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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11077
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Boat mechanic I have 2 starting batteries -when I am running

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Boat mechanic
I have 2 starting batteries -when I am running the boat can I set the battery selector to all or do I need to select battery 1 or battery 2 only - if i pick all will both batteries be charged- i have an 18 foot flats boat with a 2014 135 etec motor.

Hello, I'm Tan. Welcome to JustAnswer. I'm reviewing your question and will reply momentarily.

When running the boat you may put the switch on both. When you stop and are using accessories I would turn switch to bat # 1 or 2. This way you only drain one battery. That way you always have a good battery.

Post back with questions.

Customer: replied 2 months ago.
Now if I were to leave it on all would it still combine both batteries when starting thereby negating the need to switch to 1 or 2 and then back
Customer: replied 2 months ago.
Also when it's in all will it be charging both batteries while the engine is running?
Customer: replied 2 months ago.
On all not in all

Yes in all position is both batteries together. The only time you want to switch to 1 or 2 is when you are sitting with a drain on batteries. Switch to one or 2 engine off.

In all position the engine charges both batteries.

Customer: replied 2 months ago.
Gotcha wouldn’t 2 together be the same as 1 deeply drained plus another at near max

Well the reason you want to use just one battery is if your out in the middle of nowhere and you kill both batteries how do you get home. If you use just one battery and kill it you will still have a good battery to get home with.

2 batteries will last longer but how far do you discharge them before you realize they are to low to start the engine?

Tan and other Boat Specialists are ready to help you
Customer: replied 2 months ago.
Gotcha thanks