DC motor size??

Here is a somewhat related set of posts from last year or three... This was talking about placing a "wind turbine" on the output of an Air Conditioner Condenser... In their case ~11% coupling between fan motors and driven turbine (of course, they still need to near full volume of air through the condensing tower/unit for the A/C to work correctly).

https://forum.solar-electric.com/discussion/351987/wind-solar-charge-controler-one-battery-bank/p1

-Bill

You gave the size (AH @ volts) of the battery bank and a time limit (5 minutes?). I guessed that a 50% charge into those cells in 5 minutes (that is a C*6 rate of charge--That is very fast for any battery--You need to check the battery specifications to ensure you do not "blow up" the batteries).

Is that what how you want to charge your batteries? Otherwise, what is your energy requirement? A 5 minute charge is very fast... Generally a "fast charge" battery is something on the order of 60 minutes or so.

You have to tell us more about the energy needs for the RC car. This sounds like a school project of some sort.

For example, a 20 inch fan rated at 120 VAC and 0.8 amps will take around 0.69 amps @ 120 VAC and use ~82 Watts (just tested one of my box fans with a Kill-a-Watt meter). Interestingly, the fan consumed 82 Watts whether the air flow was unrestricted, or if the fan was flat on the floor (zero air flow). Normally, for a "fan", I would expect the fan to consume more Watts when the air is flowing, and less when air flow is blocked. It would seem that not very much of the 82 Watts is transformed into airflow with a box fan.

Anyway... A first pass approximation would be a well designed "wind turbine" would draw around 11% of the 82 Watts, or ~8 Watts charging power (that helps you size the motor).

8 Watts for 5 minutes is not very much energy:

• 82 Watts into box fan * 0.11 efficiency = 9 Watts available energy for wind turbine harvest
  
• 9 Watts * 5 minutes/60 minutes per hour = 0.75 Watt*Hours
  
• 0.75 WH / 3.7 volts (if batteries are parallel) = 0.203 AH = 203 mAH @ 3.7 volts
  
• 203 mAH * 1/2x2,200 mAH = 0.046 = 4.6% of battery capacity
  

So, if the project is a 5 minute timed charge from a 20 inch box fan, very roughly you would fill the battery bank ~5% of capacity (assuming the rest of the design is "done correctly")... Not very much.

If you look through EBay, you can find small DC permanent magnet motors (old tape drive motors and such)...

https://www.ebay.com/sch/i.html?LH_CAds=&_ex_kw=&_fpos=&_fspt=1&_mPrRngCbx=1&_nkw=permanent+magnet+dc+motor&_sacat=&_sadis=&_sop=12&_udhi=&_udlo=&_fosrp=1

Here is a nice write up on building a wind turbine (rated for several hundred watts, you would scale down for what your needs are) (Ebay link was from this project page):

http://www.mdpub.com/Wind_Turbine/

But, back to the questions... What are your energy needs and what can be realistically be harvested from a box fan??? You have to size your DC motor and fan blades to the available/needed energy. It is pretty difficult to design anything in a vacuum (no knowledge/design requirements). And you can only harvest so much energy with a wind turbine (swept area, available wind speed, etc.).

-Bill

For example, McGivor's EBay motor rating is:

Output voltage : DC 0.01v - 5.5v
Output current : 0.01 - 100mA
Rated speed : 100 - 6000 rev/min

Probably the average maximum power you could harvest is around 9 watts:

• 9 Watts / 5 volts charging = 1.8 ampsA
  
• Motor: 100 mA = 0.1 amps maximum
  

So, optimally, this motor is too small to meet the maximum expected energy harvest from a box fan (remember, lots of guesses here).

Larger motor/generators are going to require higher torque to spin... Which you simply may not be able to get from a box fan (big pm motor, will simply not spin in light wind from wind turbine).

If 100 mA (0.1 amps * 5 volts = 0.5 Watts) would be "satisfactory, then it would be a good starting point.

More or less, the RPM of the DC generator sets the voltage, and the torque sets the current. So you have to spin the motor fast enough to get >3.7 volts charging (say 5 volts), and >10 volts if the two cells are in series. The output voltage of McGivor's motor is (probably) 5.5 volts at 100 mAmps -- Enough to charge your two cells in parallel, but not enough to do it in series.

And it is possible that the maximum power (Watts @ 5.5 volts and 0.1 amps) is upwards of 6,000 RPM, you are not going to get that RPM from a box fan (wind not fast enough)... Also there is a design parameter called "maximum tip speed" --- You need a fairly small blade diameter to get high RPMs (if you see large wind turbines, they turn very slow because they are limited by tip speed).

As much as I would like to help... This is not really in my area of expertise (quick design that meets your needs). You can try these other links/forums and see if they can help you with this project (you still need to define what is a "successful project in terms of voltage and amperage to the RC battery pack). Of course, they are usually working with much larger wind systems, but they may be able to give you some good guidance for your project.

(updated post with links--See they did not copy over. -Bill B.
www.otherpower.com (good forum for DIY Wind Power)
http://www.otherpower.com/
Hugh Piggott - Scoraig Wind Electric site for tons of info (from mike90045)
http://www.scoraigwind.com/
Scoraig Wind "Recipe Book" for DYI Turbines (from Chris Olson... From his 4/11/2013 post)
http://scoraigwind.com/axialplans/index.htm
www.greenpowertalk.org (added from "russ"--Like here but more wind/less solar)
http://www.greenpowertalk.org/

-Bill

Added missing links to post #15.

-Bill