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In general
terms, the capacity of a cell/battery is the amount of charge
available expressed in ampere-hours (Ah). An ampere is the unit of measurement
used for electrical current and is defined as a coulomb of charge passing
through an electrical conductor in one second. The capacity of a cell or
battery is related to the quantity of active materials in it, and the
amount of electrolyte and the surface area of the plates. The capacity of a
battery/cell is measured by discharging at a constant current until it
reaches its terminal voltage (usually about 1.75 volts). This is usually
done at a constant temperature, under standard conditions of 25ºC (77ºF). The capacity is
calculated by multiplying the discharge current value by the time required
to reach terminal voltage.
The most common
term used to describe a battery's ability to deliver current is its rated
capacity. Manufacturers frequently specify the rated capacity of
their batteries in ampere-hours at a specific discharge rate. For example,
this means that a lead-acid battery rated for 200 Ah (for a 10-hour rate)
will deliver 20 amperes of current for 10 hours under standard temperature
conditions (25ºC
or 77ºF).
Alternatively, a discharge rate may be specified by its charge rate or
C-rate, which is expressed as a multiple of the rated capacity of the cell
or battery. For example, a battery may have a rating of 200 Ah at a C/10
discharge rate. The discharge rate is determined by the equation below:
C/10
rate (amperes) = 200 Ah/10 h = 20 amperes
Battery
capacity varies with the discharge rate. The higher the discharge rate, the
lower the cell capacity. Lower discharge rates result in higher capacity.
Manufacturer's literature on batteries will normally specify several
discharge rates (in amperes) along with the associated discharge time (in
hours). The capacity of the battery for each of these various discharge
rates can be calculated as discussed above.
The rated capacity for lead-acid batteries is
usually specified at the 8-, 10-, or 20-hour rates (C/8, C/10, C/20). UPS
batteries are rated at 8-hour capacities and telecommunications batteries
are rated at 10-hour capacities.
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