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As part of an ongoing research program to find
the optimum battery system for selected applications, Cadex
has performed life cycle tests on NiCd, NiMH and Li-ion
batteries. All tests were carried out on the Cadex 7000
Series battery analyzers in the test labs of Cadex, Vancouver,
Canada. The batteries tested received an initial full-charge,
and then underwent a regime of continued discharge/charge
cycles. The internal resistance was measured with Cadex’s
Ohmtest™ method, and the self-discharge was obtained
from time-to-time by reading the capacity loss incurred during
a 48-hour rest period. The test program involved 53 commercial
telecommunications batteries of different models and chemistries.
One battery of each chemistry displaying typical behavior
was chosen for the charts below.
When conducting battery tests in a laboratory,
it should be noted that the performance in a protected environment
is commonly superior to those in field use. Elements of stress
and inconsistency that are present in everyday use cannot
always be simulated accurately in the lab.
The NiCd Battery — In terms of life cycling,
the standard NiCd is the most enduring battery. In Figure 8-1
we examine the capacity, internal resistance and self-discharge
of a 7.2V, 900mA NiCd battery with standard cells. Due to
time constraints, the test was terminated after 2300 cycles.
During this period, the capacity remains steady, the internal
resistance stays flat at 75mW and the self-discharge is stable.
This battery receives a grade ‘A’ for almost perfect performance.
The readings on an ultra-high capacity NiCd are
less favorable but still better than other chemistries in
terms of endurance. Although up to 60 percent higher
in energy density compared to the standard NiCd version, Figure 8-2
shows the ultra-high NiCd gradually losing capacity during
the 2000 cycles delivered. At the same time, the internal
resistance rises slightly. A more serious degradation is the
increase of self-discharge after 1000 cycles. This deficiency
manifests itself in shorter runtimes because the battery consumes
some energy itself, even if not in use.
Figure 8-1:
Characteristics of a standard cell NiCd battery.
This battery deserves an ‘A’ for
almost perfect performance in terms of stable capacity, internal
resistance and self-discharge over many cycles. This illustration
shows results for a 7.2V, 900mA NiCd.
Figure 8-2: Characteristics
of a NiCd battery with ultra-high capacity cells.
This battery is not as favorable
as the standard NiCd but offers higher energy densities and
performs better than other chemistries in terms of endurance.
This illustrations shows results for a 6V, 700mA NiCd.
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