As with any other component in a PV system, efficiency is an important issue in component selection due to the relatively high cost of power generated by PV modules. The overall battery efficiency is specified by two efficiencies: the coulombic efficiency and the voltage efficiency.
The coulombic efficiency of battery the ratio of the number of charges that enter the battery during charging compared to the number that can be extracted from the battery during discharging. The losses that reduce coulombic efficiency are primarily due to the loss in charge due to secondary reaction, such as the electrolysis of water or other redox reactions in the battery. In general, the coulombic efficiency may be high, in excess of 95%.
The voltage efficiency is determined largely be the voltage difference between the charging voltage and voltage of the battery during discharging. The dependence of the battery voltage on BSOC will therefore impact voltage efficiency. Other factors being equal, a battery in which the voltage varies linearly with BSOC will have a lower efficiency than one in which the voltage is essentially constant with BSOC.
Battery efficiency for several types of batteries.