Originally Posted by Sundancer
Brian, the "20C" refers to the maximum current which the battery can be expected to supply - 20 times the capacity, which in this case is 20 x 0.8 amps or 16 amps. This is a maximum, and it is highly likely that if pushed to this maximum the voltage (and hence the power i.e. input watts - watts = amps x volts) will be forced down. You've probably had this battery a while, as these days most tend to be 25C, 30C or better. As to the prop size, reducing this will result in a lower current, which the battery will cope with better, but, of course, less watts so less power. However, a 9 x 6 will probably run at the amps as a 10 x 5. I would suggest that a 1300 25 or 30C 3S pack would do if you can squeeze it in. Should give you around 5 minutes of flight time if propped for around 15 amps. Bigger capacity batteries perform better pro-rata.
And the light bulb goes on! I've got a sort a maintenance schedule for my older planes- I check that the wheels still run freely and can't come off, that the U/C is properly attached and control horns are still solidly attached. And, if the battery is more than 3 years old and has more than 50 flights since I started keeping records, I replace it. Its startling how much more fun a tired plane with a 4 year old 10C 1050 mah pack with 100 flights becomes with a new 1300 25C pack. I thought it was just because it was newer, but now I understand!
You may find that a new bigger higher C pack is actually not much physically bigger than your older 800? They seem to get smaller and cheaper