I have since moved away from the Yongnou flash because of the wake up time and now use the trusty Nikon SB-25 or SB-28s.

I have one flash pack(Nikon SB-25 and 5200mAh 2s lipo) that has been out for about a month and a half and still has 70%. It's not a hight trafficked camera/flash set but it triggers every few days.

The 5200 mAh 2S lipo is anywhere from $12-$20 depending on the brand, it's cheaper in my mind than AA or even a C battery pack. You can usually get 100-200 charge cycles out of them.