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 columbic efficiency and the voltage efficiency.
The columbic 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 columbic 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 columbic 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.
Energy, Volumetric and Power Density
Energy density is a parameter used chiefly to compare one type of battery system to another. The energy density of a battery is the capacity of the battery divided by either the weight of the battery, which gives the gravimetric energy density in Wh/kg, or by the volume, which gives a volumetric energy density in Wh/dm3 (or Wr/litre3). A battery with a higher energy density will be lighter than a similar capacity battery with a lower energy density. In portable systems, the energy density is a critical parameter but in conventional PV systems which provide power for a stationary object, the energy density may be less important. Nevertheless, the costs of transporting batteries to remote locations are considerably high, so a high energy density battery is typically an advantage.
The power density of a battery is related to its energy density, as well as the ability of the battery to discharge quickly. While the power density is important in some applications, particularly transport, it is typically not critical in photovoltaic systems.