Discharging, like charging, depends on a number of factors such as the initial State-of-Charge, average Depth-of-Discharge, condition and capacity of the battery, load and temperature. To determine the amount of discharge time (T) for a fully charged battery at 70°F (21.1°C), the simple formula is ampere-hour rating (C) divided by the average load in amps (I). So, a 100-ampere hour battery with an average 5 amp load should last approximately 20 hours (100/5). The total number of amps that are produced when a battery is discharged over a 20 hour period is the most commonly used specification for expressing the capacity of deep cycle batteries used in most RV and marine applications.

If you increase the discharge rate to an average of 100 amps per hour, you would expect approximately one hour of capacity from a 100 amp hour battery (100/100). That is not the case. The amp hour capacity of the same battery is approximately 45 so the discharge time could become 27 minutes (45/100) for one type of battery and 56 minutes for another. This is due to the Peukert Effect. When you increase the discharge rate less power is produced. A good example of the Peukert Effect on deep cycle battery capacities at various discharge rates can be found at http://www.usbattery.com/specs3.htm. The actual formula is T=C/IN where N is the Peukert Number used for the specific battery to more accurately calculate the discharge time. The number generally is in a range of 1.0 to 1.4, with 1.0 the best performing battery.

Repeatedly discharging a deep cycle battery to 10.5 volts or below will significantly reduce the number of life cycles. New batteries often require a "break-in" period of up to 50 cycles before they will produce their rated amp hour capacity. Also, as the battery ages, the capacity is reduced as the active material flakes off the plates and some of the pores fill with hard sulphate.