Information on lead acid batteries
Ageing mechanisms
The ageing mechanisms of batteries are the actual chemical or mechanical events
that cause battery failure. These failures may be long processes of degradation
or short catastrophic events and in some instances a combination of the two. In
this text we are concentrating on the longer processes that lead to battery
failure. Regarding lead acid batteries the ageing mechanisms are quite well
known, if not completely understood. It should be noted that different battery
designs are inherent resistant to different ageing mechanisms making the choice
of that battery preferable if one ageing mechanism, or a specific combination
of ageing mechanisms are expected to be critical.
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Corrosion of the positive grid:
A process where the positive plate or the battery posts, usually near the
connection to the casing, corrodes causing a restriction in the current flow
and eventually failure either by not allowing current to pass or cracking the
outer casing.
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Hard / irreversible sulphation:
A build-up on the active mass of the plates of that prevents the chemical
exchange between the electrolyte and the active material. Some sulphation can
be reversed, but hard sulphation permanently reduces the available capacity of
the battery.
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Shedding:
A process where the active mass of the plates falls off and collects at the
bottom of the battery. This reduces the amount of active mass and thus the
available capacity of the battery. It can eventually cause battery cell
shorting if enough material builds up at the bottom of the battery case to
cause an electrical short circuit between the positive and negative plates.
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Water loss / drying out:
When a battery is being charged while at a high state of charge some of the
water in the electrolyte is split into hydrogen and oxygen (gassing) and can
escape the battery casing. In flooded batteries the top of the plates can be
exposed causing a drying out of the active material, which permanently damages
the battery. As water is lost in sealed batteries it reduces the available
electrolyte, thus reducing the available capacity of the battery.
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AM degradation:
Over time the Active Mass on the battery plates degrades and changes structure,
loosing some of the electric transfer properties and reducing the capacity of
the battery.
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Electrolyte stratification:
As a battery rests, the electrolyte in the battery separates by density with
the higher density electrolyte settling to the bottom. As the battery undergoes
charge and discharge cycles, different parts of the battery are exercised,
focusing activity on very specific parts of the battery plates causing them to
wear faster. The electrolyte can be mixed actively, using bubblers or other
means, or passively, such as by charging flooded batteries at a high voltage to
cause gassing, which mixes the electrolyte.