justmurph avatar image
justmurph asked

Starter battery voltage monitoring with isolated systems

Hi there,

I currently use the aux input on my BMV-712 and the Cerbo GX/VRM to monitor the start battery voltage in my boat. Being an aluminium boat I am making a few changes to the system to ensure that electrical systems are isolated from the hulls and that the start battery system is fully isolated from the house battery system. This means that the aux input on the shunt won't be able to read the start battery voltage any more, as they won't have a common ground.

What other ways can I get the start battery voltage into the cerbo GX and VRM for monitoring purposes?

A little more detail on the system if it helps:

- 2 x 25.6V 200AH Smart Lithium

- BMV-712

- Multiplus 5k

- 4 x MPPT 100/20

- Cerbo GX

- Orion 24/12-70 for running 12V house loads

- Swapping the Orion 12/24-20 for a Orion TR Smart Isolated 12/24-15 for charging the house from the start batteries

If only the Orion TR Smart could do communications over BT or VEDirect this would be easy.

cerbo gxVRMbattery system voltagestarter battery
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justmurph avatar image justmurph commented ·


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2 Answers
Vance Mitchell avatar image
Vance Mitchell answered ·

You could add a smartshunt/BMV to the starter battery.

I believe the ports are isolated but would ask your dealer to be sure.

I know the cerbo can have multiple BMV/shunts attached but am unsure how it displays the information. In the Advanced tab of the VRM it would be easy to tell them apart.

Another option if you just want to check the voltage on your phone is a smart battery sense or small smartsolar MPPT (you could then add a small panel for a maintenance charge).

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justmurph avatar image
justmurph answered ·

Appreciate the suggestions

I should have mentioned it's a catamaran, with two start batteries, one in each hull. They're connected in parallel. The smart shunt could be added to one battery and while it'll only show what one of them is doing, it should be representative of the voltage and SoC of both. It's a expensive option. I believe in the Cerbo I just need to tell it to display the values for the main BMV and not this one. The smart shunt figures will be available under the device list. VRM should show it all properly.

The MPPT idea is good and a lot cheaper. I'd probably just wire it up so that one of the existing 4 panels can be switched between either its normal MPPT or this one, for the situations where I need to top off the starter batteries.

For both options I'll need to confirm that the VE.Direct ports are isolated.

Thank you for the ideas!

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frank-schultealbert avatar image frank-schultealbert commented ·

Hi Murph, did u get this one working? I am in a similar situation, I want to add my 12V Battery to the VRM / Cerbo I run on my 48V system. I am converting my sailboat to electric drive with a 48V motor but still keep a 12V Battery fed by a DC DC converter. If I could get the 12V battery connected with a smart shunt I could use a Cerbo relay output to start/stop the DC DC converter.


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justmurph avatar image justmurph frank-schultealbert commented ·

No luck I'm afraid. The only options that would work would be adding another BMV or a MPPT just for monitoring voltage. Not worth it.

I think I'll just add a little blue sea systems digital voltage gauge.

In your case, using the smart shunt would work and is probably ideal.

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frank-schultealbert avatar image frank-schultealbert justmurph commented ·

I used the Start Battery watch function of my BMV702 and it opens and closes the relay inside the BMV. The positive side of the relay should be enough to start the DC-DC.

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