Amp-Hour Ratings

An indication of how much energy a battery can store
A battery’s amp-hour rating indicates the total amount of energy it will deliver at a constant rate of discharge over a period of 20 hours before it reaches a voltage at which it is stone dead for all practical purposes.A 12-volt battery, the most common nominal voltage, is fully discharged at 10 volts. A 100-amp-hour, 12-volt battery will run a 5-amp motor (or a 60 watt light) for 20 hours, and a 200-amp-hour battery will run a constant 10-amp (120 watt) load for 20 hours.That’s the theory. In fact, if a 100-amp-hour battery is discharged at a rate greater than 5 amps, it will not deliver all the advertised amp-hours before it goes dead. On the other hand, if you discharge it at a steady rate of less than 5 amps, you’ll get more amp-hours than the manufacturer’s rating shows—not a whole lot more, but some.That principle applies to most lead-acid batteries found on boats—the faster the discharge rate, the fewer amp-hours delivered. The more slowly energy is taken from a battery, the longer it will last.Sometimes a battery’s work capacity is given in reserve minutes. In the absence of any other definition, this is the number of minutes you can discharge a fully charged battery at a whopping 25 amps before its voltage drops to 10.5.See also Battery Choices; Cranking Ratings.