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The NiMH Battery — Figure 8-3
examines the NiMH, a battery that offers high energy density
at reasonably low cost. We observe good performance at first
but past the 300-cycle mark, the performance starts to drift
downwards rapidly. One can detect a swift increase in internal
resistance and self-discharge after cycle count 700.
Figure 8-3:
Characteristics of a NiMH battery.
This battery offers good performance
at first but past the 300-cycle mark, the capacity, internal
resistance and self-discharge start to deteriorate rapidly.
This illustrations shows results for a 6V, 950mA NiMH.
The Li-ion Battery — The Li-ion
battery offers advantages that neither the NiCd nor NiMH can
match. In Figure 8-4 we examine the capacity and
internal resistance of a typical Li-ion. A gentle capacity
drop is observed over 1000 cycles and the internal resistance
increases only slightly. Because of low readings, self-discharge
was omitted for this test.
The better than expected performance of this
test battery may be due to the fact that the test did not
include aging. The lab test was completed in about 200 days.
A busy user may charge the battery once every 24 hours.
With such a user pattern, 500 cycles would represent
close to two years of normal use and the effects of aging
would become apparent.
Manufacturers of commercial Li-ion batteries
specify a cycle count of 500. At that stage, the battery capacity
would drop from 100 to 80 percent. If operated at 40°C
(104°F) rather than at room temperature, the same battery
would only deliver about 300 cycles.
Figure 8-4:
Characteristics of a Li-ion battery.
The above-average performance of
this battery may be due to the fact that the test did not
include aging. This illustration shows results for a 3.6V,
500mA Li-ion battery.
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