Wow! This could be a very informative thread, if we can just clarify exactly what 'C' rating means!
'C' is a mathematical constant. Constants are used in calculations to get a meaningfull, comparable result when there are variables present. In this case, the variables are the storage capacity of the pack, usually expressed in mAH, and how rapidly the particular battery (chemistry and construction) can deliver the stored charge. Note that one mA is 1/1000th of an Amp, 1000mAH = 1 AH.
Not all batteries are created equall. Some have better discharge capability than others, it depends to a large degree on the chemistry and the physical construction of the battery. Early LiPo's were only good for 5 'C', but nowadays 20 'C' continuous is common and 35 'C' is available.
To determine the actual discharge capability of a specific battery pack you have to multiply the 'C' rating by the capacity in Amps. For example, a 12 'C' 1000 mAH (1 AH) pack can give 12 Amps (12 x 1 = 12). A 20 'C' 2500 mAH (2.5A) pack can give 50 Amps (20 x 2.5 = 50). It doesn't matter if it's 2 cell, 3 cell or whatever, the number of cells only affects Voltage, not capacity or 'C' rating.
As a very rough guide a 540 brushed motor will pull around 20 to 25 Amps at full throttle and normal load. It will pull as much as 50 Amps or more when stalled, again at full throttle. As stated in other posts above, you need a Watt meter or data logger to measure the actual current draw on your own specific set-up. Also note that motors and ESC's are rated for maximum Voltages, ignore that at your own peril!
And don't forget, you should give yourself a healthy margin (approx. 150% should do it) if you want your batteries to last. 'C' rating is a maximum and LiPo's can only give that maximum for short periods, not all day every day.
Now, put all that together and you should be able to work out what battery capacity and 'C' rating you need. I'll invite the experts to take it from here, correct me if I'm wrong, and fill in the blanks. Thanks guys.
Cheers.
