The first couple statements are true for the same power level, not for the same load. Hardly my claim, nor a mere theory. Credit traditionally goes to Georg Ohm, for which Ohm's Law is named after. One relevant derivation:

Power = voltage squared / resistance

Note the same load implies the resistance is the same, thus power is proportional to the square of voltage, meaning it changes rather quickly with voltage changes. So yes, as the battery voltage drops off from 8.4 to 7.2V during the run the power drops off. This may not be so obvious with many of today’s rather sufficiently powered RCs, lower powered examples make it far easier to detect or notice. Granted this is only one aspect, an electrical motor is a little more complex than simple resistance, there are secondary considerations, but the voltage is nearly always a dominant consideration.

The published motor power ratings are based on a specific set of conditions, not a definitive measure of the power output in all conditions. For example this motor data sheet from Novak, footnote 3 clearly specifies the voltage used to generate the power ratings. Member John Stranahan has an interesting motor dyno thread in the on road section, I can’t recall but it may include some voltage comparison data, I’ll try and take a look when I have the time.

The heat in the motor and ESC is largely a result of current-resistance losses, aka Joule’s first law. The reason the 3S example mentioned runs hotter than the 2S is because the current is higher, driven by the higher voltage. (Thus the max rating on 3S mentioned, it changes from 2S doesn’t it?)


From a practical perspective, there are many examples in RC past and present that demonstrate or are a result of the change in power with voltage changes:

- Not so obvious with good lipos which have a comparatively flat discharge voltage and typically limited discharge amount due to the high capacities and comparatively short race heats, the drop off in power during a run was far more obvious with the nickel based batteries of old. Was especially obvious with the 1200-1400mAh cells. Of course power loss due to heating up over the run is also a factor, both then and now.

- Electric 1/8 off roaders, where much of this discussion typically occurs, generally don’t use the same motor for different voltage setups. With 4S a 1900kv or so motor is common, but those running 6S tend to run 1400kv or so motors (of course there are exceptions for both). If voltage did not impact power why change motors (load)?

- 1/12 scale on road now uses 1 cell lipos, as 2S was determined to be too much power. Going back a ways, some used to run 6 nickel cells on large outdoor tracks, while 4 cell was used on the smaller indoor tracks. The difference in power was rather dramatic.

- Back in the nickel cell era, it was not unheard of to take a cell out to reduce power on slippery off road tracks. A couple examples, if I remember correctly it was the 2005 Worlds where some drivers went to 5 cells instead of 6 to make the cars easier to handle. Going back further, Gil Losi Jr. won a big race, I think in Japan, by switching to 6 cells instead of the 7 allowed at the time in modified classes. As I remember it this win was credited by some as being a significant part of the success of the launch of the original Losi, the JRX2, perhaps even part of the decision to proceed with releasing the car, undoubtedly a big gamble at the time for Pops Losi.

- All of the effort and importance the power limited stock class guys put into having the best, lowest internal resistance, highest voltage under load, max voltage charged, batteries. Been the case for something like 30 years or better. Hardly a placebo effect, there really is a difference.