 Profile Ni-MH
BatteryCycle
Life Characteristics |
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CYCLE
LIFE CHARACTERISTICS |
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 Ni-MH
BATTERY |
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 Specification
tables |
A
key determinant of the economic and practical feasibility of using
nickel-metal hydride
cells and batteries in portable electronic applications is the cell's
cycle life: the
ability of the
nickel-metal hydride cell to deliver acceptable capacity on a repetitive
basis. Nickel-metal hydride
cell cycle life has received intensive development
attention with the result that operational
life expectations are now competitive
with those for nickel-cadmium cells.
1. Limiting
Mechanisms The
life of any battery cell is determined by a combination of abrupt
failure events and
gradual cell deterioration. With the nickel-metal hydride cell,
abrupt failures, typically
mechanical events resulting in the cell either shorting or going
open-circuit, are
relatively rare and randomly distributed. Cell deterioration can
take two forms:>Oxidation
of the negative active material that increases cell internal resistance
resulting
in reduction of available voltage from the cell(MPV depression).
This also affects
the balance between electrodes within the cell and may possibly
result in reduced
gas recombination, increased pressure, and ultimately, cell venting.>Deterioration
of the positive active material results in less active material
being available
for reaction with the consequent loss of capacity.Both
phenomena result in a loss of usable capacity, but pose differing
design issues. Mid-point
voltage depression requires that the application design be able
to adapt to variations
in supply voltage from cycle to cycle. Capacity reduction simply
requires that
initial cell selection be sized to provide adequate capacity at
end-of-life for the desired
number of cells.The
actual mechanism that will determine cell life may vary depending
on application parameters
and the cell characteristics. Development work has reduced oxidation
in the
negative electrode reducing the depression in MPV as the cell ages.
1. Factors
Affecting Life The
way the nickel-metal hydride cell is designed into an application
can have dramatic
effects on the life of the cell. This is especially true of the
design of the charging
circuitry for the application to ensure adequate return of charge
while
minimizing overcharge. In fact, effective control of overcharge
exposure, time and charge
rate is the way of enhancing cell life.
2. Charge
Regime In
general, tailoring the charge regime to the application use scenario
is even more
important with nickel-metal hydride cells than with nickel-cadmium
cells because of the
increased subtlety of the voltage and temperature indications of
full charge and the
greater sensitivity of cell life to overcharge history.
Degree
of Overcharge Establishing
the appropriate degree of overcharge for a battery-powered application
is dependent
on the usage scenario. Some overcharge of the battery is vital to
ensure that
all cells are fully charged and balanced, but maintenance of full
charge currents for
extended periods once the cell has reached full charge can reduce
life. The three-step
charge process works to minimize some of the overcharge stress.
Details of the charging
process and the application context should be carefully reviewed
with the cell
manufacturer to ensure maximum cell life for the specific application.
Exposure
to High Temperatures In
general, higher temperatures accelerate chemical reactions including
those which contribute
to the aging process within the battery cell. High temperatures
are a particular
concern in the charging process as charge acceptance is reduced.
Sensing
the transition from charge to overcharge is also more difficult
at higher temperatures.
Although early data indicate that nickel-metal hydride cells may
tolerate
high-temperature charging better than standard nickel-cadmium cells,
close
consultation with the cell manufacturer is encouraged to select
a charging strategy that
meets operational requirements while maximizing cell life.
3. Cell
Reversal Discharge
of nickel-metal hydride batteries to the degree that some or all
of the cells
go into reverse can shorten cell life, especially if this overdischarge
is repeated routinely.
4. Prolonged
Storage under Load Maintaining
a load on a cell(or battery)past the point of full discharge may
eventually cause
irreversible changes in the cell chemistry and promote life-limiting
phenomena such
as creep leakage.
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| Overview |
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| Features |
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| Comparison
of Ni-MH and Ni-Cd Cells |
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| Major
applications |
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| Structural
designs |
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| Electrochemical
processes |
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| Discharge
characteristics |
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| Charge
characteristics |
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| Charging
methods |
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| Cycle
life characteristics |
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| Storage
characteristics |
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| Safety
characteristics |
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| Designing
for Ni-MH cells |
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| Battey
pack designs |
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| Battery
pack configurations designation system |
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| Precautions
for using Ni-MH batteries |
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| Battery
selection |
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Figure
23.Cycle life characteristics |
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