If the batteries are old might they may draw a lot of current to stay charged?
Many people wait until they are stranded before they change out the batteries. You might want to consider their age. If they are 5 or older you may be ready for some new ones. Check out the cables, too. If they are old, have a lot of corrosion in/on them, if they are loose, if splices are similarly effected. Try removing them and cleaning them with water and baking soda to neutralize the corrosion. You may find that the clamp bolts are corroded away and they need replacement so that you have a good solid connection. Remove them at the posts, clean out the inside of the clamp and the outer face of the posts. There are nice round wire brushes at the auto parts store that make this easy. Both for the inside of the clamp and the outside of the post.
Upon re-installation use the little felt cushions and the purple spray to deter furture corrosion. Yes, they do work.