Page 93 - Lindens Handbook of Batteries

3.12 PRINCIPLES OF OPERATION

Voltage

Continuous Intermittent
discharge discharge

Elapsed time of discharge
FIGURE 3.12 Effect of intermittent discharge on battery capacity.

3.2.7 Type of Discharge (Continuous, Intermittent, etc.)

When a battery stands idle after a discharge, certain chemical and physical changes take place which
can result in a recovery of the battery voltage. Thus the voltage of a battery, which has dropped
during a heavy discharge, will rise after a rest period, giving a sawtooth-shaped discharge, as illus-
trated in Fig. 3.12. This can result in an increase in service life. However, on lengthy discharges,
capacity losses may occur due to self-discharge (see Sec. 3.2.12). This improvement, resulting from
the intermittent discharge, is generally greater after the higher current drains (as the battery has the
opportunity to recover from polarization effects that are more pronounced at the heavier loads).
In addition to current drain, the extent of recovery is dependent on many other factors such as the
particular battery system and constructional features, discharge temperature, end voltage, and length
of recovery period.
The interactive effect on capacity due to the discharge load and the extent of intermittency is
shown in Fig. 3.12. It can be seen that the performance of a battery as a function of duty cycle can
be significantly different at low and high discharge rates. Similarly, the performance as a function of
discharge rate can be different depending on the duty cycle.

3.2.8 Duty Cycles (Intermittent and Pulse Discharges)
Another consideration is the response of the battery voltage when the discharge current is changed
during the discharge, such as changing loads from receive to transmit in the operation of a radio
transceiver. Figure 3.13 illustrates a typical discharge of a radio transceiver, discharging at a lower
current during the receive mode and at a higher current during the transmit mode. Note that the
service life of the battery is determined when the cutoff or end voltage is reached under the higher
discharge load. The average current cannot be used to determine the service life. Operating at two
or more discharge loads is typical of certain electronic equipment because of the different functions
they must perform during use.
Another example is a higher-rate periodic pulse requirement against a lower background current,
such as backlighting for an LCD watch application, the audible trouble signal pulse in the operation