Amp hours are the standard measure of the energy storage capacity of 12-volt and other batteries. When in use, a battery's voltage remains constant. The current (amperage) varies depending on how much power is being used. Twelve-volt batteries are rated based on how long a fully charged battery can provide power at a specified amperage before it is entirely drained. For example, a battery rated at 80 amp hours can deliver a 20-amp current for up to four hours.
Understand how amp hour ratings for 12-volt and other batteries are determined. A battery is brought to full charge and then completely discharged over a specified time period (20 hours is the interval used most often). The amperage required to entirely discharge the battery in that period is multiplied by the time interval. For example, if a battery takes 20 hours to drain completely at 8 amps, the amp hours equal 8*20, or 160 amp hours.
Figure out how long a fully charged 12-volt battery can operate under a particular current load. To do this, divide amp hours by the actual amperage being used. For instance, if your 12-volt battery is carrying a load of 15 amps and has 160 amp hours capacity, it will last for 10.67 hours.
Familiarise yourself with factors that affect the number of amp hours a battery can supply in normal use. Amp hour ratings tell you battery capacity when the battery is new and operating under normal conditions. If your 12-volt battery is drained rapidly (a high amperage), the battery capacity is reduced. You'll also have lower capacity in cold weather or if the battery is old or has suffered damage.
It's wise to put a 12-volt battery on a trickle charger if it isn't going to be used for several weeks or months. This prevents the battery from fully discharging, which reduces battery capacity and shortens the life of the battery. To maintain maximum battery capacity and useful life, avoid discharging the battery below the 50 per cent capacity level as much as possible.