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 investig ation aims to identify reasons for the accelerated ageing. A detailed mathe matical model of current, potential and acid distribution within the electr odes during normal operation is developed and used. Results show that the rather small currents in PV applications (on an avera ge between I-50 and I-100) and the limited charging time cause problems, wh ich are of minor relevance for standard applications. Small currents in con junction with acid stratification cause a significant undercharging of the lower part of the electrodes, which again causes accelerated sulphation. Fu rther, the number of sulphate crystals decreases with decreasing discharge current used for a full charge of the battery. This reduces the overall sur face of the sulphate crystals and results in higher polarisation during the charging. The time taken for a battery cell to be completely charged is do minated by the positive electrode because it shows a high polarisation well before the electrode is completely charged. Simulations show that the char ging time could be reduced if positive electrodes with less inner surface w ere 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. (C) 2001 Elsevier Scienc e B.V. All rights reserved.