In order for the number of individual photovoltaic installations, to increase rapidly, it is necessary for such systems to be reliable and cheaper. The analysis of these systems when running shows indeed that the weakest element is the battery, and extremely variable lengths of service life can be observed. It is, therefore, increasingly important to optimise this component and its management mode. In this study, post mortem analyses have been performed on batteries, which have been operating in PV installations. Irreversible sulphation has been identified as a major ageing effect in many investigations. Moreover, the positive electrodes were particularly affected by this degradation. The objective of this present work is the experimental observation of the evolution of the lead sulphate during deep and prolonged discharge of the positive electrodes. An experimental set-up was built to identify the influence of parameters (electrolyte specific gravity, temperature) on the lead sulphate distribution and “rechargeability”. These experiments have shown that there is a recrystallisation mechanism during prolonged discharge periods leading to low poor rechargeability.