A battery state-of-charge indicator for electric wheelchairs

Deep-discharge-type lead-acid batteries used in most electric wheelchairs require acurrate battery state-of-charge indication 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 previously implemented on a microcomputer-based circuit. This adaptive monitoring technique enables the monitor to adjust to different battery sizes as well as the aging process. Several improvements are reported. A technique has been developed to enhance the acurracy and reduce the required rest period of the open-circuit voltage measurement. The open-circuit voltage recovery curve is approximated by 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 shows it to be superior in several respects