This is a quick and easy way to make a battery pack that will allow you to use a Ryobi 18V tool for four times as long as with one battery. Sometimes a tool will die in the middle of an operation, this can mess up what you're trying to do and is dangerous in some cases. Speaking of dangerous! I get the impression that my little project is dangerous, so watch your back and don't get hurt, I believe in you.
http://www.instructables.com/id/Battery-Pack-for-18V-Ryobi-Tools/