We have a small footprint practice bot with cRIO and 4-wheel CIM motor drive interchangeable with regular or mecanum wheels. We use this battery:
http://www.all-battery.com/rectangul...ightrobot.aspx
It works great. When we add more attachments and motors to the practice bot (e.g. the shooter wheels for this season), we hooked up the regular big lead-acid battery. A picture of our practice bot with the NiMH battery can be found here:
http://www.chiefdelphi.com/forums/at...7&d=1326270647