Battery types and characteristics
• CAPACITY: The battery capacity is a measure of how much energy the battery can store. Batteries do not simply serve as a bucket into which one dumps electricity and later extracts it. The amount of energy that can be extracted from a fully charged battery, for instance, depends on temperature, the rate of discharge, battery age, and battery type. Consequently it is difficult to specify a battery's capacity with a single number. There are primarily three ratings that are used to specify the capacity of a battery:
Ampere-hour: The Amphere-hour (Ah) denotes the current at which a battery can discharge at a constant rate over a specified length of time. For SLI (starting-lighting-ignition) batteries that are commonly used in cars, the standard is to specify Ampere-hours for a 20 hours discharge. This standard is denoted by the nomenclature of C/20. A 60 Ah C/20 battery will produce 60 Ah for a 20 hour discharge. This means that the new and fully charged battery will produce 3 Amps for 20 hours - it does not mean that the battery can produce 6 Amps for 10 hours (that would be signified by a C/10 60 Ah rating).
Reserve Capacity: The reserve capacity denotes the length of time, in minutes, that a battery can produce a specified level of discharge. A value of 35 minutes at 25 Amps for the reserve capacity for a battery means that the fully charged battery can produce 25 Amps for 35 minutes.
kWh Capacity: The kWh capacity metric is a measure of the energy (Volt * Amps * Time) required to fully charge a depleted battery. A depleted battery is usually not a fully discharged battery; a 12 V car battery is considered depleted when its voltage drops to 10.5 V. Similarly, a 6V battery is usually considered depleted when its voltage drops to 5.25 V.
None of these capacity ratings completely describe the capacity of a battery. Each one is a measure of the capacity under specific conditions. The performance of a battery in an actual application may vary substantially due to different discharge/recharge rates, battery age, cycle history, and/or temperature.
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