# Bicycle Generator Need Help

#### Zefarius

Joined Nov 4, 2010
7
Hi.. i'm having a problem in understanding the processes related to my bicycle generator.. will be awesome if you guys can help me out ^_^ i am kind of confused.
My project is about green energy.. and im trying to fabricate a bicycle generator that charges a lead acid battery rating of 12V 70Ah 840Wh with a permanent magnet motor of 24v 250w output at 2850RPM, so in order to create a voltage constantly higher than 12V the motor must rotate at a speed of at least 1425 RPM.

The watt rating of the motor 250w is divided by 24volt to get the maximum current possible of 10.4 amps. Is this correct?

Can i use an AC motor such as used fan with a rating of 250V 75W 50Hz.
75W / 250V = 0.3 amps but how do you get the rpm? The only rating i saw given for permanent magnet dc motor is voltage and amps, do they usually give out rpm rating too? becase i'm thinking of getting a used car alternator as my bike generator.. whats the normal rating for alternator?

Coming back to my lead acid battery.
theoretically it will have a rating of 12V 70Ah 840Wh at 60% state of charge (around 12.2v base on internet)
let say i manage to churn out 150 watt on my generator
how do i count the time it will take to get it full charge?
i just need a rough estimation and i cant seem to comprehend this hehe
hope you guys can help out
Thanks a bunch!

This is my university project and by the end of it i hope that i can producea bicycle generator producing 100w powering a laptop of 80W and charging a lead acid battery. I havent got the stuff yet just trying to understand the calculations and circuit first.. any suggestion on the circuit is appreciated.

#### blueroomelectronics

Joined Jul 22, 2007
1,758
Are you going to strap a car battery on a mobile bicycle? You'll never be able to pedal it and generate any serious amount of electricity. Have you noticed how dim headlights are that are powered by bicycle hub generators? A few Watts tops.

#### wayneh

Joined Sep 9, 2010
16,102
It's tough for any rider to put out 150 watts for very long, it's ~1/5 horsepower, although a fit athlete certainly can do it. But then there's the efficiency problem. I think you'd do very well to convert 70% of the rider's output into electrical energy, and if you're storing it, the overall efficiency is far less. So maybe it's worth re-thinking your ambitions. FWIW, the traditional "bottle" generator for bikes is designed for 6W.

I think you can forget about AC motors for this, like what you might find in a fan or a shredder. You're right to look at alternators, but an automotive alternator is designed for efficiency at high rpm, and by "high" I mean relative to anything you'd do with a bicycle. I forget the numbers, but something like 5000 rpm would be typical for an alternator. And it would be capable of putting out WAY more power than you can generate, 50A and more. Plus you would need to energize the field windings. That alone would take a lot of the rider's energy. You need to match the alternator to the power supply, kind of like shifting the bike's gears.

A PMA, or permanent magnet alternator is a good choice. But, the problem is finding the right one. The wind-power crowd suffers the same problem. DC motors can be run backwards as generators, and you can find some of these. They tend to not be well-optimized for this task though, since they are also designed for high rpm. Motorcycle alternators are PM based, also, and might make a good choice if you could find one that you could adapt as a unit. They tend to be integrated into the design of the bike itself, not as an add-on as you find in cars.

Once you figure out a way to make AC, you'll need to rectify it (losing about 2v across the rectifier - a lot when you're struggling to get up over 12v) and then possibly invert it BACK to AC at 120v so you can run standard stuff off it. Losing power at each step. Nature can be a bitch.

#### iONic

Joined Nov 16, 2007
1,650
That's a huge battery! You'll burn quite a bit of energy just moving it around on the bike, making it even harder to generate enough current, let alone at least 13V + the proper charging current ~2A. A smaller battery would simplify things a bit, but the choice of a good generator (perhaps hand-made) will be a great challenge. Maybe a bicycle built for 5 could help too!

I'd find a good Wind power Forum and see what they have to say about that kind of power.
You may have to turn an entire tire into the generator.

#### Zefarius

Joined Nov 4, 2010
7
the bicycle won't be mobile
it will be stationary.
so its best that i find a dc or pm motor with voltage of 12v and 20amp right?
i'll look into scooter alternator then. the trick is to find low rpm dc motor in the range 12-20v with 10 amps or so..

#### Zefarius

Joined Nov 4, 2010
7
To add one more thing. i researched around and found out that most logical rpm for 26 inch bike is 80 rpm.. take the ratio of 26 in wheel and 1 in wheel. We get 80rpm*26=2080 rpm on the generator. So the rpm is most optimum to be around 2000 to 3000 rpm 15v-30v and 10-30amps

#### blueroomelectronics

Joined Jul 22, 2007
1,758
Figure out the horsepower required to turn such a generator under load.

