Battery Isolator

A battery isolator can enable you to charge two different types of batteries (deep cycle and a starter battery) from your motor's alternator without them discharging each other. This is unlike the Series or Parallel connections when you have identical batteries connected together and working in tandem. On many fishing boats with trolling motors a battery charger that can be plugged into 110 volt outlet is already installed. An isolator can be installed if you ever find your self needing an additional deep cycle battery to run added electronics. This allows a charging current to run to both the starter battery and the added deep cycle without current running between the two batteries.

Battery Isolator Diagram

Boat mechanics or parts dealers are able to provide you with an isolator, but equally important would be a diagram of where the alternator wire comes off the main motor.  They are also more than willing to help, especially if you purchase your isolator from them.   Here is a diagram of a typical installation.      The following picture shows the actual unit installed in a boat.  The single post in the middle secures the wire from the alternator.  The other two wires run to the starter battery and the deep cycle battery positive post respectively.

Bussmann RB-BI-70A 70 Amp Battery Isolator

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Boat Wiring

Boat Wiring is easily understandable. You basically are working with a positive and a negative wire for each electrical accessory. It is also advisable when adding an accessory to be sure to include an in line fuse on the red or positive wire following the manufacturers recommendation. Boats usually will have a built in fuse box or terminal, with a negative terminal built into it. Check on your boat and you may find that this terminal receives its power from the main starter battery. For added accessories it is possible to power them off your starter battery, at least to a point.

As you add accessories, you may run the risk of drawing too much power from your starter battery, particularly if you are not running your main motor for extended periods, resulting in the alternator not keeping your starter battery charged.  This is the time to begin to think about adding an additional deep cycle battery for your accessories.   You could also add a second on board 2 bank battery charging system, or consider this isolator as a viable option.

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