The process of negative plate discharge in lead acid batteries from two manufacturers has been investigated at low current densities. The discharge curves and specific capacities, at several H2SO4 concentrations (2.3-7.0 M) and current densities (0.5-3.2 mA cm(-2)), showed that the negative plate discharge includes solid state reactions. When these results were put beside those previously obtained for positive plates, they showed that the processes of positive and negative plate discharges are different and an explanatory theory has been proposed. By analysing the utilization coefficient together with the specific capacities, the different roles of the nucleation processes at the positive and negative plates were revealed. Finally, the determination of the energetic coefficient proved to be useful in evaluating the quality of manufacture of the active material.