Characterisation of photovoltaic batteries using radio element detection: the influence and consequences of the electrolyte stratification

In photovoltaic (PV) applications, electrolyte stratification is a common phenomenon, which favours the formation of irreversible lead sulphate especially at the bottom of the electrodes. This degradation leads to premature capacity losses. In this paper, the influence and the consequences (in terms of electrodes degradation) of electrolyte stratification have been studied using experimental cells. Firstly, in order to measure local current and voltage, an experimental cell has been realised, consisting of two identical systems with three electrodes (two negative electrodes and one positive electrode) connected in parallel and located one above the other in the same battery container. A cycling procedure which favours acid stratification has been performed on this experimental cell. After eight cycles, contrasting changes have been observed in the two systems. The stratification phenomenon induces a vertical potential gradient which is at the origin of the unbalanced behaviour between the two systems: the lower system is deeply discharged and partially recharged and the opposite behaviour is observed in the upper system. Consequently, the lower part of the system tends to show considerable sulphation due to the cycling in a low state of charge. This degradation has been characterised using radioelement detection. After incorporation of H235SO4 into the electrolyte, the distribution of radioactive lead sulphate (Pb35SO4) on the electrodes was followed by laser reading of photostimulable screens. This allowed recreating the kinetics by which irreversible sulphate appears, and to show its link with the phenomenon of electrolyte stratification.