A relay-based on/off controller on the other hand can be very efficient. A typical relay has a contact resistance of 3 milliOhm. At 20A, this translates to a voltage drop of 0.06 volts, with a power loss of 1.2W. The relay itself typically consumes around 0.4W. And, by using a double throw relay, one gets a brake for free.
There are two main problems with a relay-based on/off controller. One is that there is no control over motor speed; it`s either off or it`s at full throttle. Such a controller is best suited to electric gliders, which for their size, typically h