Page 56 - Battery Reference Book

Effect of cell layout in batteries on battery characteristics 1/41
500 in series is required to produce this voltage. The
greater the number of strings of 48 in series connected
together the greater will be the current. It remains
to calculate the number of strings needed to produce
400 2000 the required current. The results in Table 1.16 and
Figure 1.19 clearly illustrate the method of obtaining
the number of strings in parallel of 48 cells in series
- to achieve any required current in the range 20- 100 A
Ln
a at 96V. It is interesting to note that the maximum
s 300 1500
I - current (Zmax) of 98 A and also the maximum capacity
m
._ B of 18 797 Ah (Table 1.16) would be obtained with
”
m I 10 strings in parallel of 48 cells in series.
5:
a,
c
W This is in agreement with the statement in Equation
2 200 1000 lu‘ 1.104 that maximum current is achieved when total
-7 X internal resistance of m x n cells equals total external
6
.! resistance of battery, i.e.
m
100 500 --Rc = Rext
If R, = 0.01 Wcell, Re,, is 0.5 !2 per 48 cells, i.e.
0.05 x m x n R per m x n cells
-2.0 -1.0 0.0 +1.0 +2.0 log mln 48
48 24 1612 6 8 4 3 2 1 n Then
1 2 3 4 86 1216 24 48 m
m 0.05 x m x n
-
~ -
Figure 1.18 Various configurations of 48 two-volt lead-acid cells 0. In 48
in series and parallel. Effect of configuration on Joule heating and
wattage i.e.
I00
90 i.e. 10 files of 48 cells in series.
By these methods, it is possible to design the layout
80 of a battery to produce any required combination of
current and voltage.
70 In all the above calculations, for simplicity, standard
values have been chosen for the internal resistance per
5 60 cell, R,, and the total external resistance of the bat-
tery, Re,,. In fact, of course, the particular values of
+: the parameters that the battery designer is presented
6
50
c with will affect battery performance parameters such
0
2 40 as current obtainable, wattage, joule heating and avail-
CJ able A h capacity. The effect of variations of resist-
30 ance parameters on battery performance will now be
examined.
20 Tables 1.17 and 1.18, respectively, calculate the
total internal plus external resistance (R*oT) and cur-
10 rents and capacities obtained for an 8 V configuration
of n = 12 strings, in parallel, of rn = 4 cells in series,
L-- and a 24V configuration of n = 4 strings of m = 12
0 20 40 60 80 100 120 140 160
Number of strings of 48 cells in series, n cells in series. For any particular fixed value of the
external resistance, R,,,, of the battery an increase of
the internal resistance per cell, R,, decreases the cur-
Total number of cells, m X n rent available and capacity and also the Joule heating
effect and wattage. The same comments apply when
Figure 1.19 Selection of number of‘ strings (n) of 48 two-volt the internal resistance is fixed and the external resist-
cells in series to produce a required current (/TOT) at 96V. ance is increased.
Internal resistance (R,) per cell = 0.1 2. External resistance per
cell = 0.05 R per 48 cells, i.e. 0.05 mn/48 Wcell In general, current available and capacity decrease
as the total electrical resistance (mln)R, +Rex, is