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The NiCd and the NiMH exhibit a high
self-discharge. If left on the shelf, a new NiCd loses about
10% of its capacity in 24 hours. A problem arises if the battery
self-discharges by itself within a day, a phenomenon that
is not uncommon, especially for an older battery.
The cause of high self-discharge is a
damaged separator, a thin, delicate insulator that isolates
the positive and negative cell plates. Once damaged, the separator
can no longer be repaired. External forces that harm the sensitive
separator are: uncontrolled crystalline formation due to lack
of exercise; poorly designed chargers that "boil"
the battery and plain old age. In some instances, cell manufacturers
use a separator material that is unsuitable.
The negative and positive plates are
rolled together and placed in a metal cylinder. The positive
plate is sintered and filled with nickel hydroxide. The negative
plate is coated with cadmium active material. The two plates
are isolated by a separator which are moistened with the electrolyte.
The self-discharge of a battery can most
conveniently be measured with a battery analyzer. A battery
should be discarded if the resulting capacity loss over 24
hours at a storage temperature of 22C (70F) is more than 30%.
No amount of cycling will improve this condition.
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