#### wayneh

Joined Sep 9, 2010
16,102
the bicycle won't be mobile
it will be stationary.
so its best that i find a dc or pm motor with voltage of 12v and 20amp right?
i'll look into scooter alternator then. the trick is to find low rpm dc motor in the range 12-20v with 10 amps or so..
Well, again I don't think the power delivered by a human rider can do that, maybe half at best. Any PMA alternator will give a voltage that is proportional to rpm when unloaded. So ALL specs must be referenced to rpm to be meaningful, and you can have a useful estimate of output voltage by just ratio-ing the rpm.

The POWER produced by the PMA at a given RPM will peak when the load and the PMA impedance are matched. This will be at some value above the DC resistance of the windings themselves. However, at the low RPM of anything bicycle-powered, the AC inductance probably won't be more than the DC resistance. So impedance might be between 1X and 2X the DC resistance.

Anyway, if the windings are too big a gauge (low Ω), the peak power at a low rpm will not happen at a useful voltage. You won't have enough emf because you don't have enough turns in the windings. Tiny wire in the windings will give you more emf - in direct proportion to the number of turns - but will raise the DC resistance and the AC impedance. That will make you happy as you start getting power at low rpm, but unhappy when you can't squeeze more current and power thru the pipe by pedaling faster.

All this is why the wind guys debate endlessly on the right gauge wire with which to wind their PMAs. (say that 10 times quickly!) The analytical optimization is very complex and ultimately depends on your guesses about the power source (rider, or wind) and the load. "Rules of thumb" tend to dominate.

My hunch, and I've wound my own coils and done the math alluded to above, is that you'll want the PMA that gives you the most voltage at the lowest rpm. In other words, lots of turns in the windings. For the current you expect to produce, estimate the power that will be lost as heat in the windings. Use 2xDC resistance as a guess of the impedance. If the heat loss is too great, then maybe you need a few less turns and a heftier winding. But I'd start on the "too much voltage" side of optimization first, so you get something that works, and only then worry about getting more power thru optimization of the windings.

Fun, huh?

#### Zefarius

Joined Nov 4, 2010
7
I'm gratefull for the info guys..
wayneh the book you showed me
It explains bicycle generator in detail?

#### wayneh

Joined Sep 9, 2010
16,102
wayneh the book you showed me
It explains bicycle generator in detail?
Not sure about a generator, but I know it has great detail about the energy and physics of a human on a bicycle. Gear ratios, oxygen uptake, everything you can imagine. A fascinating book.

Have you discovered the shops out there that rewind automotive alternators with higher gauge (smaller) wire, to boost the voltage output? And there are PMA vendors as well, all for the wind crowd. This is where I was heading before I decided to invent something of my own.

If money isn't too big a problem, you can buy wind generator systems in hardware stores these days. You could cannibalize the generator out of one of those and it should be reasonably well designed for your needs, too. Low rpm electricity generation. The kits probably even include a rectifier and an inverter

#### Kermit2

Joined Feb 5, 2010
4,158
The generator may be capable of the 100-200 watt output, but most human riders are only capable of outputing 20-50 watts for anything but a very short burst. Have you considered adding more generators, or using several smaller ones that could be added or cut out to keep pedal resistance down to where a person can run the thing?

#### marshallf3

Joined Jul 26, 2010
2,358
GM alternators (the slightly older but not too old) are ideal, you can identify the proper ones as they only have a one wire connection. They take care of all the functions involved in properly charging a battey. Probably \$15 at a salvage yard.

They'll start charging around 1,200 RPM but are more efficient when you get up into the 1,600 - 2,200 RPM range. The deeper the battery has been discharged the more power required to turn them though. Considering they're usually capable of 85 or 105 amps output at about 14V and that due to efficiency losses it takes abut 1 HP to produce 500 watts you'd somehow have to limit the current output to a reasonable level you can produce.

Possibly hard to acquire, mount and wire up the alternator from an old riding lawnmower or equivalent might be more in line with what you're trying to do.

#### wayneh

Joined Sep 9, 2010
16,102
I think powering the field windings alone - for any automotive alternator - will overwhelm most riders. The field winding current is controlled to control the output of the alternator. But at very low current in the field windings, which you'd need to match up to a rider's capacity, I'm not sure if you'd get much at all out. People do take advantage of the cheap or free nature of these, but they generally need to rewind the coils to make them useful at low rpm.

#### blueroomelectronics

Joined Jul 22, 2007
1,758
As an experiment it's interesting. As a practical source of electricity no.