05. Discharge Methods 4

GUIDE: Batteries in a portable world. 5. Discharge Methods 4

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5.3 Pulse Discharge

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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