The influence of different operating conditions, especially over-discharge, on the lifetime and performance of lead/acid batteries for photovoltaic systems

A study is made of the influence of different operating conditions (i.e., cycling, self-discharge, floating) on the lifetime of different types of lead/acid battery (vented: flat plate, tubular; valve-regulated: gel, AGM) recommended by manufacturers for solar applications. A bench test has been conducted for more than six years and is based on practical photovoltaic operating conditions. The best performance is exhibited by gel batteries and by vented, pasted-plate batteries with electrolyte agitation. The over-discharge of series-connected cells in large solar battery packs influences the lifetime. Results are given for the discharge and over-discharge characteristics of lead/acid batteries, i.e., battery voltage, cell voltage, positive and negative electrode potentials, gassing rate, oxygen evolution, and sulfuric acid density. The same characteristics are also examined for the recharge of over-discharged batteries. In the over-discharge period, inactive PbO₂ and Pb are activated and can react in a further electrochemical discharge or/and in a chemical comproportionation reaction.