Reserve Minutes

by Don Casey

Revised by BoatUS editors in April 2012

Deep Cycle AGM 32Boaters traditionally think of battery capacity in terms of amp-hours, but if the battery will be used to power an inverter, the reserve minutes rating can be more telling. Reserve minutes, also called reserve capacity, is the number of minutes a fully charged battery can sustain a designated constant load--usually 25 amps--before it is fully discharged. For a 12-volt battery, that means battery voltage will have fallen to 10.5 volts. Recalling that we divide watts by volts to get amps, supplying a 300-watt inverter load from a 12-volt battery requires 25 amps.

So a battery with a reserve capacity of 120 minutes can theoretically handle this load for two hours, but the practical limit is just one hour since it is never a good idea to discharge a battery more than 50%. Running a 1,000-watt coffeemaker through an inverter will completely drain this same battery's usable capacity in less than 15 minutes. If you plan to install an inverter, you need a battery with a high reserve minutes rating.

Don Casey has been one of the most consulted experts on boat care and upgrades for 30 years, and is one of the BoatUS Magazine's panel of experts. He and his wife cruise aboard their 30-footer part of the year in the eastern Caribbean. His books include Don Casey's Complete Illustrated Sailboat Maintenance Manual, and the recently updated This Old Boat, the bible for do-it-yourself boaters.

 

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