[kby]

Car/boat batteries are lead/acid technology; the electrolyte is sulfuric acid.

Alkaline cells used to used a potassium hydroxide solution (I think) as an electrolyte (basic).

A battery for an old radio is more likely carbon-zinc, which used ammonium chloride or zinc chloride solution as the electrolyte (weakly acidic).

The thing you're trying to get off, though, is probably not just dried electrolyte, though (especially not with a car/boat battery), but the reaction products of the electrolyte + whatever it spilled/leaked on, e.g. copper or brass contacts, wire, etc.