Originally Posted by
WalrusLike
Assuming both batteries have enough cold cranking amps to start your motor....
I would recommend my setup which is battery's both go to isolator then everything runs from there. In other words use one battery at a time for everything and second is always ready as a spare.
I go out on 1, stop motor, switch to 2 and then use that all day and come home still on 2. That way both batteries are exercised and recharged and I always have at least one good one if I somehow flatten the other.
Shouldnt happen, but if two flat batteries, then can try starting on 'both' setting. That's a desperate last resort. 'Both' setting is risky in case you have one bad battery it can pull down the other if connected as both.
If you split the tasks across batteries then you risk both becoming bad at once. Used in above fashion you always have two known-good batteries for starting. All other equipment can fail but you _must_ have a starting battery or be able to pull start your outboard. My two cents worth.
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