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Depends on the battery. Most batteries use an alkaline paste as the electrolyte. Try water(probably won't work, but try anyway).
Try vinegar(make sure to wipe away any excess, and then wipe with water). Try some fine steel wool or a fine emery board(be sure to wipe away residue). Just don't get too vicious and try the new batteries between each attempt. The thing only draws a few milli-amps and just needs minimal clean contact to work properly.
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