Flight time from different C batteries

What makes a higher C battery higher C is a lower internal resistance. For a given Current draw, voltage drop across the battery is proportional to the internal resistance of the battery. A battery with a lower internal resistance will have a lower voltage drop for a given load. The output voltage of the battery will be higher by that same amount. Motor speed is a function of voltage, so the motor will run faster for a given Current load, with a lower internal-resistance (=higher C) battery. To keep the model flying exactly the same with this higher C pack, you will have to reduce throttle to compensate for the higher output voltage (which results from the lower internal voltage drop, which results from the lower internal resistance, which is a property of a higher C battery). Since the motor resistance is constant and V=IR, voltage and current are proportional so lowering throttle also reduces current draw. You are pulling less current from the battery per unit time and the battery takes longer to discharge.

The C-rating is simply the highest current that the battery can put out at any instant. (The maximum instantaneous current draw.) The capacity rating (rated in mAh) determines the flight time. Period. Think of the mAh as being the size of the gas tank in the car. The bigger the tank, the longer you can drive, or in this case fly. Now think of the C-rating as the size of the carburator (or fuel injector)... The bigger the carb, the more power you can pull from the engine.

Given a 2000mAh 3s 11.1v lipo battery the following can be derived.

2000mAh => 2 Amps/hour or 4 Amps/30min or 10 Amps/12min. You get the idea.
30C = 30 x 2A = 60 Amps available as the max instantaneous current draw.
Going to a 40C battery simply increases the max power available.
40C = 40 x 2A = 80 Amps available as the max instantaneous current draw.

The thing to remember when adding higher capacity batteries of a similar C rating (20C-25C) will produce ever diminishing returns. The end result being you end up expending more energy just to compensate for the extra weight of the larger battery.

My way of overcoming this was to use a lipo 5000mah, only slightly heavier than the stock 2200mAh 25C battery. To accommodate this larger battery only a small modification to the battery bay was required. Essentially using a Dremel type tool to widen the opening to the battery bay by about 1mm all round. Once the battery door is closed there is no sign of the modification.

My Phantom 1 setup uses the larger props from the P2 Series and has a Walkera G-2D Gimbal and iLook camera. My last flight with the Phantom was a few months ago in a mild breeze at a temperature of 7C and flight times from four Gens Ace 4000mAh 10C packs produced an average flight time of about 18 minutes, and this was aggressive flying.

The 10C rating of these packs may seem rather low but they are rated at a sustained current draw of 40Amp, the Phantom pulls less than 30Amp at full throttle.

I also use the Gens Ace 5200mAh 10C packs, two in parallel, on my heavy lift hexacopter (AUW of 2.8kg) and flight times were about 16 minutes, again in the same weather conditions I last flew the Phantom. The combined weight of these two batteries is only slightly more than that of a single 5000mAh 25C battery which only provided about 6 minutes of flight time.