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brian-de-beer asked

BMV 712 - Incorrect SOC House/ Starter Battery and Isolator Switch

On my yacht, I have a BMV 712 connected to monitor my "house" battery and to display the "starter" battery as the aux display. (This is all connected as per the Victron installation specs). My two batteries can be selected via a manual 1, 2, both or off switch. I have the settings on the BMV set to the house battery specs as this is the battery I want to monitor. Under normal use, I switch to only draw the house loads, (lights, navigation, etc). This displays the SOC correctly if I only charge via the solar panel. The problem I have is when I opt to charge both batteries through the alternator from the engine. The current flowing into the batteries is then obviously divided to charge both batteries, but the BMV counts the current flow and then allocates it show that the house battery is being charged. This results in the BMV displaying a 100% charged "house" battery, but the voltage is only 12.5V which is nowhere near fully charged.

How can I wire/ setup my monitor to correctly display the SOC for my house battery, but still allow me to be able to isolate the two batteries and connect them in the case of charging via the alternator?

BMV Battery Monitor
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2 Answers
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JohnC answered ·

Hi @Brian De Beer

Just make sure the BMV shunt is the only thing wired to the neg terminal of the batt you're monitoring for SOC. Just the batt wired to the BATT terminal. The other one may be labelled LOAD terminal, but that one really means 'everything else'. So do all your charging/switching/whatever from the LOAD side of the shunt.

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brian-de-beer answered ·

Hi @JohnC Thanks for your reply. That makes sense yes, and I have connected it up as you mention.

My problem, though is as follows: House Battery = 105AH. Starter Battery 75AH. Both connected in parallel with the 1, 2, both, off switch to isolate batteries manually.

My BMV is setup with the specs of the House battery alone.

Soon as I start the engine and charge both batteries, the BMV counts current and assumes all of it goes to the house battery, it then shows close to full charge with the voltage still at 12.5V (or about 50%).

You might ask why don't I then program the BMV with the two battery capacities added together as one bank (105+75). I can't do this though, because I only use the house battery to run loads from and by adding the two battery capacities and keeping them in parallel switched to both, although will work for the BMV, will cause me to drain my starter battery.

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

Connect the minus of the starter battery to the "load side" of the shunt, so the BMV only measure the current in/out of the house battery.

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JohnC avatar image JohnC ♦ Matthias Lange - DE ♦ commented ·

Yes, essentially the shunt will go between the negs of each battery.

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