I can be wrong here, but 13 flights seems a bit too much. On one of my planes I have a 4.8v 2000/2200mah (can't remember now off the top of my head) battery running 5 servos on a ghz system. I check my battery before every flight and have never gone pass 5 flights and only did this once (when I was on mhz) - then the reading starts getting too low for my liking. I do not have the precise voltage since the load tester only has a low, two medium and two high reading lights. After 5 flights it usually loads to about the last medium light (just before low). My flying time is anywhere between 7 - 10 minutes.

I'm sure someone with more indepth knowledge will give a more correct and appropriate answer.