I understand the "C" rating on LiPo battery packs (e.g. 15C, 20C, 25C, etc.) refers to the "discharge current". How does that translate into differences in performance? For instance if I have 2 different packs for the similar plane, both 11.1v & 1800mAh, but one is rated 15C and one is 25C, what's going to be the noticeable difference in performance?
You are right, "C" rating refers to how much the battery can discharge and how many amps you can pull from it. The amps you can pull from a battery are directly proportional to the capacity of the pack. So for instance, you've two packs, both are 1800mah, but one is 25C and the other 15C. The 25C 3S 2200mah pack should be able to deliver about 45amps where the 15C will only give you around 27amps.
So if you have a plane that is drawing around 27amps and use the 15C pack you may find your performance down a bit because you are working the battery at its limits of what it can deliver. Any extra load and the 14.8v lipo may actually cut out on you. If you use the 20C pack, it still has amperage in reserve (for lack of a better term) and isn't operating at its upper most limits. It will be able to more easily dump its current, and will be unlikely to heat up as badly as the 15C pack.
If you are using these batteries in a vehicle that doesn't draw much current you may still see a difference in performance even though you are not pushing the batteries to their limits. The standard of the battery pack cell and its internal resistance play a huge roll in how easily the battery will dump its voltage and model helicopters are the most demanding application of all.
In theory, any battery will provide all of its rated capacity in one hour when discharged at 1C. Its voltage during this one hour 1C discharge, will drop from the peak 'open circuit'voltage when freshly charged down to the minimum allowable voltage per cell of that particular battery's composition. The nominal voltage of a cell is typically the average of the peak, charged open circuit voltage and the minimum allowable or minimum useful voltage.
In the case of LiPo's the nominal voltage quoted is 3.7V per cell, which is the average of the 4.2V peak charged voltage and 3.2V, which we take to be the min working voltage.
So a 1000mAh LIPO battery could theoretically deliver 1 amp current at 1C for 1 hour while maintaining an average of 3.7V.
For all practical purposes, and for 8 to 10 minute flight times, leaving enough extra capacity for a go around landing and taxi back, we should really only be drawing an average of around 5C out of any given pack in any given flight or drive. So what good are 20C or 30C packs then?
The C rating is also a good indicator of low internal resistance. The higher the C rating, the lower the internal resistance (IR). Low IR tells us that voltage will remain high during the discharge in comparison to high IR cells and that low IR cells will run cooler, since the higher resistance the cells convert more energy to heat, this is why the better cells have closely matched IR¡¯s. We want the lowest IR cells we can afford.