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How to Restore and Prolong Nickel-based Batteries
The effects of crystalline formation are most
pronounced if a nickel-based battery is left in the charger
for days, or if repeatedly recharged without a periodic full
discharge. Since most applications do not use up all energy
before recharge, a periodic discharge to 1V/cell (known as
exercise) is essential to prevent the buildup of crystalline
formation on the cell plates. This maintenance is most critical
for the NiCd battery.
All NiCd batteries in regular use and on standby
mode (sitting in a charger for operational readiness) should
be exercised once per month. Between these monthly exercise
cycles, no further service is needed. The battery can be used
with any desired user pattern without the concern of memory.
The NiMH battery is affected by memory also,
but to a lesser degree. No scientific research is available
that compares NiMH with NiCd in terms of memory degradation.
Neither is information on hand that suggests the optimal amount
of maintenance required to obtain maximum battery life. Applying
a full discharge once every three months appears right. Because
of the NiMH battery’s shorter cycle life, over-exercising
is not recommended.
A hand towel must be cleaned periodically. However,
if it were washed after each use, its fabric would wear out
very quickly. In the same way, it is neither necessary nor
advisable to discharge a rechargeable battery before each
charge — excessive cycling puts extra strain on the battery.
Exercise and Recondition — Research has
shown that if no exercise is applied to a NiCd for three months
or more, the crystals ingrain themselves, making them more
difficult to break up. In such a case, exercise is no longer
effective in restoring a battery and reconditioning is required.
Recondition is a slow, deep discharge that removes the remaining
battery energy by draining the cells to a voltage threshold
below 1V/cell.
Figure 10-2: Exercising and reconditioning
batteries on a Cadex battery analyzer.
This illustration shows the battery
voltage during a normal discharge to 1V, followed by the secondary
discharge (recondition). Recondition consists of a discharge
to 1V/cell at a 1C load current, followed by a secondary discharge
to 0.4V at a much reduced current. NiCd batteries affected
by memory often restore themselves to full service.
Tests performed by the US Army have shown that
a NiCd cell needs to be discharged to at least 0.6V to effectively
break up the more resistant crystalline formation. During
recondition, the current must be kept low to prevent cell
reversal. Figure 10-2 illustrates the battery voltage
during normal discharge to 1V/cell followed by the secondary
discharge (recondition).
Figure 10-3 illustrates the effects
of exercise and recondition. Four batteries afflicted with
various degrees of memory are serviced. The batteries are
first fully charged, then discharged to 1V/cell. The resulting
capacities are plotted on a scale of 0 to 120 percent
in the first column. Additional discharge/charge cycles are
applied and the battery capacities are plotted in the subsequent
columns. The solid black line represents exercise, (discharge
to 1V/cell) and the dotted line recondition (secondary discharge
at reduced current to 0.4V/cell). On this test, the exercise
and recondition cycles are applied manually at the discretion
of the research technician.
Figure 10-3: Effects of exercise and
recondition.
Battery A improved capacity on exercise
alone; batteries B and C required recondition. A new battery
with excellent readings improved further with recondition.
Battery A responded well to exercise alone and
no recondition was required. This result is typical of a battery
that has been in service for only a few months or has received
periodic exercise cycles. Batteries B and C, on the other
hand, required recondition (dotted line) to restore their
performance. Without the recondition function, these two batteries
would need to be replaced.
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