Abstract
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 strati®cation cause a signi®cant
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.
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