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hartrh avatar image
hartrh asked

monitor dual boat cranking batteries

Two cranking batteries wired thru 3 way switch: Bat 1, Bat 2, both. Negative leads parallel to negative bus. I understand the shunt goes in the negative lead. The shunt power wires to Bat 1; 2nd 702 input wires to Bat 2. 702 set to monitor voltage on 2nd battery. Question: Should total AH entered be sum of both batteries? When engine runs switch is set to "Both" so both batteries charge. When engine is off switch is moved to either Bat 1 or Bat 2. Will this set up work to give me some idea of status of both batteries? Or does someone have a better setup method?

battery charging
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Kevin Windrem answered ·

The BMV will never give you an accurate SOC number because you are changing the switch position so the battery capacity is changing: 2x in both while charging, then 1x when stopped and discharging.

You might consider leaving the battery switch in the both position at all times. But this doesn't give you a safety against draining both batteries and loosing the ability to start the engine(s). A low SOC warning might be useful to avoid the dead battery situation.

The best solution would be to connect battery monitors to each battery independently. If you don't need a dedicated readout and can use a smart phone/tablet for status, the SmartShunt is less expensive than the BMV series.

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