Page 79 - Battery Reference Book

1/64 Introduction to battery technology

2. Handling the pressure excursions that occur at cer- At the end of 10 min the internal pressure would be
tain parts of the chargeldischarge cycle due to the 10 x 0.1 x 760 = 760mmHg
presence of excess over stoichiometric amounts of
unrecombined oxygen or hydrogen. and at the end of 30,min the pressure would be

30 x 0.1 x 760 = 2280 mmHg
Even small departures from stoichiometric gas com-
position at certain periods of the chargeldischarge Le. three atmospheres. Clearly, the occurrence of
cycle would cause excessive gas pressure build-up internal pressures between one-tenth of an atmosphere
within the cell as shown below. and three atmospheres or higher has implications in the
Suppose that the dead space above the electrolyte problems of designing cells capable of withstanding
in a sealed cell was 100cm3 and that this space such pressure cycling.
was filled with gas having a composition 70% vlv It is very important, therefore, when designing
hydrogen and 30% oxygen, Le. 70 cm3 hydrogen sealed cells to avoid these problems as far as possible
and 30 cm3 oxygen. With effective recombination the by reducing gas production to an absolute minimum
30 cm3 of oxygen would consume 60 cm3 hydrogen during charge and discharge so that pressure excur-
leaving 10 cm3 of hydrogen unconsumed. If 100 ml sions are kept to a minimum. The non-stoichiometry
of 70:30 vlv hydrogedoxygen were being generated problem cannot be avoided, but, by keeping gas evo-
per minute the dead space would contain 1Oml of lution to a minimum, maximum internal cell pressure
unconsumed hydrogen, i.e. the internal cell pressure can be kept low and gas loss by venting avoided so
would be that in a subsequent stage of the charge/discharge cycle
excess oxygen and hydrogen can be recombined back
10 to water, thereby avoiding any long-term reduction in
- 0.1 x 760mmHg
=
100 electrolyte volume.