BMV-712 firmware issiue?

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heidi asked Jul 17, '20

BMV-712 firmware issiue?

Hi,

the BMV 712, BMS 12/200 and Li-Battery is installed in a trailer.
Input (AB) fuse is 15A.

I'm trying to measure the starter battery voltage. The ground is connected through the BMS (AB) and the shunt (secondary input) is connected to the starter battery .

Is this a firmware bug or isn't it possible to measure the voltage across the BMS, because of the diode reverse current protection? Primary battery measuemend is fine.

The discrepancy is exactly 12V (24,4-12,4 = 12V), that looks a bit like a firmware issiue.

Thanks!

BMV Battery Monitor BMS voltage

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Answer 1 · 2020-07-17T19:22:46Z

dazey77 answered · Jul 17, 2020 at 07:22 PM

Questions like this are an interesting puzzle. I don't have a BMS so I really dont follow this part of your post

The only way I can think it is possible to get a measurement of double is if you have crossed wiring between the trailer and the vehicle. i.e. if you have +ve in vehicle connected to -ve in the trailer (and vice versa) then the PD between vehicle +ve and trailer -ve is double. Actually achieving this wiring scheme without blowing any fuses is puzzling in itself.

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Answer 2 · 2020-07-17T19:58:58Z

heidi answered · Jul 17, 2020 at 07:58 PM

As you can see (hopefully) it is not V(+,starter battery) + V(+,Li) in series, and I‘ve measured the correct voltage and polarity between AB(-,starter battery) and V(+,starter battery).

The V(ignition+, starter battery) from the vehicle is connected to V(+,Li), but the vehicle was turned off during this test and it utilized a relais.

I‘m using the permanent plus from the vehicle only for this measurement, and the ignition+ for charging to eliminate at least half of the voltage drop between them.

Odd number (24,4V), maybe it is building a bootstrap network (diodes/capacitors) inside of the BMS. Wiring is more or less straightforward.

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