09andot avatar image
09andot asked

BMV-712 overcounting drawn amps

I've just installed a BMV-712 to monitor a 105AH AGM auxiliary battery in my car and am having issues with it overcounting / overestimating the amount of amps drawn by my accessories. The result of this is that at the end of a four day trip my State of Charge was down to 22%, whereas my Redarc BCDC1225D charger was in float mode as the battery was completely full.

The reasons I believe the BMV-712 is underestimating amps are:

- The Redarc BCDC1225D charger believes the battery was full whilst the BMV-712 believed there should have been an 80AH deficit. I trust the Redarc charger as it is plug and play (ie. I couldn't have done something wrong here), and the battery voltage aligns with the idea that the battery is full (ie. settling at 13+ volts after some time of no load / charge running through)

- I put an inline digital watt meter on my 12v fridge (the largest draw) and checked its read against the BMV. The watt meter showed draw that was ~60% less than that showing on the BMV-712

- The current draw that the BMV-712 shows on the 12v fridge is materially larger than what the design specifications say the fridge should run at

To be clear, the BMV-712 doesn't seem to be overestimating charging amps. It is showing a very clear 25amps when my 25amp DC charger is running on boost. It just seems to be the load side that has the issue.

For your reference, I have wired the shunt as follows:

- Battery side: Negative from auxiliary battery

- Load / charger side: Negative to Redarc BCDC1225D, Negative to chassis (the aux battery needs to earth somewhere), Negative to fuse box, which powers the fridge. The negative to fuse box is attached to the small ~M6 phillips-head bolt on the front of the load/charger shunt side rather than the ~M10 bolt.

Any idea as to what I am doing wrong here? Hoping this is just a simple mistake I have made in the wiring.


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2 Answers
JohnC avatar image
JohnC answered ·

Hi @09andot

Don't use the philips head screws. Only use the big bolts on top.

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09andot avatar image 09andot commented ·

Thanks JohnC - will give that a go. Can I ask what the phillips bolt is for, then?

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ 09andot commented ·

The small screws are only attach the BMV PCB to the shunt, nothing else.

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09andot avatar image 09andot commented ·

Thanks JohnC and Matthias - confirming this was the issue.

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klim8skeptic avatar image
klim8skeptic answered ·

You might like to review your shunt / earth's wring.


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