title_body: Why do higher voltage batteries need lower KV motors? I've noticed that higher voltage batteries need lower KV motors, why is this?
upvoted_answer: They don't necessarily, however a given amount of power will be required to rotate a motor at a certain RPM with a given propeller.
  
  kV, the voltage constant of a motor, is the maximum RPM it can turn at per volt of power supplied. Example:
  
  A 4S (15V to make math easy) battery will turn a 2000kV motor at 30,000RPM (with no load, inefficiencies, etc)
  a 6S (20V) battery will need a 1500kV motor to achieve the same RPM
  
  In theory, the result is the same amount of power (watts) required to achieve the same RPM/power output/etc. A 4S 2000kV motor will draw say 20 amps, or 300 watts. To get the same power output, a 6S 1500kV motor will draw 15 amps (still 300 watts), and spin at the same RPM. Higher voltage but lower current generally results in less heat, because of ohm's law. Current creates energy loss over resistances (ESC components have internal (small) resistance, which is why they heat up, which is why they have a current rating in the first place).
  
  A lot of Bardwell's videos go into the details of 4S vs 6S, but they work out to be pretty much the same. The reason larger models go with higher voltages (and lower kV to achieve the desired RPM) is because they are much higher wattage, and achieving that higher wattage strictly with higher current is unrealistic (Large scale planes would be drawing 500-600 amps at 3-4S voltages, dangerous and expensive).
  
  To reference an earlier comment, the voltage "rating" of the motor is just the suggested voltage. The actual rating of the enamel coated wires would be much higher, but you will destroy components from drawing too much power under load if you went up to those voltages.