I used a LiFe 220mah on my pan car with 1S and brush ESC / motor.
With the rx battery fully charged, I can play 3 x 8min run safely. Most of time I charge between 4th and 5th packs. http://ernrc.blogspot.com/2010/03/xray-12link.html
Hope this helps.
Yup, I did that too for my brush ESC - remove the red wire from ESC to rx when connecting to the rx batt. It works fine so far. But never try this on brushless ESC.