This is just a tip (which I learned today through others on this site, thanks guys)
All Vex electronics are self regulated.
Which means that even though they say you must use their battery's, you can run any battery between 6v and 12v without drama.
For example, I'm running a 12v nimh 5000 pack on my robot with a regular Molex/Tamiya connector.
https://www.robotshop.com/letsmakerobots/node/27575