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Battery chemistries react differently
to specific loading requirements. Discharge loads range from
a low and steady current used in a flashlight, to intermittent
high current bursts in a power tool, to sharp current pulses
required for digital communications equipment, to a prolonged
high current load for an electric vehicle traveling at highway
speed. Because batteries are chemical devices that must convert
higher-level active materials into an alternate state during
discharge, the speed of such transaction determines the load
characteristics of a battery. Also referred to as concentration
polarization, the nickel and lithium-based batteries are superior
to lead-based batteries in reaction speed. This reflects in
good load characteristics.
The lead acid battery performs best at a slow 20-hour discharge.
A pulse discharge also works well because the rest periods
between the pulses help to disperse the depleted acid concentrations
back into the electrode plate. In terms of capacity, these
two discharge methods provide the highest efficiency for this
battery chemistry.
A discharge at the rated capacity of 1C yields the poorest
efficiency for the lead acid battery. The lower level of conversion,
or increased polarization, manifests itself in a momentary
higher internal resistance due to the depletion of active
material in the reaction.
Different discharge methods, notably pulse discharging,
also affect the longevity of some battery chemistries. While
NiCd and Li-ion are robust and show minimal deterioration
when pulse discharged, the NiMH exhibits a reduced cycle life
when powering a digital load.
In a recent study, the longevity of NiMH was observed by
discharging these batteries with analog and digital loads.
In both tests, the battery discharged to 1.04V/cell. The analog
discharge current was 500mA; the digital mode simulated the
load requirements of the Global System for Mobile Communications
(GSM) protocol and applied 1.65-ampere peak current for 12 ms
every 100 ms. The current in between the peaks was 270mA.
(Note that the GSM pulse for voice is about 550 ms every 4.5 ms).
With the analog discharge, the NiMH wore out gradually,
providing an above average service life. At 700 cycles,
the battery still provided 80 percent capacity. By contrast,
the cells faded more rapidly with a digital discharge. The
80 percent capacity threshold was reached after only
300 cycles. This phenomenon indicates that the kinetic
characteristics for the NiMH deteriorate more rapidly with
a digital rather than an analog load.
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