The active material of the battery plates is contained in a number of filled tubes or pockets, securely mounted in a supporting and conducting frame or grid. The support is in good electrical contact with the tubes. The grid is a light skeleton frame, stamped from thin sheet steel, with extra reinforcing width at the top. The grids – as also all metallic parts of the cells – are nickel plated to prevent corrosion. The elements must remain covered with electrolyte; if they dry out, the negative plates will oxidize and will require a very long charge.