If your boat batteries discharge too quickly, adding a second battery can help. By having 12-volt batteries connected in parallel, they both can be charged by a single alternator while still providing the 12 volts that most marine accessories require. If two 12-volt batteries are wired in series they will produce 24 volts, which can damage your boat's electronics. Before installing a second battery, make sure that your compartment has enough room to safely accommodate a pair of batteries.
Disconnect the positive and negative battery cables of the existing battery. This will prevent current from running while you install the second battery.
Lubricate the four battery terminals with petroleum jelly. This will prevent corrosion.
Strip the insulation from both ends of a battery cable. Slide one end of the cable into the opened end of a battery cable clamp and tighten the two locking bolts and nuts with a crescent wrench. Crimp a 3/8-inch ring terminal onto the other end of the cable. Repeat this process for a second cable.
Slide the battery cable clamps over the second battery terminals. Tighten them with a crescent wrench.
Slide the ring terminal of the cable connected to the positive post of the second battery over the positive post of the existing battery.
Slide the ring terminal of the cable connected to the negative post of the second battery over the negative post of the existing battery.
Slide the existing positive cable clamp over the first battery's positive post, then connect the negative cable clamp. Tighten them securely with a crescent wrench.
Tighten battery straps over both batteries to prevent them from tipping over and spilling in rough seas.
Remove any jewellery before working on your boat's battery system.