Page 86 - Lindens Handbook of Batteries

FACTORS AFFECTING BATTERY PERFORMANCE 3.5

To further clarify this nomenclature system, the designation for a C/10 discharge rate for a battery
rated at 5 Ah at the 5 h rate is
0.1 C 5
In this example, the C/10 rate is equal to 0.5 A, or 500 mA.
It is to be noted that the capacity of a battery generally decreases with increasing discharge cur-
rent. Thus the battery rated at 5 Ah at the C/5 rate (or 1 A) will operate for 5 h when discharged at
1 A. If the battery is discharged at a lower rate, for example the C /10 rate (or 0.5 A), it will run for
more than 10 h and deliver more than 5 Ah of capacity. Conversely, when discharged at its C rate (or
5 A), the battery will run for less than 1 h and deliver less than 5 Ah of capacity.

Hourly Rate. Another method for specifying the current is the hourly rate. This is the current at
which the battery will discharge for a specified number of hours.
E-Rate. The constant power discharge mode is becoming more popular for battery-powered appli-
cations. A method, analogous to the C rate, can be used to express the discharge or charge rate in
terms of power
P = M × E n
where P = power (W)
E = numerical value of the rated energy of the battery in watthours (Wh)
n = time, in hours, at which the battery was rated
M = multiple or fraction of E
For example, the power level at the 0.5E or E /2 rate for a battery rated at 1200 mWh, at the 0.2E
5
5
or E/5 rate, is 600 mW.
3.2.3 Mode of Discharge (Constant Current, Constant Load, Constant Power)

The mode of discharge of a battery, among other factors, can have a significant effect on the per-
formance of the battery. For this reason, it is advisable that the mode of discharge used in a test or
evaluation program be the same as the one used in the application for which it is being tested.
A battery, when discharged to a specific point (same closed-circuit voltage, at the same discharge
current, at the same temperature, etc.) will have delivered the same ampere-hours to a load regardless
of the mode of discharge. However, as during the discharge, the discharge current will be different
depending on the mode of discharge; the service time or “hours of discharge” delivered to that point
(which is the usual measure of battery performance) will likewise be different.
Three of the basic modes under which the battery may be discharged are:
1. Constant resistance. The resistance of the load remains constant throughout the discharge (the
current decreases during the discharge proportional to the decrease in the battery voltage).
2. Constant current. The current remains constant during the discharge.
3. Constant power. The current increases during the discharge as the battery voltage decreases, thus
discharging the battery at constant power level (power = current × voltage).
The effect of the mode of discharge on the performance of the battery is illustrated under three
different conditions in Figs. 3.4, 3.5, and 3.6.
Case 1: Discharge loads are the same for each mode of discharge at the start of discharge.
In Fig. 3.4, the discharge loads are selected so that at the start of the discharge, the discharge cur-
rent and, hence, the power are the same for all three modes. Figure 3.4b is a plot of the voltage dur-
ing discharge. As the cell voltage drops during the discharge, the current in the case of the constant
resistance discharge, reflects the drop in the cell voltage according to Ohm’s law:
I = V/R
This is shown in Fig. 3.4a.