<P> Alkaline batteries offer up to eight times the battery life of zinc chloride batteries, especially in continuous - use or high - drain applications . </P> <P> Manufacturers recommend storage of zinc--carbon batteries at room temperature; storage at higher temperatures reduces the expected service life . While batteries may be frozen without damage, manufacturers recommend that they be returned to normal room temperature before use, and that condensation on the battery jacket must be avoided . By the end of the 20th century, the storage life of zinc--carbon cells had improved fourfold over expected life in 1910 . </P> <P> Zinc carbon cells have a short shelf life as the zinc is attacked by ammonium chloride . The zinc container becomes thinner as the cell is used, because zinc metal is oxidized to zinc ions . When the zinc case thins enough, zinc chloride begins to leak out of the battery . The old dry cell is not leak proof and becomes very sticky as the paste leaks through the holes in the zinc case . The zinc casing in the dry cell gets thinner even when the cell is not being used, because the ammonium chloride inside the battery reacts with the zinc . An "inside - out" form with a carbon cup and zinc vanes on the interior, while more leak resistant, has not been made since the 1960s . </P> <P> This picture shows the zinc container of fresh batteries at (a), and discharged batteries at (b) and (c). The battery shown at (c) had a polyethylene protection film (mostly removed in the photo) to keep the zinc oxide inside the casing . </P>

What is the purpose of the carbon rod in a standard d cell