Page 88 - Lindens Handbook of Batteries

FACTORS AFFECTING BATTERY PERFORMANCE 3.7

voltage), the power output is the same for all of the discharge modes and at the level required for
acceptable equipment performance. During the discharge, depending on the mode of discharge, the
power output equals or exceeds the power required by the equipment until the battery reaches the
cutoff voltage.
In the constant-resistance discharge mode, the current during the discharge (Fig. 3.6b) follows the
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drop in the battery voltage (Fig. 3.6a). The power, I × V or V /R, drops even more rapidly, following
the square of the battery voltage (Fig. 3.6c). Under this mode of discharge, to ensure that the required
power is available at the cutoff voltage, the levels of current and power during the earlier part of the
discharge are in excess of the minimum required. The battery discharges at a higher current than
needed, draining its capacity rapidly, which will result in a shorter service life.
In the constant-current mode, the current is maintained at a level such that the power output at
the cutoff voltage is equal to the level required for acceptable equipment performance. Thus both
current and power throughout the discharge are lower than for the constant-resistance mode. The
average current drain on the battery is lower, and the discharge time or service life to the end of the
battery life is longer.
In the constant-power mode, the current is lowest at the beginning of the discharge and increases
as the battery voltage drops in order to maintain a constant-power output at the level required by
the equipment. The average current is lowest under this mode of discharge, and hence, the longest
service time is obtained.
It should be noted that the extent of the advantage of the constant-power discharge mode over the
other modes of discharge is dependent on the discharge characteristics of the battery. The advantage
is higher with battery systems that provide a wide voltage range to deliver their full capacity.

3.2.4 Example of Evaluation of Battery Performance Under Different
Modes of Discharge
In evaluating or comparing the performance of batteries, because of the potential difference in per-
formance (service hours) due to the mode of discharge, the mode of discharge used in the evaluation
or test should be the same as that in the application. This is illustrated further in Fig. 3.7.
Figure 3.7a shows the discharge characteristics of a typical AA-size primary battery with the
values for the discharge loads for the three modes of discharge selected so that the hours of discharge
to a given end voltage (in this case, 1.0 V) are the same. This is the same condition shown in Fig. 3.5b.
(This example illustrates the condition when a resistive load, equivalent to the average current, is
used, albeit incorrectly, as a “simpler,” less costly test to evaluate a constant-current or constant-
power application.) Although the hours of service to the given end voltage are obviously the same
because the loads were preselected, the discharge current versus discharge time and power versus
discharge time curves (see Figs. 3.5a and c, respectively) show significantly different characteristics
for the different modes of discharge.
Figure 3.7b shows the same three types of discharge as Fig. 3.7a, but on a battery that has about the
same ampere-hour capacity (to a 1.0 V end voltage) as the battery illustrated in Fig. 3.7a. The battery
illustrated in Fig. 3.7b, however, has a lower internal resistance and a higher operating voltage. Note,
by comparing the voltage versus discharge time curves in Fig. 3.7b, that, although the voltage level
is higher, the hours of discharge obtained on the constant-resistance discharge to the 1.0 V cutoff in
Fig. 3.7b are about the same as obtained on Fig. 3.7a. However, the hours of service obtained on the
constant-current discharge and, particularly, the constant-power discharge are significantly higher.
In Fig. 3.7a, the discharge loads were deliberately selected to give the same hours of service to
1.0 V at the three modes of discharge. Using these same discharge loads, but with a battery having
different characteristics, Fig. 3.7b shows that different hours of service and performance are obtained
with the different modes of discharge. To the specified 1.0 V end voltage, the longest hours of service
are obtained with the constant-power discharge mode. The shortest service time is obtained with the
constant-resistance discharge mode, and the constant-current mode is in the middle position. This
clearly illustrates that, on application tests, where performance is measured in hours of service,
erroneous results will be obtained if the mode of discharge used in the test is different from that used
in the application.