Optimum battery design for applications in photovoltaic systems — theoretical considerations

In comparison to standard applications, lifetimes of lead–acid batteries in photovoltaic (PV) systems are shorter than one might expect. This investigation aims to identify reasons for the accelerated ageing. A detailed mathematical model of current, potential and acid distribution within the electrodes during normal operation is developed and used. Results show that the rather small currents in PV applications (on an average between I50 and I100) and the limited charging time cause problems, which are of minor relevance for standard applications. Small currents in conjunction with acid stratification cause a significant undercharging of the lower part of the electrodes, which again causes accelerated sulphation. Further, the number of sulphate crystals decreases with decreasing discharge current used for a full charge of the battery. This reduces the overall surface of the sulphate crystals and results in higher polarisation during the charging. The time taken for a battery cell to be completely charged is dominated by the positive electrode because it shows a high polarisation well before the electrode is completely charged. Simulations show that the charging time could be reduced if positive electrodes with less inner surface were to be used in batteries for PV systems. It is worth mentioning that the requirements for power are rather small in PV systems. This paper focuses on the qualitative results of the simulations and their interpretation. No models are explained in detail.