 There is a direct correlation between the depth of discharge (DOD) and the cycle life of the battery, with differences between 500 and 1300 cycles depending on depth of discharge.Originally a kind of gel cell was produced in the early 1930s for portable valve (tube) radio LT supply (2, 4 or 6V) by adding silica to the sulfuric acid.By this time the glass case was being replaced by celluloid and later in 1930s other plastics.