Well ok that's really detailed
Thanks for the answer

Since 20-30A was what I was hoping for, I guess I will just try using it and additionaly make sure if it burns, it can't make destroy anything else :)
 
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