Dont buy ebay batteries- the magority of them are junk- and 15c is not anywhere near what amperage you need.
see here
http://hobbycity.com/hobbycity/store..._35C_Lipo_Pack
the only advantage to running 2 2s packs in series is that you can use the packs in other cars.
Benjamin White
R/c Monster Team Driver
Jq the car, LST, Sportweks turmoil pro
Unconventional Techniques, Superior Results