Imperfectness as a useful approach in battery monitoring

Ability to monitor the state of health and state of charge has become increasingly important. The earlier methods were entirely based on cell or block voltages. In the case of valve-regulated batteries the information of plain cell or block voltage can not be used straightforwardly as a sign of failure any more due to high normal deviation. It might be the long term trend of the voltages which contain the useful information. Battery impedance based methods are the most often discussed subject of today. Even if they look promising, there are still too much controversial matters in the air about their usefulness to substitute e.g. capacity testing. Discharging of battery is still the only reliable method by means of which the state-of-health and state-of-charge can be verified. This paper will describe a practical and simple method as well as circuitry to be used as a battery monitor. The method is based on measuring and comparing of the voltage differences of series connected battery blocks in a way which reduces the number of measuring channels as well as comparisons. For a typical 24-cell battery string containing four 12 V monoblocks the measuring channels and comparisons needed are three. The monitoring process contains also periodical discharge of battery. The discharge interval and time can be chosen to ensure sufficient reliability and availability performance of battery system.