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ventus asked

BMV-712 Shunt Location and Wiring for Two Separate Batteries on Boat

On my boat I have a single 4D as my house battery and a Group 24 as my starting battery. Both are in plastic battery boxes on either side of my engine so the distance between them is about 6-10 feet. These batteries are wired separately. I was going to connect the shunt to the house battery as battery 1. I understand the wiring for that, simply removing the negative cable currently on the battery and placing that on the load side of the shunt and running a short jumper from the battery side of the shunt to the battery, then the small red wire to the positive of that battery.

For connecting the Starting battery, do I simply run the other small red wire to the positive of the starting battery? If needed, can I make a butt connection and extend that wire to reach the other battery? I see in the diagram the negative cable from that battery looks like it should go to the load side of the shunt, but since these batteries are wired separate do I even need to worry about that?

I also have the temp sensor but I think if I am monitoring two battery banks I cannot use that, correct?

Also, where should the shunt be installed? Inside the top cover of the battery box or elsewhere. It is on a boat so not sure about keeping it protected from the bilge area.

BMV Battery Monitorinstallation
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3 Answers
Mark avatar image
Mark answered ·

For your arrangement the BMV would be setup to only measure current in/out & state of charge (SOC) of the house battery.

The 2nd BMV input can be setup to monitor only voltage of the starter battery. This just requires a fused positive cable to be run from the starter battery - providing that both batteries share a common negative.

There is only 1 additional BMV input so you must choose between monitoring the 2nd battery voltage or battery temperature.

You could buy a 2nd BMV for the starter battery if you were really keen...

The current shunt should be located as close as possible to the house battery negative terminal and the 'battery' side connected directly to the negative battery terminal.

There must NOT be any other connections on the 'battery' side of the shunt or the battery negative terminal.

All other connections MUST be made on the other side of the shunt.

Refer to extract below from quick install guide;

I am not a boater, but to protect from water I would install it in a water tight electrical project box and use watertight cable glands for all cables/wiring coming into the box.

A clear box or lid might be best to easily see if moisture ever gets in & forms as condensate.

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Mark avatar image Mark ♦♦ commented ·

You could if you want but the disadvantages are;

- Battery voltage may be a little inaccurate due to resistance/voltage drop in the cable/switches/connections/fuses up to the connection point of the BMV lead

- SOC may be off if there was ever a current in or out of the battery while the BMV is off. But then again if the battery is totally isolated this should not be possible.

Just note that if you have everything off for an extended period you would probably have to wait for a 'synchronization' event to occur before you can trust the SOC reading again

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ventus avatar image ventus commented ·

Got it. Thanks, could I connect the positive connections on the other side of the battery switches instead of the positive terminal directly on the batteries so when the switches are turned off the monitor is turned off as well so it’s not on all the time?

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Michael Riley avatar image
Michael Riley answered ·

We are on a yacht in QLD AUST. It is my experience that in any situation in this geography we find that thermal changes almost 'guarantee' moisture (then condensation) will get inside any box/enclosure that is not hermetically sealed (also becomes a maintenance issue). As a consequence we actively manage this by putting a small drain hole on the bottom side of any closed box (can have insect protection). In this way any lack of maintenance or oversight of moisture ingress does not result in frying your electrics when the condensation eventually builds up.

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luckybeanz avatar image
luckybeanz answered ·

I'm looking to use a smart shunt with a GX device (cerbo / multi gx) on a boat too. But will have lithiums and two different voltages. a 12v and a 48v. How does the cerbo / gx distinguish between the two batteries?

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