What are Amp Hours?

The Amp Hour (AH) specification provides a measurement of battery capacity.  In other words, it is an indication of how much energy can be stored by the battery.

A typical Amp Hour specification might read, “100 AH @ 20HR”.

The specification is saying that the battery will provide 5 amps of current at a useable voltage continuously for 20 hours.  The “5 amps” was calculated by dividing 100 by 20.

Similarly, a battery with a specification that reads “150 AH @ 15 hours” will provide 10 amps of current at a useable voltage continuously for 15 hours.

It should be noted that a useable voltage is considered to about 10.5 voltsand above on a battery that is under load (or has devices connected

To ensure that ratings are given in a realistic way, lead-acid batteries have a few parameters on how they get that “AH” rating. In order to get an AH rating, the battery that is being tested has to be drained down to 0 over the course of a specified amount of time.  The amount of amperage that it took to get it down to zero, over that specified amount of time constitutes the AH rating.

Because of the Peukert effect (aka, the faster a battery is drained, the less overall amperage is available), if you discharge a battery over the course of 100 hours, the AH rating looks higher than if you discharge that same battery over the course of 1 hour. So, there has to be a standard.

For deep cycle batteries the standard rating is 20 hours. So, if a battery has a rating of 100AH @ 20Hr rate, then that battery was discharged over 20 hours with a 5 amp load.  Starting batteries, on the other hand, are typically rated at 10Hr rate, because they are used faster, so the 20Hr rate is not as important.  So, that weird 20Hr rate that you see after the AH rating on batteries tells you that the rating in question is the realistic, common rating—rather than an over-inflated number to make the battery look better than it really is.

As indicated above, the Amp Hour specification on 12 volt batteries is normally based a twenty hour rate. In fact, the specification is so standardized that battery labels often do not include this information.

That said, it is important to be aware that deviations from this norm are not uncommon.

Some battery manufacturers will establish different rates for their amp hour specification. For example five and ten amp hour rates are not uncommon.

In the case of a battery manufacturer who specifies a 100 AH based on five hour rate, the claim is that the battery will provide 20 amps for five hours before dropping below 10. 5 volts.