Battery Application & Technology
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 25C (77F). 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 (25C or
77F). 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:
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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