4
Capacity varies with the rate of dis-
charge as shown in Figure 3. Testing
for how discharge rate affects capacity
is discussed later in more detail.
Generally, lower discharge rates over
longer periods of time yield higher
values of total capacity. It is impor-
tant to realize that since discharge
rate affects how the value of C is
determined, battery manufacturers
must decide on a standard time of
discharge. Since different values for
capacity can be obtained for the same
battery, capacity is generally deter-
mined over a “standard” period of
time—from 5 to 20 hours at discharge
rates from C/5 to C/20. A complete
specification for capacity should
therefore have a C rate and the period
of time that was used to determine
the capacity. For example, Capacity:
450 mAh @ 5 hour rate.
Average and maximum capacities are
obtained by putting the battery through
five successive charge/discharge sta-
bilizing cycles. The batteries are given
five stabilizing cycles where they are
charged, discharged and rested at an
ambient temperature of 23 degrees C.
Batteries are charged at C/10 A for
a period of from 20 to 24 hours and
rested for a period of from 2 to 4 hours.
The batteries are then discharged at
a constant current of 1C amperes to
an EODV of 0.9 volts.
The value of the capacity used in the
following tests is the value obtained
in the fifth stabilizing cycle. Also, the
capacity obtained in the last three
cycles must not be less than that
stated by the manufacturer as rated
capacity (1C).
Capacity Retention
This test characterizes how much of a
fully charged battery’s capacity is
retained over a long period of time
under specific conditions. This time
is sometimes referred to as the “shelf
life” of the battery. This test is not to
be confused with an attempt to char-
acterize the self-discharge effect of
the spontaneous internal chemical
actions in batteries. Self-discharge
occurs regardless of the battery’s
connection to an external circuit.
The procedure to determine the effec-
tive capacity retention of a battery is
relatively simple. Immediately follow-
ing the 5 cycles of capacity measure-
ment, the battery is fully recharged. It
is then stored open circuit for a peri-
od of days at a specific temperature.
Then it is discharged at a constant
current rate to an EODV of 0.9 V as
before. The capacity obtained should
not be less than 37% of the rated
capacity for the battery. The number
of days of shelf life are typically pro-
vided for values of temperature from
23 degrees C to 50 degrees C.
Figure 3. Effect of Discharge Rate on Capacity