If you have a boat with just one battery, it’s wise to add another, giving you twice as much battery capacity in case, for instance, you inadvertently drain a battery with the stereo while the engine is off. You can use the second battery to start the engine and then switch to the low battery to give it a charge while you’re running.
Adding a battery is an easy project using a dual-battery selector switch from a company such as Blue Sea Systems (bluesea.com) or Perko (perko.com), which we used here. When properly installed, these switches let you choose one of the two batteries, combine them or disconnect both with the turn of a dial. More complex systems incorporate automatic charging regardless of switch position, but this setup is simple and upgradeable and gets the job done.
Skill Level: 2.5/5
Time to Complete: >4-5 Hours
Tools and Supplies:
*Second marine battery meeting engine specifications
*Perko 8501 battery selector switch
*Battery box with tie-down or battery bracket
*2/0 marine-grade cable, red & black
*2/0 marine-grade cable lugs, either crimped or soldered
*Heat-shrink tubing to seal cable-lug connections
*If you prefer, pre-made marine cables in various lengths are available from Ancor (marinco.com) and others.
*Terminal caps for each positive battery terminal
*Power drill and drill bits
*Stainless-steel mounting screws
*Socket wrench set
*Heavy-duty crimper or soldering gun and solder
*Cable clamps and tie-wraps
American Boat and Yacht Council (ABYC) standards permit a 3 percent voltage drop for critical electrical systems. The length of the run (measured as the length of positive wire from the source to the device and back to the source along the negative wire) and the diameter or “gauge” of the cable determine the amount of voltage lost or dropped. To determine the cable size you need, visit boatingmag.com/cable-size.