You can make a paste of baking soda and water, apply the baking soda paste, scrub with an old toothbrush and let it sit for a short time, then flush with fresh water. You can also apply the paste to the battery cable clamp. Auto parts stores sell a battery post cleaning tool that can clean off a battery post and also includes a small wire brush to clean the inside of the battery cable clamp. Wear eye protection and rubber gloves as the battery acid residue is sulfuric acid based. Concentrated sulfuric acid is highly caustic to skin and clothing. The acid can be nuertalized by the chemical action of the baking soda and water. Repeat as necessary and flush clean with plenty of fresh water.