Battery State of Charge Determination in Photovoltaic Systems

The ability to determine the battery state of charge in a system at any time is very important from system design point of view. Knowing the state of charge continuously makes system energy management possible. In addition, for a stand-alone photovoltaic system the required battery capacity can be more accurately determined, which implies higher reliability of the energy supply and lower system cost. The present study introduces a new on-line method of battery state of charge determination in a working photovoltaic system. The proposed method is based on extending the known open-circuit voltage-charge relation to the operating battery voltage-charge relation and combining it with the ampere-hour accounting method for finite periods of time and averaging out random measurement errors. The method uses electrical measurement of voltage and current at sampling times. The new method was compared experimentally with charge determination methods for a battery at rest and shows good agreement at the control points. The method was developed and validated experimentally for operating conditions that prevail in autonomous photovoltaic systems.