Answer:
C/20 is the formula most commonly used to describe a battery according to its amp hour rating; for instance a 400Ahr rated battery means that a 20 amp load placed upon the battery delivers at that rate of energy for 20 hours upon reaching the manufacturer's per cell rating, usually about 1.7 volts or 10 volts for a dead, discharged battery. There are different discharge or load ratings such as a 10 hour or 100 hour rating resulting in a different Amp Hour rating for the very same battery. A battery is an electrical potential energy source whose capacity (amount of energy density delivered over time) is determined by the rate/time over which the battery will deliver a set rate of load/discharge. The lesser the load, the longer the rate, the higher the load the shorter the rate.