A battery isolator can enable you to charge two different types of batteries (deep cycle and a starter battery) from your main 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. When I added my third deep cycle battery to run the majority of my electronics I wanted to be able to tap into the alternator on my main motor to charge this deep cycle battery while running the main motor.
Battery IsolatorI spoke with my marine mechanic who recommended this device. He also looked up the diagram of my Mercury motor in his directory and showed me where the alternator wire came off the motor and showed me how to hook it up properly. Here is a diagram of what he told me to do.
The following picture shows the actual unit installed in the 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.
Actual Installation On Boat
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 have built into them a fuse box with a negative terminal built into it. In most new freshwater fishing boats this terminal is powered off the starter battery. It is possible to add devices/accessories by tapping into this fuse box for power. However, you can very quickly run the risk of demanding more power from the starter battery than it was designed to handle. This is especially true if you plan to use these accessories when the main motor is not running and you will not have the advantage of the alternator powering your equipment. This is the time to begin to think about adding an additional deep cycle battery for your accessories. Click on the following link to see a diagram example of wiring accessories to an add on deep cycle battery.