I use 3-cell flightpower 1800mah lipo pack. Fits in the battery bay without any mods. You just need to solder J connector. Gives you about 6-7 hours runtime. What jsenicka talks about looks a very good deal though with a right connector and it's price. There are definitley direct fit packs out there specifically made for 9C transmitter. You will not have a hard time to find one. I used 25C flightpower pack because I wanted to be able to use the lipo for other applications incase I get rid of the tx in the future.
