The structural changes accompanying capacity loss in the PbO2/PbSO4 electrode were followed using differential thermal analysis. The thermal decomposition mechanism of formed plates was found to differ, depending on the method of manufacture. All cycled plates, however, gave the same decomposition mechanism after a few cycles. The major changes in the DTA curves, as the positive electrode was cycled to failure, was the gradual disappearance of the exothermic peak at 200*C and the endothermic peak at 358~ It is believed that these peaks are associated with an electrochemically active amorphous form of PbO2. As the electrochemically active PbOs is cycled to failure it is converted to an electrochemically inactive form of PbO2. This latter form of PbO2 gives DTA results similar to those obtained on reagent PbOz. The continual conversion of electrochemically active PbO2 to the electrochemically inactive PbO2 is one of the major factors that accounts for the loss in battery capacity and ultimate failure. ElectrochemicalSociety Active Member.