Experimental Characterization of Lead – Acid Storage Batteries used in PV Power Systems

Abstract

Storage batteries are indispensable in all standalone solar electric systems (PV power systems). Their efficiency and life time affects significantly the overall PV system performance and economics. Batteries specified especially for use in PV systems have to be distinguished with standing of a very deep discharge rate and high cycling stability. A lead acid battery has been exposed to experimental tests to determine its characteristic parameters by charging and discharging processes. The internal resistance of the battery is a reliable key for determination of its state of charge, its value increases with increasing of the stored energy. At the same time the specific gravity of the electrolyte decreases linearly with the degradation of ampere hour capacity. The experiments have shown that the battery internal temperature doesn’t change significantly from the ambient temperature during charge and discharge processes. The experimental tests have proved that a regular lead acid battery cell will be not more rechargeable if it is fully discharged. This issue requires using always a controllable battery charger within the PV power systems to protect the storage batteries against deep discharge and extremely over charge. Such equipment will extend the life time of the battery and consequently improve the economic feasibility and reliability of the PV power systems. In addition, the tests have shown that the watt hour efficiency of a battery is considerably less than the ampere hour efficiency, which advices to depend more on the watt hour efficiency when designing storage battery systems to secure higher reliability. Moreover, depending on an earlier developed algorithm for determination of the ampere hour capacity of a battery cell,a new similar algorithm based on specific gravity and cell voltage have been developed which enables also the determination of the ampere hour capacity from the implemented tests on the new battery. This algorithm enables the correct settings of the limits of charge -discharge hysteresis of the battery charger in order to avoid extremely deep discharge and over charge of the battery.