Theoretically, you could use a 2S Lipo on the stock setup, you'd just need a LVC alarm, so that you don't run the lipo too low and kill it. I might actually go that route, instead of investing in 7-cell NiMhs. Cuz you're gonna want to upgrade to brushless eventually anyway. Buy a 2S lipo or two and an LVC alarm. BE SURE TO CHANGE OUT THE STOCK BATTERY CONNECTORS. Change everything to Traxxas connectors. The crappy Molex connector will NOT hold up to Lipo or 7-cell Nimh, and will melt. Trust me.