By Don Casey
Revised by BoatUS editors in April 2012
A marine starting battery is essentially an automotive battery--sometimes "ruggedized", sometimes not. Like an automotive battery, a marine starting battery is designed to start the engine(s). After that, the battery is being continuously charged. STARTING BATTERIES ARE NOT DESIGNED TO BE DISCHARGED--PERIOD. So can you run the electrical equipment on your boat from a starting battery? If you run it only when the engine is running, the answer is yes. Power drawn by appliances is concurrently replaced by the alternator, so the battery stays fully charged.
If, however, you expect to operate electrical equipment when the engine is NOT running, you need a deep-cycle battery. Deep-cycle batteries can be discharged to about 50% of their capacity without damage. In other words, you can expect a 100 amp-hour battery to supply around 50 amp-hours of power between charge cycles without suffering damaging. Provided it has sufficient cranking amps, a deep-cycle battery can also serve as the starting battery, but most boaters elect to have a dedicated starting battery so discharging the "house" battery cannot leave them unable to start the engine.
A dual-purpose battery has appeal for those who wish to avoid the complication of multiple batteries. Something of a hybrid, this type of battery typically is more starting battery than deep-cycle, but it will tolerate modest discharge without damage. If you only want to run the stereo and maybe a couple of lights when the engine is silent, this can be a good choice.