# Given a source of mechanical input, can an RC motor charge it's own battery without extra cicuitry involved?

## Background

I took my RC quadrotor to a windy field this weekend to fly it around. After the battery died, I retrieved it and while carrying it back home, I noticed that the wind was spinning the propellers like mad.

Each propeller is attached to little electrical motor which, of course, runs to a circuit board and then on to the lithium battery. This made me think, "Hmm--it would be cool if I could mount my QR to a stake in the ground and let the wind charge it back up while I eat lunch". But I wasn't sure if this would fry the battery or anything.

I am not an Electrical Engineer, but have some familiarity with how electricity and circuitry works.

## Question

Given a source of mechanical input, can an RC motor charge it's own battery without extra cicuitry involved?

Thanks!

• Reminds me of "regenerative braking" used in electric and hybrid cars. "Energy harvesting" might be a good term to google. – DarenW Apr 16 '12 at 17:33

You will be able to get some charge with relatively minor additions to the electronics but the amount will be small compared to the energy required for a full charge.The motors are shown as "brushed motors" so the main requirement would be to provide conduction paths from the motor to the battery. this is technically easy but can be somewhat complicated by what else is connected.

Your quad-rotor rotors are optimised to move air at high velocity. The wind is far lower velocity and coupling to the blades and area of wind intercepted will be poor. If you were extremely lucky your 4 x 140mm blades MAY make as much as 10 Watts in a 10 m/s = 36 kph wind. Probably only a few Watts. 10 m/s is quite a stiff wind. Power changes with velocity cubed so at say 5 m/s wind you'd get about 1 Watt max and probably rather less.

A reasonably efficient wind turbine can produce 200 Watts per square meter of rotor disk area in a 10 metres/second wind = 36 kph or about 22 mph. 100% efficiency is about 600 Watt/m^2 and the theoretical upper limit possible is about 360 Watts (known as the "Betz limit"). As your blades are very slightly ducted you may notionally be able to exceed the Betz limit but in practice even 200 Watts/m^2 at 10 kph is "hopeful".

Power = 0.6 x Area x Efficiency x V^3

Power i n Watts
Area in m^2
V in m/s Efficiency 0 ... 1.
where 0.3 is superb, 0.2 is good, 0.1 - 0.2 typical.

If you measure the rotor area and wind-speed you will find that the power generation capability is low compared to the power required to fly.
Surprisingly the figures suggest that this may actually give enough energy to be useful. Charging times MAY be under an hour at 10 ms but probably not.

• But I don't see where the "cool factor" enters into your equation ;). – Matt Cashatt Apr 16 '12 at 18:45
• enough energy to fly a micro plane with micro stepper motors. – Standard Sandun Apr 16 '12 at 18:50

you probably need extra circuit involved. Something like a simple joule theft configuration.

Even with that extra circuit it's not simple as you think, because some RC motors are brush-less stepper motors. In that case it will output three phrase supply. Then you need a converter in advance.

• Also there are problems with LiPo batteries. It's not that simple to charge them. Also in this particular case, the motors are brushed. – AndrejaKo Apr 16 '12 at 18:06