The capacity of the positive plate of the lead-acid battery is determined by the number of the active centers in positive active material (PAM) where the reaction PbO2-->PbSO4 proceeds and by the resistance of the interface corrosion layer (CL) 1 AMCL (thin PAM layer interconnecting the PAM and the CL). It was established that the number of the active centers is high if the positive plate discharge is performed with low current and if the charge (at the initial period) is performed with high current. During discharge the active centers from the outer layer of the plate are exhausted and the reaction of the PbO2 reduction approaches the inner part of the plate near the interface and the interface itself. If the pH of the pore solution is low the reduction of the PbO2 proceeds to PbSO4. At the subsequent charge with sufficiently high current a maximum appears on the potential transient. When the H2SO4 flux to the interface is impeded, the pH of the solution in the pores in the interface is high and the reduction of the PbO2 proceeds to PbOn (1 < n < 1.5). As the PbOn has high electrical resistance, during the subsequent charge with high current a peak appears at the beginning of the potential transient. Its height depends on the resistance of the interface. On the basis of the appearance of a maximum or a peak the state of change and the state of health of the positive plate can be determined.