Page 85 - Lindens Handbook of Batteries

3.4 PRINCIPLES OF OPERATION

1
2 4
Voltage 4 3 Voltage 2 3
End
5 1 voltage

0 Elapsed time of discharge 0 Elapsed time of discharge
FIGURE 3.3 (a) Battery discharge characteristics—voltage levels. (b) Discharge characteristics of a battery dis-
charged sequentially from high to lower discharge rates.

curve for the lower discharge rates, as shown by the dashed portion of the each curve, obviously is
lost. In some instances, a time interval is allowed between each discharge for the battery to equili-
brate prior to discharge at the progressively lower rates.
C-Rate.* A common method for indicating the discharge, as well as the charge current of a battery,
is the C rate, expressed as
I = M × C
n
where I = discharge current, A
C = numerical value of rated capacity of the battery, in ampere-hours (Ah)
n = time, in hours, for which rated capacity is declared
M = multiple or fraction of C

For example, the 0.1C or C/10 discharge rate for a battery rated at 5 Ah is 0.5 A. Conversely, a 250 mAh
battery, discharged at 50 mA, is discharged at the 0.2C or C/5 rate, which is calculated as follows:
1 0 050
.
M = = = 02
.
C n 0 250
.
*Traditionally, the manufacturers and users of secondary alkaline cells and batteries have expressed the value of the current
used to charge and discharge cells and batteries as a multiple of the capacity. For example, a current of 200 mA used to charge
a cell with a rated capacity of 1000 mAh would be expressed as C/5 or 0.2 C (or, in the European convention as C/5 A or 0,
2 CA). This method for designation of current has been criticized as being dimensionally incorrect in that a multiple of the capac-
ity (e.g. ampere-hours) will be in ampere-hours and not, as required for current, in amperes. As a result of these comments, the
International Electrotechnical Commission (IEC) Subcommittee SC-21A has published a “Guide to the Designation of Current
in Alkaline Secondary Cell and Battery Standards (IEC 61434),” which describes a new method for so designating this current.
In brief, the method states that the current (I) shall be expressed as
I (A) = C (Ah)/1(h)
n
t
where I = is expressed in amperes
C = is the rated capacity declared by the manufacturer in ampere-hours
n
n = is the time base in hours for which the rated capacity is declared
For example, a battery rated at 5 Ah at the 5 h discharge rate (C (Ah)) and discharged at 0.1I (A) will be discharged at 0.5 A
t
5
or 500 mA.
For this Handbook, the method discussed in the text, and not in this footnote, will be used.