Hydration: This failure mode was a severe problem in
military batteries. Hydration comes about in battery designs
where the sulfuric acid is the limiting reagent. That is, the
limiting reagent is that component which determines when
the chemical reaction has gone to completion. Whenever
sulfuric acid is the limiting reagent, the electrolyte in a
lead–acid battery approaches that of pure water when the battery is fully discharged. This is a common occurrence in
military vehicles because of the long storage times. In this
case the natural self-discharge completely discharges the
battery. This is rarely the case in commercial and passenger
vehicle applications. The result of this hydration condition
is that lead sulfate is dissolved as lead solubility increases
considerably in the low specific gravity electrolyte. The
problem arises when one attempts to pass a current though
this solution. There is a tendency for dendrites to form creating
high resistance shorts in the battery. These dendrites
find their way though the pores of the separator resulting
in micro-shorts between the plates. The observed effect
is that these batteries cannot hold a charge for extended
periods of time. Two solutions can be used to avoid the
hydration effect. The first is to add excess sulfuric acid
to the electrolyte. The other is to add a supporting electrolyte
(for instance, sodium sulfate) to the battery electrolyte
to ensure that the ionic strength of the electrolyte
never reaches that of pure water. This failure mode is not
reversible