The Amp Hour or Ampere hour (abbreviated Ah) rating tells you how much amperage is available when discharged evenly over a period of an hour (60 minutes). his term is used to express the storage capacity of deep cycle batteries. This is the maximum sustained amperage drawn from a fully charged battery over a certain time period to a point where the battery is at 100% DOD (depth of discharge), for all purposes dead! This amperage is then multiplied by the discharge period to give the battery its Ah rating. The period of time used to drain and measure the battery is usually 20 hours, although longer or shorter time periods may be used depending on the application.
As an example, a 200Ah (20 hr rate) battery can sustain a 10-Amp draw for 20 hours before reaching 100%DOD. 10 Amps X 20 hours = 200Ah. You may say that this same 200Ah battery should last 4 hours with a 50Amp draw, but this is not the case at all. The reason for this non-linearity is that the capacity of a battery actually decreases when the load increases. Take a look at fig. 1 to understand how the Ah ratings vary depending on the discharge period.