I am planning to use a BMV-712 to monitor the SOC of two Battle Born 12-volt Li batteries connected in series to run a 24-volt trolling motor. The batteries are charged individually by 12-volt chargers.
I can find multiple diagrams for using a BMV-712 to monitor:
12-volt batteries wired in parallel, and charged separately,
6 volts in series to make 12 volts and charged with one 12-volt charger.
6 volts in series to make 12 volts, with several connected in parallel to increase capacity.
This is what I have in my travel trailer, which is charged through the trailer system, or can be switched to be charged by a Sterling 60-amp battery to battery charger I can connect to the truck with an Anderson connector. This is monitored with BVB 712.
What I do not find is how to use one (or two) BMV-712’s to monitor series connected batteries being charged individually.
My thinking is this:
I have 12-volt batteries A and B connected in series to yield 24 volts.
Both batteries A and B are charged individually.
Connecting to measure both batteries:
If I connect the BMV shunt between the negative terminal of battery A and the trolling motor, the shunt will detect the energy supplied to the trolling motor by BOTH battery A and B, but will only detect the energy charging battery A. With each use and charge, the calculation of the SOC will be off by 50%. The only ways I can think of to compensate for this is to adjust the peukert exponent to be about .52, or the charging efficiency factor to about 200%. The ranges shown in the manual do not include these drastic settings so I doubt either is possible.
Connecting to measure ONE battery of the pair.
If I connect the BMV between battery A and B the shunt measures the current flowing from battery B to battery A. When charging, the shunt will measure the energy used to charge battery B. The SOC should be accurate for battery B. Batteries A and B are “matched” per Battle Born- and under the same load from the trolling motor, so the SOC of A should be very close to battery B.
Is there a way to measure the SOC of both batteries?
Is there a better way to measure the SOC of one of the batteries?
The rest of the story:
I have two Battle Born 12-volt 100 AH Li batteries connected in series to produce 24 volts to run a trolling motor. The batteries are charged individually by two different systems. The primary means of charging is a Dual Pro charger (15 amp per battery) The batteries will also be charged by a Minn Kota MK-2-DC onboard alternator charger. Its output is up to 10 amps per bank with a 25-amp alternator, but my outboard only puts out 20 amps.
It would have been simpler to have two 24-volt Li batteries in parallel charged by 24-volt chargers- but that is not what I have…