I have a 1700mah NiCd in mine placed right under the hatch. It is a 6v setup i got from my LHS. I recently started to upgrade to NiMH packs that I found on line. I got a 2300mAh NiMH from
www.all-battery.com to power my GP Reactor for $15 shipped. I will be changing all my batteries out to this setup as it is insanely powerful and it makes even the cheap standard servos I have seem alot stronger. Heres the Link:
[link=http://www.all-battery.com/6v2300mahnimhsidebysiderxreceiverbatterypackswhite cconnectorforrcaircraftswalkingrobot.aspx]Battery[/link]