I personally tested some Zippy batteries, they were junk. The C rating is not a continuous discharge rating. I discharged a few of their packs at their C rating with a continuous discharge and every pack swelled. Make sure your batteries are not swelling on discharge.

Your better off going with a name brand whose C rating is rated for continuous discharge rate, not a burst rate like almost all of these cheap LiPos. The quote stands with batteries, "you get what you pay for".