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diy solar

Shunt inside BMS circuit?

andyP

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Aug 31, 2020
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Now I'm thinking I'll get a low (100amp) BMS on my 12v battery bank, and connect a 3000W inverter directly to the battery terminals, ignoring the BMS, (but for protection I'll use a relay that disconnects the inverter if the BMS output switches off).

But I'd also like to use a shunt to measure the input and output of the battery. So can I put the shunt inside the BMS circuit? If I put it inside then I can't measure the charge being used by the inverter, but then the BMS will be reading all the cells as having a lower voltage because of the voltage drop on the shunt, and might overcharge them. Is this latter problem significant?

The voltage drop on the shunt is rated at 50mV. Can't find a value for it's resistance. What tolerances do the battery and BMS need to stay within?
 
Shunts are almost always on the negative feed. I would put it after the pack, and have the BMS reading the cells directly. The BMS uses an internal shunt typically, so it will not provide any overcurrent protection.

The BMS doesn't control charging. Its there to protect the cells from over/undervoltage. Its a simple on/off in almost all cases.
 
V = I * R
.05V = 500A * R

R = .05/500A = 0.1mΩ

Thanks, I meant I couldn't find the rated resistance, to calculate back to verify that 50mV figure. But interestingly I hadn't thought that the 50mV drop only occurred at maximum amps. And also, that the 50mV is divided over the 4 cells so 0.012v on each cell. I think that's OK if each cell is undercharged, and overdischarged by that much.
 
The balance leads will still connect to the individual cells.
 
Oh, yes, of course! Thanks, that's cleared up my thinking further.
 
What works for me. battery negative -> shunt bat -> shunt load ->BMS -> negative buss bar. Also shunt load second cable to inverter negative. Inverter negative bypass the BMS. There is a separate LVD that shuts down inverter And other loads. BMS is emergency shut down, not control device.
 
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