A BATTERY STATE-OF-CHARGE INDICATOR FOR ELECTRIC WHEELCHAIRS

Deep-discharge type lead-acid batteries are used in most electric wheelchairs. An accurate battery state-of-charge indicator is essential to prevent stranding and to provide economical operation of the wheelchair. A monitoring technique combining the open circuit voltage and the coulometric measurements had been implemented by the authors on a microcomputer-based circuit. The technique employs the coulometric measurement under loading conditions and open circuit voltage under no-load conditions. A 30-min rest period was used in measuring the open circuit voltage. An adaptive monitoring technique that enables the monitor to adjust to different battery sizes as well as the aging process was examined. Several improvements are made in this paper. A new technique has been developed to enhance the accuracy and to reduce the required rest period of the open circuit voltage measurement. The technique approximates the open circuit voltage recovery curve with two asymptotes on a semilog scale. The open circuit voltage is then extrapolated from the slope of the first asymptote before it fully stabilizes. The accuracy of the monitor has been verified in field tests, and comparison with a commercial battery monitor showed that the monitor in development is superior in several aspects.