DISCARD this puffed Lipo before it starts a fire and causes major damage. Some likely reasons for this are most likely one or more of the following: discharging below 9v cutoff; overheating battery; not balance charging; not allowing a cool down rest period before recharging; defective motor drawing too many amps; defective battery.

Are you using a lipo charger with built-in balancer and charging via the smaller 4-pin connector. Don't charge through the larger discharge connector unless you have a Blinky Balancer connected to the 4-pin connector. With your lipo you could balance charge as high as 1.3 amps (1300mAh = 1.3amps).

An 18C 1300mAh lipo should be able to handle a continuous discharge of 23.4 amps (1.3 x 18 = 23.4) and short 5 second bursts as high as 35amps without excessively overheating the lipo. As mentioned previously this may be too highly rated by the manufacture, so 20 amps continuous discharge with 5 second bursts at 30amps followed by at least 30 seconds below 20 amps) is more realistic.

I have a 20oz plane with a brushed 480 motor that draws 14amps at full throttle during takeoff/climb with a fully charged 11.1v lipo. Normally, when cruising the motor draws only 4 amps at half throttle. I use a 2000mAh 8C lipo with a continuous discharge rating of 16 amps with 24 amp bursts. This 8C battery doesn't even get warm as the pilot and plane prefer a more conservative flying pattern once airborne. So, a $25 - 11.1v 2000mAh 8C lipo offers plenty of capacity in this plane - provides at least 12 minutes of enjoyable flying.

This 8C Lipo would be an accident(overheating) waiting to happen and a much shorter flying time in any 20oz sport/racing plane powered by a brushless motor drawing 20amps with a pilot that prefers full throttle most of the time. But when powering a brushed 480 motor--its the motor that overheats and eventually gets fried, while the battery barely gets warmer than your hand with a moderate flying pattern..