Nasa bm1 with 2 batteries

[steve yates]

ihave been reading some interesting old threads about fitting a bm1. from these i get that the battery neg terminal musthave no other connections, justthe shunt, and that all other negs must lead to theother side ofthe shunt.
The batteries will have an old 121 isolator, so what is the best way for me to deal with the battery negatives? Given the above? I presume I cannot run a negative from each battery to the shunt if the above is true?

can I daisychain them together at the batteries or does that break the circuit when the other battery is selected?
Or is it just not feasible to monitor 2 batteries with this unit?
thanks.

 

[VicS]

If you regard and use one battery chiefly as an engine start battery and the other as the house battery you could put the BM shunt in the neg connection of the "house" battery and use it to monitor just that one. Monitoring just the volts on the other one

I wonder if
you regard the two batteries more or less as equals you could connect their negatives together, with the shunt in the common connection and link their positives with a VSR to charge both whichever is selected as the duty battery with the selector switch. The monitor would then monitor them as though the two were part of the same bank.
Not sure if that would work properly or not. The problem might be that the monitor positive has to be connected to just one of them

Perhaps now is the time to separate engine and house electrics, fit individual isolator switches, a crossover switch and a VSR.

 

[Tranona]

Read the manual here https://www.nasamarine.com//wp-content/uploads/2015/12/Clipper-BM1BM2.pdf The BM1/2 will only fully monitor one bank, although as you see you can connect with a wire to the negative of a second battery to give you just the voltage on that one. All you need if you have two independent banks.