Page 70 - Battery Reference Book

Heating effects in batteries 1/56

convection and conduction to the surroundings when the resistance between two cells A and D in parallel is
J - AH is positive and a gain of calories from the denoted by Rz and the current by 12. Such conduction
surroundings when J - AH is negative. This factor between cells can occur, for example, if free electrolyte
is ignored in the calculations that follow but is easily has been spilt on the battery top, providing a low-
allowed for in any such calculations. resistance conducting path for the current. A particular
Consider the case of a 2V lead-acid battery that example of this is batteries used in electric locomotives
requires charging to 30Ah at 20°C during 2h. The used in coal mines. Such batteries, in addition to free
cooling effect is: sulphuric weight, have coal dust on the battery tops.
A slurry of coal dust in acid is a very good conductor
3600
Afj = __- 30 (2.01 - 293 x 0.000 25) and, as will be seen, such low-resistance conducting
':
4.18 paths can conduct high currents and lead to localized
= 50 041 'calf2 h
high ohmic (Joule) heating effects on the battery top,
= 6.950 calk which in certain circumstances can generate so much
heat that battery ignition is a possibility.
The heating effect, J, will depend on the charging
current and the cell resistance. If J - 6.950 is negative
the cell will cool down; if J - 6.950 is positive the
cell will heat up. Table 1.23 tabulates the heat losses or
heat gains obtained with cells of four different assumed
internal resistances, namely, 0.002, 0.005, 0.015 and
0.027 Q, when charged at various currents between
20 and 1000A. Cell cooling, during charge, occurs
only at relatively low charging currents of the order of
20-50A. Cell heating would not be a serious problem
at charging currents under 200-300A with cells in the
0.002-0.027 Q electrical resistance range, especially
when the natural heat losses from the cell due to
convection and conduction are taken into account.
E = I, R, = I,R2
1.22.3 Localized heating effects in batteries I = I, 4- I2
Additional h[eating effects of a localized nature can I,=l- R2
arise in batieries due to low-resistance short-circuit R, + R,
conducting paths being formed between cells in bat- R,
teries arranged in series. Figure 1.55 shows such an 12=1-
effect where the resistance between batteries A and B Rl + R2
in series is denoted by R1 and the current by 11, and Figure 1.55 Distribution of current in a divided circuit on a battery

Table 1.23 Heating and cooling effects during charge of 2V lead-acid battery at 20°C. Influence of charge current and cell resistance
Charging Assumed cell resistance (a)
-
current 0.002 0.005 0.015 0.021
(A)
Cell heating (+) e.m.5 Cell heating (+) em.$ Cell heating (+) e.m.5 Cell heating (+) em.$
or cooling (-) change* or cooling (-) change* or cooling (-) change* or cooling (-) change*
(calls) (calfs) (calls) (calls)

20 -6.76 0.04 -6.41 0.10 -5.52 0.3 -4.31 0.54
50 -5.76 0.10 -3.96 0.25 f2.02 0.75 +9.20 1.35
100 -2.17 13.20 -5.01 0.5 +28.93 1.5 +57.64 2.7
200 +12.17 13.40 +40.89 1 .o +136.57 3.0
300 f36.07 13.60 +100.69 1.5
400 +69.53 10.80 +184.41 2.0
500 +112.55 1 .o +292.05 2.5
600 +l63.13 1.2
1000 +471.05 2.0
1500 +717.0 3.0
*Current x cell resistance