Dual-Purpose Marine Batteries for Small Powerboats and Sailboats

Small powerboats often use a single battery for both starting and running loads with the engine off. Small Sailboats often use two identical batteries interchangeably for starting and house electrical loads. Dual-purpose batteries are a good compromise, but true Deep Cycle batteries are highly recommended to handle the inevitable deep discharges. When the boat is plugged into shore power, the 12-volt battery system is recharged by a battery charger. The alternator is the sole charging source when away from the dock. Maintenance-free spill-proof batteries are highly desirable in marine applications.

Dual-purpose batteries must be able to consistently deliver reasonable Cold Cranking Amps (CCA), operate in high and low temps and withstand vibration. With the engine off, High Cycle Reserve Capacity (RC) is required to support moderate electric loads such as electronic equipment, radios, house lights, bilge pump, small inverters, etc. From low to high, dual-purpose batteries can be designed with micro-cycle (17.5% Depth of Discharge - DoD) or high cycle (30% Depth of Discharge - DoD) life characteristics. Typically, the higher the cycle life ability, the lower the CCA rating will be in a dual-purpose battery. A battery with high CCA, High Reserve Capacity “and” long life should be viewed with suspicion.

Dual-purpose battery specifications: Cold Cranking Amps (CCA); the amps a 12V battery can discharge for 30 seconds at -18°C / 0°F while maintaining its voltage above 7.2 volts. Because lead-acid batteries produce more energy at higher temperatures, we also rate marine batteries in Cranking Amps (CA), similar to CCA but measured at 0°C / 32°F and Marine Cranking Amps (MCA), similar to CCA but measured at 27°C / 80°F.

Engine size, type, and ambient temperature determine what size of cranking battery is required. Higher cranking power is required for cold temperatures, diesel engines, or high-compression gas engines where high cranking voltage must be maintained for the fuel ignition systems to function. This requires maximum Amp Hours or Reserve Capacity: Minutes a battery can maintain a load of 25 amps before it drops to 10.5 volts (at 27°C / 80°F).

Lead-acid battery failure is most commonly caused by acid stratification, extreme temperatures and destructive vibration. Acid stratification naturally occurs in flooded lead-acid batteries and leads to a decline in capacity and charge acceptance. AGM technology and acid mixing technology for flooded lead-acid starting batteries will mitigate acid stratification.

Starting batteries are designed for maximum repeated discharges of 1-3%. Dual-purpose and high-cycle batteries are designed for maximum repeated discharges of 17.5 – 30%. Deep Cycle batteries are designed to withstand repeated depths of discharge of up to 50% to 80% depending upon technology.