It's not a good idea to store an old automobile battery in the garage, because the plastic case can deteriorate and leak, and you don't want what comes out on your floor. The substance is usually sulfuric or muriatic acid, better known as battery acid, and besides being hazardous to children, pets and you, you won't like what it does to the concrete. If it sits on the floor long enough, it will eat a hole. You may be able to prevent permanent damage to the concrete by cleaning the mess early, starting by neutralizing the acid.
Mix a paste of baking soda and water. Baking soda is sodium bicarbonate, which is alkaline. It will neutralize the acid. You can also use lime from your garden shed -- which is calcium carbonate -- but if the spill is a large one, the lime may combine with the acid and solidify.
Apply a generous amount of paste to the stain with a paint scraper. Work it into the stain with a wire brush, then wash the area with plenty of clear water.
Wash the area with a cleaner that contains ammonia.