I use the the stock 700mh in my 9C. I can fly 7-8 /10min flights with no problems. If 28 minutes is all your getting maybe the pack is bad. Have you tried to cycle it? I pull mine in the spring and cycle it. It will not cycle through the TX it must be pulled. I am going on 3 years and have only had to field charge the TX twice.Both times were at a Fly In with above normal # of flights.

David