Formation processes. Processes of formation of the negative plates proceed in two stages. In first stage, PbO and basic lead sulfates are partly reduced to Pb, and partly react with H2SO4 to form PbSO4. The solution in the plate pores is neutral to alkaline. During the second formation stage, PbSO4 is reduced to Pb and the pore solution becomes acidic. Formation starts on the grid surface yielding a grey (Pb+PbSO4) zone. This zone then grows along the two surfaces of the plate. After covering the whole plate surface, the (Pb+PbSO4) zones grow towards the plate interior until the entire plate volume is formed. Structure of negative active material (NAM). The structure of the lead active material formed during the first stage comprises a network (skeleton) of bonded particles of irregular shape. During the second formation stage, small Pb crystals are formed on the skeleton surface. These crystals take part in the discharge process and build the energetic structure of NAM which determines the capacity of the plate. The ratio between the skeleton and energetic structures has an impact on the performance of the negative plates. Effect of expander on the formation processes and on NAM structure. Lignosulfonate contributes to the formation of the energetic structure of NAM built of small Pb particles providing high active mass surface. The size of the Pb particles depends on the type and amount of expander used. The stability of the energetic structure depends on the expander. At temperatures higher than 60oC, expanders disintegrate and the plates lose capacity.