I am setting up an electric boat that uses a 48V motor. The boat also has a 12V system from the factor to run the navigation lights and fish finder. Instead of also having a 12V battery I’ve been using these 48V to 12V converters common on golf carts. But my question is how can I set the BMV-712 to monitor both? I’m not sure how to tie in the negative from the converter to the BMV-712 and then to the negative post for the boat negative wires to connect to. With the ground for the converter already coming from the busbar that’s hooked to the BMV shunt do I need to do anything more then connecting the small wire that came with the BMV to the 12 negative post at the bottom of the picture? Thank you for your help.
The BMV will measure the current taken from the 48V battery and send it to the 12V system. If you check continuity you will find the ground is common on the converter it’s not isolated. For safety, you should be using an isolated converter. If that step-down converter has an internal fault you could see 48v on the 12v system. When you set up an isolated system the grounds are kept separate. You might be able to use a VSR like the battery protect to disconnect if there is a fault.
As the 48-12 converter is already connected to the negative bus bar you are OK in that respect.
The BMV +B1 terminal connects to the 48V positive. The BMV +B2 terminal connects to the positive 12V terminal exit the 48-12 converter. Then in the BMV settings tell it that the aux terminal is measuring the starter battery voltage.
It is all in the manual.
Keep a close eye on that 48-12 converter. Over on DIYSolarForum.com there are plenty of posts where the cheap step down (and buck) converters haven’t lasted very long. I would keep one as a spare.