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Mark Webster avatar image

BMV 712 - Monitoring 2 batteries

My situation - Camper van with a Starter battery grounded to vehicle frame and Camper Auxiliary Battery grounded to vehicle frame (different location). I have the BMV 712 shunt setup so that all Camper Auxiliary battery peripherals are going through the shunt. On the Shunt, +B1 goes to the Camper Auxiliary Battery positive and +B2 goes to the Starer battery positive terminal. I DO NOT have the Starter battery negative attached to the Shunt. I only require reading the voltage of the Starter battery. Here is my problem - When i detach the Camper Auxiliary battery ground from the frame of the vehicle, my BMV high voltage alert goes off as i watch the voltage of the Starter Battery continuously rise (as high as 21v before i returned the ground wire). However, a volt meter attached to the Starter battery shows ZERO increase in voltage. So, the voltage spike seems to only be evident in the BMV monitor itself.

Questions:

1. Why am i seeing voltage spike on the BMV but not an actual voltage spike on the battery?

2. Will i damage anything? (i won't be removing the Auxiliary battery ground very often if at all - i was testing for parasitic drain)

BMV Battery Monitor
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1 Answer
WKirby avatar image

The BMV measures the Voltage of the battery on +B1 with respect to ground. It also powers its self from the battery connected to +B1 and ground, it uses very litttle energy to power its self.
If you remove the ground, the current will return to ground via an alternative route if there is one available. It finds an alternative route to ground via the starter battery whos ground is connected to - and + is connected to +B2. This means that the +B2 is 12V (or the starter battery Voltage) above ground and so this Voltage adds to the Voltage measured at +B1.
The Voltages of the two batteries don't fully sum up due to the Voltage drop of the BMV powering it's self via +B2 which is a high impedance route.

It's never a good idea to lift the ground in any system.
In your case the the sum of the two 12V batteries is below the maximum operating Voltage of the BMV. If you had 48V batteries then you'd probably have a problem.
You could also have a problem if you lift the ground while a charger is running, you could add it's Voltage to that of one of the floating batteries and then cause some damage to equipment.

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