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Sizing Batteries


Amp-hours on board are measured just like kilowatt-hours in your home. How much current we consume and for how long are the factors that determine the overall capacity needed in a boat’s house bank of batteries.

Starting batteries are easy to size since the current is high but the duration is typically very short: It takes seconds rather than minutes or hours to start a boat’s engine. By contrast, smaller loads with low current that are on for hours at a time — such as lighting and fans — can really strain capacity. Still other types of loads, including refrigeration, can vary greatly in consumption patterns during hot and cool days, and at other times.

Your goal in tallying all these needs is to make sure your boat’s house batteries never discharge below 50 percent of their capacity, so voltage will stay at an acceptable level. At a minimum, batteries have to be at least twice the size of the expected loads between charging intervals.

For extended cruising, a factor of three times the expected loads is more prudent because recharging can be a challenge, and batteries may not have enough time to recharge to 100 percent. If you do a lot of long-distance cruising, consider having multiple charging sources. They can supplement the engine alternator and provide security when the engine isn’t running.

Other charging sources, such as solar panels, and wind and auxiliary water generators, can be deducted from the consumption total, helping you achieve a balanced boat — meaning the current required for a safe, comfortable trip is easily met, and all systems operate at the highest level. Consumption and recharging can be monitored by battery voltage, and meters can track amperage flowing either way over time so that actual amp-hours in the bank are shown in real time.

If you need to reduce consumption, some of the best product advances have been in LED lighting. The amps required for long-term LED use are a fraction of what they are with traditional lighting. Also, consider newer, more efficient refrigeration compressors.

This article originally appeared in the December 2017 issue.


Alternator Pulley Side View Illustration

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