In Part 1, I briefly outlined a process of determining overall power, how to deliver the power, how big a motor is required, and finished with suggesting last of all choosing a motor with the Kv to do the job.

This entry will talk a bit more about how to deliver the power, and how to find the elusive Kv that supports the power system design goals.

Pick a preferred cell count (voltage) and pack capacity for how to deliver the power

Voltage. The easiest rule of thumb to apply in power system design is watts input power per pound all up weight as it gives a single number in Watts, eg 6lb at 100W/lb = 600W. But 600W can be delivered by a 3S pack with nominal 11.1V at 54A, or by a 6S 22.2V pack at 27A, so how many cells to use? And what about pack size? Unfortunately, it's hard to give a good rule of thumb; rather some considerations. One driver is that ESCs bigger than 60A capacity start to get bulky and expensive. So allowing for some headroom on ESC capacity, it can be cost effective to add a cell and drop current. For example, using the 600W target, 40A on 4S is comfortable for a 60A ESC, while 54A on 3S is close to the limit. But going up the scale, ESCs capable of handling over 6S tend to be classified as 'high voltage' with a step change in pricing. Maybe re-using an existing ESC sets an upper current limit. Existing charger capabilities or commonality with existing packs may also be valid considerations. So choosing cell count up front might be somewhat arbitrary and...Continue Reading