8000mah zipo lipos packs from hobbyking. Available in 2s and 3s which allows you to run 4,5 and 6s.
I bought four 2s and four 3s when i had my savage. U will need to take the lid off the battery trays and get sone velcro battery straps.
Well worth doing for 35min+ runtimes and crazy speed.
2s - http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=32213
3s - http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=32214