To remove battery acid from your car upholstery, rub a paste of baking soda and water into the spot right away, leave for two hours, then wipe off with a damp cloth. Repeat if necessary. Any remaining residue can be cleaned off with a commercial upholstery cleaner, or mix 1/2 teaspoon liquid dishwashing detergent in a quart of warm water. Beat with a mixer, then clean the upholstery using only the suds. Work on a small area at a time, overlapping areas to avoid spotting. Change the rinse water frequently to keep it clean. Let dry thoroughly.