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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.

For example, if a battery takes 20 hours to drain completely at 6 amps, the amp hours equal 6*20, or 120 amp hours.

P= V * I = 12 * 6 = 72 W during 20 h ( = 72 J /s )

E = 72 * 3600 * 20 = 5.184 10^6 J

Read more: How to Calculate 12 Volt AMP Hours | eHow.com http://www.ehow.com/how_5567042_calculate-volt-amp...

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