Graco swings

My sons swing has been almost stopping by itself lately. I believe it's the batteries.. only the super expensive ones make it work well. It's suppose to hold up to 30lbs, my sons 17lbs. I don't understand but it's really frustrating when he falls asleep and then it shuts down, so he wakes up! I never had a problem till recently but it's ridiculous to keep buying expensive batteries. Anyone else have this problem?