as indicated by the shapes of their curves. It is clear that some of the discharge curves
are essentially flat, others have more than one flat region, and still others have a
slanted and stretched S-shape, at times with an appreciable slope. These variants
can be simplified into three basic types of discharge curve shapes, as depicted in
Fig. 1.7. The reasons behind their general characteristics will be discussed later.
1.4.5 Cycling Behavior
In many applications a battery is expected to maintain its major properties over
many discharge - charge cycles. This can be a serious practical challenge, and is
often given a lot of attention during the development and optimization of batteries.
Figure 1.8 shows how the initial capacity is reduced during cycling, assuming three
different values of the Coulombic efficiency - the fraction of the prior charge capacity
that is available during the following discharge. This depends upon a number of
factors, especially current and depth of discharge in each cycle.
It is seen that even minor inefficiency per cycle can have important consequences.
For example, a half percent loss per cycle causes available capacity to drop to only
78% of the original value after 50 cycles. After 100 cycles, only 61% remains at that
rate. The situation is worse if the Coulombic efficiency is lower.
Applications that involve many cycles of operation require cells to be designed
and constructed such that the capacity loss per cycle is extremely low. This means
that compromises must be made in other properties. Supercapacitors are expected
to be used over a very large number of cycles. They typically have much lower
values of specific energy than electrochemical cells which are used for applications
in which the amount of energy stored is paramount.