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Tuco avatar image

How to monitor the starter battery with my BMV-712? Wiring seems very complicated for my installation.

0 CB_Midkiff asked · Dec 10 at 4:27 PM


I have a housebank, which is charged by a Smart Solar MMPT 100/50 and connected and supervised by a BMV-712 via shunt. In addition I have a starter battery which is not connected at all to this house system. However I would like to monitor the starter battery also via the auxiliary input of the BMV-712.

Fig. 3 of the BMV-712 wiring diagram

Issue is now that the starter battery is about 5m away from the housebank and a cable connection for high loads would be quite complicated. From the BMV-712 manual I did see that normally I would have to connect the minus of the starter battery also to the shunt via a very thick load cable and an additional thin plus cable to the B2 port of the shunt.

Is there an easier way to connect and monitor the the starter battery via BMV-712?

Would it be possible to use the Smart Battery Sense device and connect it via bluetooth with the BMV-712 as an auxiliary input for monitoring the starter battery?

BMV Battery MonitorSmart Battery Sensebmv 712starter battery
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3 Answers
Grim Reaper avatar image

In the diagram above, the negative of the Starter/Auxiliary battery doesn't use the shunt to get to ground/chassis earth so the shunt isn't monitoring the starter battery current drain, it's just monitoring voltage. (It's not the best way of illustrating its use, might be better to redraw that with the negative lead not connected to the shunt but make it obvious that they share a common earth point elsewhere)

There's no reason in this case to route the starter battery negative all the way to the BMV shunt, just leave it as it is in its present location and connect the sensor wire to the positive pole of the starter battery to allow voltage monitoring. So long as both the ground connection (top) of the shunt and the ground connection of the starter battery share a common connection point (in your case the chassis of the motorhome) you're good to go.

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I agree with grim reaper. I setup my configuration as he described, but my 712 reports nearly the same voltages for the starter and coach batteries under all circumstances. The starter battery voltage decreases and increases consistent with the chassis battery voltage. Are you experiencing the same thing?

mvader (Victron Energy) avatar image

Hi, using the Smart Battery Sense is a good idea. You cannot read it out on your BMV.

But, you can read it out on your smartphone (or other device) directly.

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houstonstroker avatar image

Does the negative of the starter battery have to be connected to the shunt? Since it will only read voltage, correct?

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Yes it does. A smaller wire should work in this case, as it's voltage only.

If both batteries share a common chassis ground that may also work, but is probably not recommended.

Thank you Mark !

Please don't forget to add a fuse (rated for the wire size) in this wire, if other ground connections have higher resistance all current might go through the small wire...