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Choosing a Marine Battery

Choosing a marine battery that will be right for your boat can be a tough undertaking today due to the numerous types and wide variety of design duties of batteries on the marine market.

Two basic types of marine battery perform two distinctly different jobs. A starting battery is built to crank the starter on your boat’s engine by delivering a high number of amps in short bursts of 5 to 15 seconds. Once the engine starts the onboard alternator quickly recharges the battery.

Marine electrical experts recommend that boat owners have a dedicated starting battery for each engine. The starting battery should be independent of other systems requiring electricity.

The marine cranking amp (MCA) or cranking amp (CA) rating on the battery indicates its starting power. Your engine manual should tell you the recommended MCA/CA for your engine. Get a battery that meets or exceeds that rating.

A house battery or house battery bank supplies power to all the components and systems that run on DC electricity on your boat. Deep-cycle batteries are used for house power. This type of battery is designed to supply a steady flow of power over a long period of time.

The windlass, the bilge pump, the stereo, the trolling motor and the navigation lights are all powered with the house batteries. These batteries are designed and constructed to deliver energy for longer periods and are depleted to a greater degree than starting batteries. They need to be re-charged at a slower rate over a long period of time to assure a long service life.

The chemistry of starting and deep cycle batteries is different. Starting batteries have more plate surface area due to a greater number of plates. This allows for the quick bursts of high energy. Deep-cycle batteries have fewer, but thicker, plates that deliver energy and recharge at a slower rate.

Dual-purpose batteries are used on smaller powerboats that have room for only one battery. The dual-purpose serves as both the starting and house battery.

More Marine Battery Choices

Do you want a traditional wet-cell, gel or AGM (Absorbed Glass Mat) battery? Each has strengths and weaknesses.

Wet-cell or flooded-cell batteries are less expensive, last longer if maintained, and are less likely to be damaged by overcharging. On the downside, their distilled water levels must be maintained and they need sufficient ventilation. There’s also a threat of a battery acid spill.

AGM batteries are completely sealed so there’s no acid spilling, and they require no maintenance. They have better shock and vibration resistance than their flooded counterparts. The cons: They are heavier, cost more and are more likely to be negatively impacted by overcharging.

Like AGM batteries, gel batteries are sealed and require no maintenance. They also have a high resistance to damage done by overcharging. Plus, gels have a low discharge rate so they can be stored for long periods, making them a good choice for boat owners who have a tendency to leave their batteries in storage without recharging. Gels are expensive and they have to be recharged carefully, from 13.8 to 14.1 volts, so chargers with a gel setting are recommended.

Choosing a Marine Battery—Calculate Energy Use

There are more factors to consider when choosing a marine battery. You should determine the size and/or the number of batteries for your house power. This depends a great deal on the amount of electricity the systems on your boat will likely consume over a given period of time.

Figuring out the planned amount of energy use requires some math. Energy use is measured in amp hours. How much electricity will your DC loads use? The current, or amps, multiplied by the time of operation, or hours, equates to amp-hours (Ah) per day. For example, if you have two running lights that each draw 1.5 amps, and those lights are operating for five hours a night, the Ah per day consumption would be 2 x 1.5 x 5 = 15 Ah/Day.

A small powerboat, usually rigged without a separate generator, generally consumes from 60 to 200 Ah per day. A large cruiser or sportfisherman could easily consume upwards of 400 Ah per day.

To calculate what your boat needs make a chart and list the consumption amounts of all systems, everything from the head, to the bilge pump, to the GPS unit. Now, if your boat has AC power, too, that must be factored in if an inverter is used aboard to turn AC current into DC current. More math is required to translate AC usage into DC usage. AC watts x hours/10 = Ah/Day. Now add all of the consumption figures up to come away with a gross energy consumption Ah/Day.

With this figured out, you can determine the battery capacity that meets your energy needs and begin the process of choosing a marine battery. Many experts say your battery capacity should be three to four times your daily energy consumption. So a boater using an estimated 160 Ah per day should have a house battery bank capacity of 560 Ah of capacity.