Page 76 - Battery Reference Book

Pressure development in sealed batteries 1/63
or at 25T, with E$: in volts, battery, this gas production represents a loss of water
from the electrolyte, which, unless the water is replen-
(1.126) ished, makes the acid more concentrated with con-
sequent adverse effects on battery performance and,
By means of these equations the equilibrium constant eventually, physical deterioration of the battery. The
of any reaction can be readily calculated from the concept of a sealed battery is to reduce or eliminate
standard e.m.f. of the reversible cell in which the gassing during charging and discharging of the battery
reaction occurs. so that water loss does not occur from the electrolyte.
The reaction in the Daniel1 cell, for example, is In this situation the battery can be fully sealed during
manufacture (with the proviso that a pressure relief
Zn(si + cu2- = Zn2+ + CU(S) valve is supplied to relieve gas pressure if untypica!
conditions develop) and will not require topping up
for the passage of 2F, i.e. n = 2, and the equilibrium with water during its life.
constant is given by Two main approaches have been made in the design
of sealed batteries. In one approach, gassing is con-
(1.127) trolled at a very low level by attention to battery design
such as using calcium-lead grid alloys and avoiding
the presence of free electrolyte by attention to charging
where the subscript, e, is used to show that the activ- methods. In the other method, any hydrogen or oxy-
ities are the values when the reacting system attains gen produced is recombined back to water by means
equilibrium. The activities of the solid zinc and cop- such as catalytic conversion or methods based on the
per are, as usual. taken as unity. The standard e.m.f. third electrode principle. Reconversion of hydrogen
of the cell, sLs seen above, is equal to E& -E;", Le. and oxygen to water does, however, present problems.
1.101 V at 25°C; hence, by Equations 1.126 and 1.127, It is a fact that towards the end of charge, and on over-
charge, the hydrogen and oxygen are not produced in
exactly the stoichiometric amounts as indicated by the
equation
H20 = H2 + 402
Were this the case the electrolysis gas would contain
The ratio of the activities of the zinc and copper ions 66.7% v/v hydrogen and 33.3% vlv oxygen (i.e. the
in the solution at equilibrium will be approximately stoichiometric composition) and, provided the recomb-
equal to the ratio of the concentrations under the ination device were efficient, complete recombination
same conditions; hence, when a system consisting of of these gases to water would occur:
metallic zinc and copper and their bivalent ions in H2 + 102 = H2O
aqueous solution attains equilibrium, the ratio of the
zinc ion to the cupric ion concentration is extremely In fact, as will be discussed later, in the later stages of
large. If zinc is placed in a solution of cupric ions, the charge and the earlier stages of discharge, the hydro-
latter will be displaced to form metallic copper until gen-oxygen mixture in a sealed battery has a com-
the czn2-/ccu2+ ratio in the solution is about In position which is non-stoichiometric. An excess of
other words, the zinc will displace the copper from the either hydrogen or oxygen remains over the stoichio-
solution until the quantity of cupric ions remaining is metric composition and this excess does not, of course,
extremely small. react. Therefore, a pressure build-up occurs in the cell.
Whilst cells can be designed to tolerate a certain pres-
1.24 Pressure development in sealed sure build-up, there is a practical limit to this. During
a complete charge/discharge cycle, the total amounts
of hydrogen and oxygen produced are stoichiometric;
This discussion is concerned with the development and consequently, in the long term, complete recomb-
of gas pressures due to hydrogen and oxygen in ination would occur and internal cell pressure would
sealed lead-acid batteries, although the comments be relieved, i.e. cell pressure would not continually
made would zipply, equally, to other types of batter- increase with continued cycling of the battery but a
ies where hydrogen and oxygen are produced. As a maximum pressure excursion would occur within each
lead-acid battery approaches the end of charge, i.e. single chargeldischarge cycle.
its voltage exceeds 2V, and also on overcharge, an
increasing proportion of the charge current is used up 1.24.1 Overvoltage
not in charging the plates but in electrolysing the sul-
phuric acid to produce hydrogen and oxygen. Besides At a platinized platinum cathode, hydrogen is liber-
being wasteful in charging current and damaging to the ated practically at the reversible hydrogen potential