Electrical: Batteries

Boat Battery Basics
The batteries on your boat are responsible for starting the boat’s engine, and for powering lights, electronic devices, and other accessories.
Generically, there are three types of boat batteries: starting batteries, deep-cycle batteries, and dual-purpose batteries.
Starting Batteries
When you turn the ignition key, the starting battery sends a burst of electricity to the engine’s starting motor. After the engine starts, the engine’s charging system sends electricity back to the starting battery—recharging it for the next time you want to start the engine.
The starting battery expends a lot of energy for a few seconds at a time. Although a starting battery is strong, it doesn’t have a lot of power in reserve and discharges (loses its reserve of electrical energy) quickly.
Frequently discharging a starting battery will eventually rob the battery of its ability to recharge—rendering it useless.
Deep-Cycle Batteries
A deep-cycle battery’s purpose is to keep your boat’s lights lit, the trolling motor trolling, and the blender blending.
The deep-cycle battery is the workhorse of your boat’s electrical system, delivering plenty of power over extended periods. The engine’s charging system replenishes the deep-cycle battery’s electrical stores for later use.
Dual-Purpose Batteries
Many smaller boats don’t have room for a starting battery and a deep-cycle battery; this is where a dual-purpose battery comes in.
A dual-purpose battery starts the engine, and maintains a steady flow of electricity to your boat’s electrical components by combining features of both a starting battery and a deep-cycle battery.
The Battery Code
Shopping for a new battery is tough, because they all look alike—not to mention the myriad of abbreviations and numbers on each battery’s label. The label helps to quantify a battery’s power delivery in a variety of ways.
Every electrical component on your boat requires a certain amount of electricity to function. This electrical energy is measured in amperes (amps). A battery’s electrical output is defined by how many amps it can deliver at certain temperatures and over pre-determined periods of time.
Starting Batteries
Cold Cranking Amps (CCA) tells us how many amps a 12 volt battery can provide to the engine’s starter motor for thirty seconds, at 0°Farenheit, while keeping its voltage above 7.2 volts.
Marine Cranking Amps (MCA) is the same measurement as CCA, but at 32°Farenheit.
You want as much electrical energy as possible to be available to start the engine, so look for a starting battery with the highest CCA/MCA ratings.
Reserve Minutes is how long a 12 volt battery can power a 25 amp load (load: electrical energy leaving the battery) before the battery’s voltage drops to 10.5 volts.

Deep-Cycle Batteries

Since a deep-cycle battery provides electricity over extended amounts of time, we want to know how much electrical energy a deep-cycle battery holds and how long the battery can supply the boat’s accessories.
Amp-hours (Ah) defines the length of time that a deep-cycle battery can power a 20 amp load before battery voltage drops to 10.5 volts.
As with a starting battery, a deep-cycle battery’s Reserve Minutes rating is the number of minutes that the battery can power a 25 amp load prior to its voltage dropping to 10.5 volts.
Battery Care
  • Have your engine’s charging system checked regularly for optimum output.
  • Wear eye protection whenever you’re working around batteries.
  • Tighten battery terminal connections with a wrench or pliers.
  • Clean corroded battery terminals with water and baking soda
  • During the off-season, remove the batteries from your boat and store them in a cool, dry place—setting them on a piece of wood in the garage or basement works well