You may have to get the batteries out in pieces. I'd use vinegar and the right size applicator after that. For something that uses AAA or AA, I'd use a Q-tip. It'll take some experimentation for D batteries. You should see it foam up like hydrogen peroxide. It's neutralized when it doesn't foam anymore. Depending on where it is (i.e. on the tail cap end of a Mag-lite), I'd follow that with a water rinse.