Someone may have already answered this, but I can't seem to find it. My question is, why a 3S 15C battery runs less rpm than a 3S 20C? The prop charts on this scorpion 3008 32 say motor kv 1086 rpm per volt, so if the voltage is the same, one would think (one who doesn't know better) that the rpm should be the same, but I see a big difference. I am running a Will H. timer set to 80 percent on an inexpensive BP ESC. I am planning on going to a new setup for next year, thinking 4S3000 or 4S4000 for the turnigy 35 48 - 900 motor but don't know whether I need more than 15C.

Hello Jim,
In general, the higher the "C" rating of a cell, the lower the series electrical resistance of that cell will be. Once current flows, this series resistance results in a voltage drop within the cell itself. This produces the difference in voltage you actually see at the motor and the resulting RPM difference. You can easily see a 10% drop!

For a 6-minute Stunt flight, consuming 75% of battery capacity, you are running 7.5C average current, and the peaks and valleys in current consumption are right about 10C and 5C.
Good quality 15 C batteries suffice nicely.
20 and 25C batteries are desirable, but both additional battery capacity and "C" rating all cost more weight. You have to compare carefully.

On top of that, one manufacturer's 15-C rating is not always the same as another. Price is not always a good indicator.
As Bob says, recommendations from those you trust are helpful.

best regards,
 Dean P.