Best motor suited for a diy generator that can provide 48V?

Ok, out of all places, I've seen this particular question be asked once and the discussion trailed off topic somewhere. Before we start, Yes it's a bit of a backwards thought but it's still a project, experiment and I'd do it for the sake of doing it.

I have an electric scooter - lack of knowledge for proper battery care during winter lead it to 4 dead batteries. In my area, as long as the scooter is moved by an electric motor and makes less than 40km/h it's a street legal and non insurance or license requiring vehicle. Only issue for me is the limit of the battery capacitance and the eventual depreciation of the battery capacity.

So I want to try an experiment of strapping a small weedwacker motor or even a hobby airplane fuel motor to an electric motor to act as a power source for the scooter. Don't laugh, It's an experiment. Yes, I'm aware I can just buy batteries. they are 50 bucks a battery + 12 bucks shipping and mine ran on 4x of these suckers.
Buying batteries would work but would not satisfy the curiosity of an experiment..

Now then:

My scooter, when bought, could do about 12 blocks (1 block = 2km ) trip on a full charge. Before this winter, it could do 8 barely. It ran on 4x Lead Acid 12v 18Ah batteries.

Now the batteries are connected in series so that's 4x 12v 18Ah making it 48v 18Ah. At this point we can drop the need to have the generator produce exactly 18Ah of current because that originally is the total battery capacitance, determining how long we can ride before draining of the batteries. So with a generator, all we need is to draw sufficient Current at the moment of generation.

Which motors are best suited for generating 48v. I'm obviously missing some que variables here but I just want a point in the right direction. I'll most likely need extra head room and make a Step Down Buck Converter.

Every time A talk of a diy generator comes up, people seem to mention Stepper motors. Seem like good choices? Brush-less, multi lead motors, yes - no?

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Toga_Dan4 months ago

Cordless lawn mower.

48 v cordless tools.

maybe someone will give one away once their battery quits, too.

I am laughing, but also am curious about the outcome.

iceng4 months ago

A weed eater engine comes in at 25cc with 1.3HP...

I found an online CC to hp calculator based on 32.5 cc per horse.

It said my 31cc weed wacker is 0.953846 hp.

Going by most generators are at best 50% efficient it might run a 355 watt generator.

MilesT9 (author)  iceng4 months ago

Just about enough to power the scooter with a good bit of overhead for other electronics, provided the load doesn't stop the weed eater motor via the generator

I like electric Ebikes

OK so you want to make a weed whacker generator.

With most generators at best you get 50% efficiency.

Pick a generator or motor 1/2 the power of the weed whacker motor.

What is the power of the weed whacker?

1 horse is 746 watts; so if the weed whacker is 1/2 horse that is 373 watts, so at best the weed whacker can power a 185 watt generator.

A home made Axial Flux Generator might be exactly what can fit your bill with the best efficiency, but if you want to use a motor I wood go with a 48 volt DC motor 1/2 the power of the weed whacker. If you use a brushless DC motor you will need to remove the hall effect sensors and add a rectifier like in a car alternator.

Jack A Lopez4 months ago

Can the existing electric motor on your scooter act as a generator?

I mean, I guess the way to tell would be to come up with some way of actually measuring the electric current flowing through it, and the battery in series with it.

It's probably a pretty big current, around 10 amperes or so. That's just a guess, based on the size of the batteries you are using.

Anyway, if you had a way of actually measuring the current, and this ammeter was small enough and light enough to travel with the bike, and also if you could find a road on a hill somewhere...

... then ride the bike down the hill. Then put the bike's motor in gear. Then look at the ammeter to see which direction the current is flowing.

I am guessing that in a circumstance like this, with mechanical power flowing into the bike's motor from the gravitational descent, that the bike's motor will act like a generator, pushing current (and energy) into the battery, recharging it.

Also guessing you could make the same thing happen by adding a gas-powered, "weed whacker", small engine that is mechanically connected to the bike's drive train somehow.

Or maybe you intentionally wanted to avoid a mechanical connection, and go with a purely electrical connection instead? And that is why you are asking about which electric motors, e.g. stepper motors, can be used as generator.

MilesT9 (author)  Jack A Lopez4 months ago

Yes, I am looking into a purely electrical connection and also this is meant to power the e bike During the actual trip, not just for charging. As per the user manual, it does say that the bike auto chargers when throttle is not pulled and the wheel is in motion so the drive embedded into the back wheel indeed does act as a generator.

As per the listing of the e-bike, it's a "500W engine" no idea if it's valid or just some number they put in to make up for content on the page:


Well, I am guessing the motor-generator needed would be something about the same size as the electric motor in the bike.

I noticed that site,, has a page for "48 volt motors", here:

Also they have a page for "60 volt motors", here:

That kind of motor, is a brushed DC electric motor,

and those will work well as a generator, provided you can put enough mechanical power (speed and torque) into them.

That reminds me of something important to mention.

Electrical power is the product of two numbers: voltage and current.

Mechanical power is the product of two numbers: angular speed and torque.

The best, easiest, way, to build a generator, is to pick a (gas powered) engine that turns at just the right speed that the motor generator wants. That way you can connect them directly, shaft to shaft, without any gears or pulleys needed to change the speed from one shaft to the other.

So how fast does the shaft of a small gas engine turn? Is it 100s of RPM, or 1000s of RPM?

I mean the DC motors on electricscooterparts' pages, those have rated speeds of 2500 to 3000 rpm, and that is about how fast you would want them to be turning for to be used as generator. Actually the one rated as a 60 volt DC motor, that one might work better if it can give you higher voltage at lower speeds.

rickharris4 months ago

Ok A petrol (gas) engine has one of the highest energy densities vs weight there is. IE there is a LOT of energy in the fuel for you to use and it doesn't weigh much.

If your going to connect a DC motor to the weed eater engine essentially you will get back more of less an amount of electricity proportional to the speed of the motor. IE faster = more.

HOWEVER - When you connect that generator to a load that load gets passed back to the weedwacker. so the motor will slow down how much depends on the load.

I would expect you will need to gear down the weedwacker somewhat to increase the available torque to a usable level.

HOWEVER you should check regulations because they often, (at least here in the UK), refer the the primary source of power, so if running your scooter using a gas engine would be illegal then that is still so even if there is a generator and electric motor between the gas and the wheels.

Running a petrol generator to recharge batteries as you go would be a popular option, in fact some hybrid cars do that, but the motive power would still be gas with all the legal implications.

Any DC motor will do what you want - Obviously you need to find one that operates on more or less the same voltage as you original battery pack provided.

I should add you can't get something for nothing - if the scooter needs 250 watts then no matter how you play with it you will have to provide 250 watts. 746 watts=1 HP.

MilesT9 (author)  rickharris4 months ago

500Watt engine is listed in the specifications. Maybe a bit more with the electronic amenities like the lights and controller.

Check your weedeater - it must supply at least that much, actually perhaps a bit more because there will be losses.