Max length for wires on BMV-712 Smart battery monitor

DIY
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1

I have the Multiplus II 12v/2kw inverter/charger unit and the BMV-712 Smart battery monitor. I wired them on a board with the shunt next to the inverter unit and put it in the cubby under my bed in the trailer.

The wall unit doesn’t show the proper voltage/current/etc and I’ve finally figured out that it is because my batteries (2 12v in parallel) have their negative lines connected to the tongue BEFORE going under the trailer and up into the cubby to connect to one end of the shunt and then the other end of the shunt goes to the inverter negative terminal.

Because the shunt doesn’t have the complete and only negative line going from the batteries directly to the shunt first I think that’s the problem.

So I want to move the shunt to the battery box so it completely encompasses the full negative line from the batteries before connecting to the frame to ground the trailer, if that makes sense.

So finally my question is, how long can I make those wires that connect to the shunt and go into the inverter? The two tiny wires with pokey ends and the ethernet-looking wire that goes into the inverter. Because the shunt will then be about ten feet away from the inverter due to the routing of wires.

current wiring diagram

trailer-inverter-wiring-diagram
trailer-inverter-wiring-diagram1261×824 149 KB

3

The BMV power supply wire should not go to the inverter at all. It goes to the battery.

The RJ12 UTP cable also does not go to the inverter, it goes to the BMV display head.

See the “basic electrical connections” section of the user and installation manual here: 3. Installation

4

Yes, your chassis ground is wrong, you have figured this out, but just in case

Why is my shunt misreading
Why is my shunt misreading1280×720 59.2 KB

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5

You are absolutely correct and I wrote it wrong in my post. That is how I have it wired in now, so my question is moot. I posted it due to the values on the BMV panel looking “weird” to me and constantly negative even when nothing is running in the trailer, but even if there is some kind of tiny drain (such as the silly USB panels that have a green LED in them) then the values are still wacko on the panel. So that’s why I think the ground as it is now is wrong and I need to just move it to after the shunt. Hoping that will correct the odd readings I get from the BMV. Otherwise everything else works great and the inverter works flawlessly, can lose shore power and not even know it…

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