Finding the right brush-less Direct Current (BLDC) motor to use as generator

Since I got my Honda GX25 motor, the task of finding the right generator to make the most efficient and compact generator for the batteries in the robot has lingered somewhere in the back of my mind.

First I thought about using an car alternator for this, but have since found perhaps an even better alternative, namely high-end radio controlled helicopter motors. It turns out alternators weigh a lot and are clunky and heavy, while BLDC motors from modern RC helis are not.

So how would I go about finding the correct BLDC for my project? I got some help here and ended up with the following:

• Lead-acid batteries have optimal charging voltage of 13.8 to 14.4 volts.
• The GX25 motor has optimal RPM (RPM with highest torque) @ 6000 RPM.
• The GX25 motor has an effect of around 0.7kW.

With this as input finding a motor for our project is easily calculated as follows:

• Desired kv (rpmvolt) = 6000 rpm/14.4 volt ~= 417 kv
• Desired continuous effect is above 700 W. We will go with at least the double to put the least amount of strain on the generator.
• Desired continuous amp is 700/14.4 ~= 48 A

(disregarding all the voltage losses etc).

Next I just chuck these parameters into hobbyking's motor finder to get some alternatives, for example the Turnigy SK3 5055-530kv with the following specs:

Turns: 12TVoltage: 5~8S LipolyRPM/V: 430kvInternal resistance: 0.019 OhmMax Loading: 70AMax Power: 1750WShaft Dia: 6.0mmBolt holes: 25mmBolt thread: M4Weight: 378gMotor Plug: 4mm Bullet Connector

I must investigate this further to ensure that I don't go buying one of these motors that wont work because of some parameter I did not think of.

Generator update

I have been stuck with progress for some time on the generator. It turns out alternators are hard to come by for free/cheaps. While i continue to look for alternators i have turned my attention to the "other end" of the motor, where I will be fitting the starter motor in place of the recoil starter.

After initial investigation, i made a makeshift tool to remove the wheel attached to the shaft that the recoil starter grabs durin a pull, and verified that the shaft is right way threaded M6 (5mm ⊙) with 1.0mm threads.

I have purchased two one way bearings of ebay and the plan is to make a 10mm ⊙ adapter with M6 hole to fit on the shaft and then slide on a one way bearing which in turn will be driven by a small electric motor somehow. Not sure how yet, i need to get some experience with one way bearings first. Peculiar thigs they are in deed, and from what i have read the can sometimes be fickle.