Before you ditch your battery, clean the posts, cable ends and top of the battery thoroughly with water and baking soda. Even if a batter looks clean a light, invisible film on top can create a draw from one post to the next slowly draining a battery. You might also check your alternator output to be sure it is charging the battery properly. Some chargers have an alternator check on them or maybe a battery store will check it (If you trust them to tell the truth vs selling you something). And of course if it is a wet cell, check the water level inside.