Imperfectness as a useful approach in battery monitoring

Ability to monitor secondary battery state of health and state of charge has become increasingly important. Earlier methods were entirely based on cell or block voltages. In the case of valve-regulated batteries, information of plain cell or block voltage can no longer be used as a sign of failure due to high normal deviation. Discharging a battery is still the only reliable method by which battery state-of-health and state-of-charge can be verified. This paper describes a practical and simple method, as well as the circuitry used, for a battery monitor. The method is based on measurement and comparison of the voltage differences of series connected battery blocks in a way which reduces the number of measurement channels as well as comparisons. For a typical 24-cell battery string containing four 12 V monoblocks, the measuring channels and comparison needed are three. The monitoring process also contains periodical battery discharges. The discharge interval and time can be chosen to ensure sufficient reliability and availability performance of the battery system