Originally Posted by Ivegotklas

http://en.wikipedia.org/wiki/Low-pass_filter

From an electrical stand point the cap really acts as a low pass filter. Kinda like on a speaker crossover. The cross over send all the high pitch noise to the tweeter and the low pitch to the woofer. In this case the capacitor acts like a speaker cross over with no tweeter. The high frequencies produced by the ESC during acceleration and breaking get filtered by the capacitor. The low frequencies (i.e. DC voltage) come from the battery.

So how in return do you get better run time and acceleration. The key here is the effect of the cap on the battery. Lipo's are DC, under a PWN driven motor (ESC) the power required to drive the system has lots of transients (i.e. high frequency noise). Batteries have a hard time with producing that power. With out a capacitor the battery would heat up and have high voltage drop accross it. Thus lower run time and less acceleration. The impedence (i.e. resistance and reactance) of a capacitor decreases as a function of the frequency of the signal applied to it.

No from that perspective you have a battery that doesn't have a lot of resistance under low frequencies DC in our case. The capacitor has low impedence under high frequencies. So electricity always follows the shortest and easiest path. The high frequency signal will flow through the capacitor and the low frequency DC goes through the battery.

If I had a FFT spectrum analyzer I could easily show this with a graph or two maybe need a third. Not only that I always see this question on this forum. But when it comes down to it a bigger capacitor makes life easier for you lipos. Thats why caps are a good thing.