I'm surprised that some of you are so insistent on putting essentially car batteries in your machine. Portable machines like this are designed to be, well....portable. Which means light enough to easily transport. What good is it to have a machine that gives you 10 hrs of playing time if it weighs 70 lbs and collapses under it's own weight?? Or try and put something like lithium batteries that cost so much more and may have other issues? The majority of ball machine users are on the court for 2 hrs or less. Sure there are exceptions, like teaching pros, but again, they represent a small percentage.
Keep your SLA battery and charge it properly and you'll be fine.