State-Of-Charge Determination for Lead-Acid Batteries in PV-Applications

Almost all stand-alone PV-systems require an energy storage unit to provide energy during cloudy days or at night. Lead acid batteries are generally used. To extend battery lifetime, the battery must be protected from deep discharge and overcharge. Advanced battery operation control is based on the battery state of charge (SOC) rather than on voltage measurements. Up to now the typical method to determine battery SOC is the ampere-hour balance. But, in PV-systems long time periods of low battery SOC make the necessary recalibrations for this method difficult. This paper presents three alternative methods for the determination of the SOC of lead acid batteries.