It depends on the size of the batteries meaning the amp hours of each and the amount of discharge you have incurred during the night. You probably will not lose too much voltage if it is only overnight so both batteries together should have no problems at all in starting the engine.
I have 600 amp hours of battery and I use about 90 amp per 24hr period. We are a power hungry boat. My solar panels would recharge this loss
I probably would recommend that you use only 1 battery for starting then turn to both to charge both.
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Last edited by St Anna; 10-11-2012 at 05:35 PM.