Page 53 - Battery Reference Book

1/38 Introduction to battery technology

As As the maximum current, I,,, is obtained when
NnE, (1.102) (rn/n)R, = RCxt it is possible to obtain I,, graphically
by plotting
-
NRc +Rex,,* - ITOT
m
--Rc
(1.104) __
Rext
or, better:
m
-Re
log n
Rat
and
against the calculated value of ITo~ to read off the
(1.106) maximum current, I,,, corresponding to
m
If we consider the example of a battery comprising -Rc
48 two-volt cells (i.e. N = 48), each cell having an L=1
e.m.f., E,, of 2 V, it is required to calculate the number, Rext
n, of files in parallel of rn cells in series to achieve the or
m
maximum current, I,,,, from the battery. It is assumed -Rc
that the battery has a total external electrical resistance, log n
0
=
Rex,, of 0.05 Q and that the internal resistance per cell, Rext
R,, is 0.1 Q, i.e. the total internal resistance of a 48- The data for the above battery are tabulated in
cell battery is RT~T rn/n x 0.1. The total internal plus Tables 1.13 and 1.14 and plotted in Figure 1.16. It
external resistance of the battery is R = (rn/n)R, + can be seen clearly from the log plot in Figure 1.17
ReXtQ. Considering all the possibilities between rn = that a maximum achievable current of 96A will be
48, n = 1, i.e. 48 cells in series, and rn = 1, n = 48, obtained from a battery comprising 12 strings of four
Le. 48 cells in parallel, and all possible configurations cells in series (i.e. n = 12, rn = 4), or eight strings of
of cells between these limits, it is possible to calculate six cells in series (i.e. n = 8, rn = 6). If currents other
the total current, ITOT, produced by the battery: than the maximum are required, the values of n and
rn required to achieve these currents can be deduced
(1,101) from Figure 1.17. The other characteristics of these
two battery configurations are shown in Table 1.15.

900 0
800 -

700 -
-
600
5 - 500 -3
L k-
6 400 -2

0 5 10 15 20 25 30 35 40 45 50
Number in series = 2
Number of strings n
Figure 1.16 48-cell 2V/cell lead-acid battery. Demonstration that maximum internal resistance of battery, (m/n)R, equals total external
resistance (Rea), i.e. (m/n)R, = Rext or mRclnRea = I