Page 41 - Battery Reference Book

1/26 Introduction to battery technology

During discharge, the e.m.f. decreases from 32.9% is quite small. However, on a 500-cell 100-V
2.0263V to 2.0082V, Le. by 0.0181V. Both the lead-acid battery this difference will cause a differ-
temperature increase and the lowering of acid ence in e.m.f. of approximately 3V which, in some
concentration contribute to a reduction in e.m.f. types of battery application, will be significant.
Alternatively,
8.312(273 + T1)2.303
Ecl = Eo - 1 .I 1 End-of-charge and
2 x 96500
end-of-discharge e.m.f. values
8.312(273 + T~)2.303
E,, =Eo -
2 x 96500 As seen in Table 1.5 the thermodynamically calculated
e.m.f. of a lead-acid battery at the end of charge is
Ec1 - Ec2 = 8'312 2'303 { (273 + TI)logacl about 2.03 V at 25T, corresponding to a sulphuric acid
96 500
concentration of about 29% by weight. The thermo-
dynamic e.m.f. is that of the charged battery on open
-(273 + TF) log aC2 circuit. Correspondingly, the open circuit e.m.f. of a
discharged battery (corresponding to 21% sulphuric
= 0.000 1984(28810g0.842 - 31310g0.436) acid concentration by weight) is about 2.00V at 25°C.
= 0.0181 v The actual, as opposed to the thermodynamic, e.m.f.
values differ from these values because of various
In general, charge and discharge effects. During passage of cur-
rent, polarization effects occur which cause variations
Eq - Ec, = 8'312 2'303 [ (273 + TI) log a,, of the voltage from the thermodynamically calculated
96 500
values during charge and discharge. Figure 1.8 shows
typical charge and discharge curves for a lead-acid
battery. During charge, the e.m.f. rises rapidly to a little
over 2.1 V and remains steady, increasing very slowly
- R2 [ (273 + TI) log acl as the charge proceeds. At 2.2 V, oxygen begins to be
-
F liberated at the positive plate, and at 2.3V hydrogen
1 is liberated at the negative plate. The charge is now
-(273 + TF) log a,, (1.73) completed and we enter the region of overcharge. Fur-
ther passage of current leads to the free evolution of
and gases and a rapid rise in e.m.f. The overcharge state
R x 2.303 is not only wasteful in charging current, which is now
E -Eo-- 2F (273 + TI) (1.74) entirely devoted to gas production rather than charging
c1 -
of the plates, but it causes mechanical damage to the
R x 2.303
E -Eo-- 2F (273 + TF) battery. If the charge is stopped at any point during
c2 -
charge (i.e. open circuit), the e.m.f. returns in time,
The effect of a change of sulphuric acid concentra- when free gases have become dislodged, to the equi-
tion and consequently activity on cell e.m.f. at 25°C is librium thermodynamic value for the particular state
shown in Table 1.5. The e.m.f. difference of 0.062V of charge (Table 1.5). During discharge, the cell e.m.f.
obtained between acid concentrations of 47% and drops rapidly to just below 2 V. The preliminary kink
Table 1.5 Effect of activity of sulphuric acid on cell emf. of a lead-acid battery at 25°C
Sulphuric acid Molality, rn Activity Activity e.rn.5
concentration (g H2S04/1000g water) coeficient, y (a = vm) (V)*
(% by weight) (see Table 1.3)
0.097 0.01 0.617 0.006 17 1.8999
0.196 0.02 0.519 0.010 38 1.9133
0.487 0.05 0.397 0.019 85 1.9299
0.970 0.1 0.313 0.031 3 1.9417
1.92 0.2 0.244 0.048 8 1.9530
4.67 0.5 0.178 0.089 1.9685
8.92 1 0.150 0.150 1.9819
16.39 2 0.147 0.294 1.9992
22.72 3 0.166 0.498 2.0127
28.16 4 0.203 0.812 2.0253
32.88 5 0.202 1.010 2.0308

*2.03059 - O.O295710g(l/u~~,,~)