Lead sulfate is a normal byproduct of a lead/acid battery discharging, but as long as it stays submerged it re-dissolves back into the electrolyte easily during recharging. When it dries out it forms a very stable crystalline form that is hard to dissolve. A battery that stays in a discharged state for a long period of time can sulfate, even if the electrolyte level is not low.
A sulfated battery can cause a voltage regulator to go to its highest setting almost immediately after startup because the internal resistance of the battery indicates it's fully charged. Unless you have an adjustable set of points in your regulator, simply installing a good battery might solve your over-voltage problem.
Jerry
