Dual Boat Battery Wiring

[Gerry Zagorski]

If you have a small boat and want to limit your exposure to running down your starting battery, read on....

The best set up is to have 2 batteries wired separately run through an isolator and a parallel switch. One battery is dedicated to starting and the other a house battery, which runs all your electronics. Having said that 90% of the factory wired boats with 2 batteries are set up with an A/B/Both switch. Although this is a good set up it has its issues.

Here is why an isolator set up is the way to go as compared to the typical A/B/Both switch set up:

An isolator allows current to flow individually from your alternator into each battery for charging purposes yet it isolates one from the other in terms of draw. As mentioned above, most boats with 2 batteries are wired with an A/B/Both switch. If the switch is left in the both position while you’re not using the boat and one battery is low it will drain the other to even out the state of charge between the 2. You can also set the switch to 1 or 2 but you have to remember to switch between 1 and 2 so each battery gets charged.

The isolator eliminates the need to have an A/B/Both switch and you never have to think about alternating between 1/2 to charge each battery or running both batteries down if you leave the switch on both.

The parallel switch allows you to temporarily parallel 2 batteries together if you need the extra juice from both batteries to get started.

So, the isolator isolates one battery from the other and the parallel switch allows you to temporarily parallel them together if you need to. No A/B/both switches to mess with and no need to worry about how the switch is set.

Here is a great article on the subject
www.thejump.net/fishingarticles/Ship-Shape.htm