On-board battery charger guru's...???
 

I have a MarineCo 3-bank on-board battery charger. It has one lead that goes to the starting battery and two other separate leads that go to each one of my two accessory batteries. Both the accessory batteries are also wired together. My question is, can I wire an additional accessory battery in and still use the 3-bank charger, or do I need to get a 4-bank charger? Thanks for any info...

Sorry, it's a ProMariner charger...

I'm no expert, but I think you can do this. The only problem I see is that it will take longer to charge all three accessory batteries. I have a two bank charger and in the off season when the batteries are in my garage, I connect one bank to the starting battery and connect the second bank to the accessory batteries that are wired together. For those two batteries I connect the positive to one battery and connect the negative to the other battery.

Another option would be to disconnect the bank that goes to the starting battery and connect that bank to the third accessory battery. Use your alternator to charge the starting battery. This is what I do during boating season and I have yet to have problem with starting the boat in three years. I just make sure that when we are hanging out with the boat off that the switch is turned to the two accessory batteries.

Since your accessory batteries are already wired together, just leave one lead from the charger hooked up to them, and it will charge them both. Use the 3rd lead from the charger to charge the new battery.

@moon, if the batteries are wired together (+ to + and - to -), it doesn't really matter which battery you hook your charger to

jrw, thanks. Like I said, I'm no expert.

Keep in mind if they are hooked together, a smart charger may not sense voltage from each battery equally if they are different capacities or drained differently. Even though they are hooked in parallel, they will charge at different rates and the charger will feed them the same amount of amperage so one battery may be under or over charged.

If they are same capacity and age and use (run parallel) then it probably won't make a difference, but best way to do it is to have each isolated and on a separate circuit during charging.