I bought a Tanic 2100 mah 11.1 volt LiPo battery pack for my Crazy 8. I only charge the thing for about 20-30 minutes and have short flights (don't really care until I get a Poly Charge 4 and another battery). I just started thinking about that when the battery 1: sometimes the battery charger (Electrify Triton) announced it was charged after, at the most, 5 seconds of charging, and 2: after the 20 minutes the motor could go at full power for all of 20 seconds. Anybody have any ideas?

What current are you charging at? Did it always do that or did it just start? Have you tried charging a different battery/charger? You're definitely not charging it all the way so something is wrong somewhere. I wouldn't use the battery until you get the problem solved since you're using up its useful battery life. LiPos are not the cheapest things in the world.

Those are important questions to help figure it out. Also with a battery that size or any bigger I would recommend a balancer every so often at least. If you've only been charging the pack for 20 minutes repeatedly it is more than likely out of balance among other problems.

The battery should be charged at 2.1 amps for one hour or just a little bit more. So at 20 minutes of charging it's not unusual that the motor would shut down very quickly.

What is the setup in the plane? Motor, esc?