My 2c
I don't know much about batteries, but I'll tell you what works for me.
I have a NiMh 8 cell 720 Mah homemade pack that weighs 3 ounces. It doesn't seem too heavy for the airplanes, and with thes stock motor it will do ha;f an hour on idle. The whoe airplane with it weighs about 13 ounces.