Page 55 - Battery Reference Book

1/40 Introduction to battery technology

Total -5 of six cells in series. Both supply a maximum current
resistance 0 of 96 A, the former at 8 V and the latter at 12 V. The
110 - higher e.m.f. battery has a higher available wattage
100 - 4 7100 and a greater Joule heating effect in parallel with its
greater internal and total electrical resistance. There is
x-x
90 - more available capacity from the battery configuration
80 - comprising longer strings of cells in series, i.e. higher
values of rn (Table 1.13). If a voltage greater or less
70 - than 12 V is required, two choices are available:
1. Arrange the configuration of cells such that a higher
(or lower) voltage is obtained at the cost of not
operating at maximum current producing efficiency.
2-40 % 2. Increase (or decrease) the total number of cells in
0 the battery.
m
30 - -30 3
-1
20 - - 20 1.19.1 Operation at lower current efficiency
10 - - 10
As shown in Figure 1.17, the maximum current that
ow-91 I I I I I -0
-2.0 -1.0 0.0 +1.0 f2.0 log mln could be expected for a battery consisting of 48 two-
48 241612 68 4 3 2 1 n volt cells with an internal resistance, R,, of 0.1 Wcell
1 23486121624 48 m and a total external resistance, Rex,, of 0.lOQ is
105 A. Because of the Dractical difficulties of arrann-
Y
Figure 1.17 Various configurations of 48 two-volt lead-acid cells ing 48 cells in series-parallel, the maximum current
in series and parallel. Effect of configuration on current, voltage achieved was 96A (i.e. 91.4% of theoretical maxi-
and total electrical resistance. Internal resistance, R, = 0.1 Wcell.
Total external electrical resistance (Rea) = 0.05 Q mum) at 8V (12 strings of four cells) or at 12V (eight
strings of six cells). Table 1.14 shows the currents and
voltages that would be achieved with a wide range of
Table 1.15 Characteristics of two battery configurations other cell configurations for a 48-cell battery.
It is seen (Figure 1.17) that, depending on cell con-
12 strings 8 strings figuration, voltages between 96V and 2V can be
of 4 cells* of 6 cells? obtained. When the voltage is 96V, the current pro-
duced is only about 19% of the maximum value of
Imax (A) 96 96 96A that can be obtained as discussed above. Con-
- mE, versely, when the voltage is 2V only 37% of the
-
(m/n 1% + Rext maximum current is produced. Figure 1.17 indicates
External resistance, ReXt (9) 0.05 0.05 that intermediate voltages, say 24V, can be obtained
with not too serious a loss of current from the maxi-
Internal resistance, RTOT mum. There is a steady decrease in wattage available
= (m/n)Rc(S22) 0.0333 0.075 in going from an all-series battery (1899 W) to an all-
Total resistance, RTOT parallel battery (76.8 W). Joule heating and wattage
decrease in the same order (Figure 1.18).
= Rext + (m/n)Rc(W 0.0833 0.125
Voltage, ETOT = ImaxRTOT (v) 8 12
Wattage, V x I,, 768 1152 1.19.2 Change in number of cells in a battery
Joule heating, I~J~oT 183.6 275.6 It has been shown that, for a battery consisting of 48
(cas)
4.18 two-volt cells (internal resistance, R, = 0.1 Wcell and
~
total external resistance of battery, Rex, = 0.05 Q), the
Ah capacity, ~ mhEToT (Ah) 1536 2304 maximum achievable current (Zma) of 96 A (105 A the-
RTOT
(where h = duration of discharge of each cell = 4 h) oretical) is obtained with 12 strings of four cells in
series or eight strings of six cells in series. The volt-
* = 12, m = 4, N = 48 ages, respectively, of these two - 1 cell configurations
+n = 8, m = 6, N = 48 are 8 V and 12 V. The following treatment applies if it
is required to obtain the maximum current at a higher
(or lower) voltage, or a higher maximum current at the
It is seen that, with a set of 48 two-volt cells, to same voltage.
obtain near maximum current we have a choice of If the required battery voltage is 96 V, then, clearly,
using 12 strings of four cells in series or eight strings a basic configuration of 48 two-volt cells (i.e. m = 48)