You'll need to remove the batteries, clean area with baking soda and water, sand, and repaint to stop the corrosion. I've seen people use the bottom half of marine battery boxes, once saw someone who used plastic dish pans, anything to isolated the battery from contact with the metal. Instead of paint you can also use the Spray and Seal stuff advertised on tv, it's actually pretty good stuff!
Dave & Myra
One day your life will flash before your eyes,
make sure it's worth watching!