Hangtimes (as well as other sources) sells transmitter packs to upgrade their capacity to match the receivers. I gave my brother a 1650 mah pack for his Futaba 9C. When warm weather returns we'll get to give it a workout. This time of year we're not able to get out very much. North Texas winters are pretty mild but the sun is so far South that visibility is poor most of the time.

http://www.hangtimes.com/txpacks.html