Bike2Charge

[midasgossye]

Here you see my latest project;
http://www.indiegogo.com/projects/bike2charge/x/4813282

It's an open source bike charger that let's you charge your phone whilst your cycling.

[Dongulus]

Make had a weekend project video for the same concept:


It's a pretty easy DIY to make on the cheap without a PCB. This project looks like a good deal for your average person who has no soldering experience, but I would just make my own.

This project doesn't seem to include the dynamo, which probably costs more than the assembly they are hoping to sell.

[midasgossye]

You're right, the project doesn't include a generator.
This project is met for people who don't have any electronic skills, they just want to charge their cell phone.
Also, the Bike2Charge supports Ipod/Iphone Charging.
See: http://www.indiegogo.com/projects/bike2charge/x/4813282

[Stonent]

So generator + bridge recitifier + capacitor + 7805 + USB Port?

[rollatorwieltje]

How do you get roughly 8W out of a dynamo? Even the high power types only do about 6W. My Shimano hub dynamo barely reaches 2.5W at reasonable speed.

[tom66]

Personally, I would have used a TPS2511 instead of four resistors - the TPS device lets you charge Samsung and standard USB devices as well as Apple devices as it automatically figures out what device is connected.

I would also make sure to use several TVS devices (for safety/protection) and a DC-DC converter (poss. SEPIC type for maximum energy) instead of an LDO/7805 to generate 5V, that way you can get the most power out possible. The capacitor you used is also rather small, will this provide enough ride-through when pedalling slowly up a hill, for example? Don't want the phone to rapidly enter and exit charging as that could be damaging to the firmware.

[midasgossye]

The cap is 1000 uF.

Is that too small?

[tom66]

Well, before you ask for other people's money... perhaps you should check?! I am assuming the dynamo either produces AC or rectified DC. Either needs a storage capacitor. The frequency of the AC will be important to the amount of filter capacitance required. This information can be gleaned from measuring the output of a dynamo (with an oscilloscope), or careful research. I think 1,000uF is too small myself. Something on the order of 6,800 to 10,000uF seems more realistic.

Here, you can see you can get around 10W at 40kph cycling (25mph), but any more is difficult:
http://www.pilom.com/BicycleElectronics/HubDynamo.htm

Assuming 20% conversion loss, 5W to 8W charging is possible.  However, that does require a lot of cycling pedal power!

I'd suggest you investigate a supercap charging circuit (1.0F or so), and some hysteretic comparator to ensure the phone does not get interrupted too often. This would also allow you to charge at, say, 10W with a 50% duty cycle and maybe 2 minute on/off which should not be damaging to the phone. I haven't done the energy storage calcs to see if this would work though.

[midasgossye]

I did test it on a oscilloscope, the frequency is variable with your pedaling speed.
I checked and tested the prototype for months with a standard dynamo and it worked perfect with both a Sony Xperia U and an ipod Touch
On the video of make, they also used 1000 uF.

Thanks for sharing your thoughts of the project. I will think about putting a DC-DC converter in it for better performance and power loss.

[tom66]

Out of interest at normal pedalling speed what frequency did you get?

[midasgossye]

At medium speed, i got about 150 Hz. See screenshot oscilloscope in attachment

[tom66]

If you say 100Hz and 10Vpk minimum (Motor Volt = Freq * KV) and 7V min for 7805 operation, at I=1A, capacitance must be C = 1/(2*100*3) or 1,667uF. If you're including diode drops, then the Vpk might be 8.5V and Vmin 7V, giving 3,333uF.

I'd double each of those and round to nearest highest E12, to give some extra margin.

[midasgossye]

So 4700 uF or 6800 uF would be good? I will check if I can find cap's of that value with the same footprint. Or I change the PCB layout. Thanks for calculating, I didn't know the formula.

[midasgossye]

I've found cap's with a value of 4700 uF that will still fit on my PCB. Is that value large enough?

[tom66]

What I'd also add is a comparator. The comparator should look at the input voltage. If it's more than 10V, it should allow charging. If it falls below 7V, it should stop. Add a heavy time delay (say 10 seconds) on the start-up part. This way, it won't start charging if the input voltage is inadequate.

You should work out everything BEFORE you start the campaign on indiegogo. This includes the final PCB layout.

[Stonent]

You should work out everything BEFORE you start the campaign on indiegogo. This includes the final PCB layout.

What? You can't expect me to make decisions without a wallet full of cash first! 

[Marco]

No it's not, you need a supercap for the intermittent charging approach ... that's not the only problem though, if you want a circuit which can just be attached to an existing dynamo system you will need an input system which optionally limits it's power draw (so the bicycle lights stay on) and you will need a boost converter, since some dynamos deliver max 6 Volt which would give you very little headroom.

The Make approach with a dedicated dynamo makes most sense if an extremely simple circuit is necessary.

[midasgossye]

Indeed, my bike has LED lights. But I can imagine that it's not possible to charge your phone AND power up the light. It's a good idea to add a super cap. Because now, every time you stop, it pause the charging.

[Marco]

I think you're missing the point of the supercapacitor and what tom66 is saying ...

[tom66]

Plus constant start/stop charging has a serious risk of corrupting firmware/filesystem of the phone if the battery is discharged.  This has been observed with some other cheap chargers.

[midasgossye]

I've added the full schematics and PCB files. See the project page and you can download them: http://www.indiegogo.com/projects/bike2charge/x/4813282

[tom66]

I'm not sure you need a schematic to say bridge rectifier plus 7805.

[midasgossye]

New version will come out very soon. New website as well: users.telenet.be/bike2charge

[AndyC_772]

This is a perfect example of a situation where a linear regulator is absolutely NOT the right tool for the job.

Travel uphill and the output voltage falls outside the USB spec, rather than cutting out completely. Downhill, the 7805 has to drop the extra voltage and can overheat.

The circuit is crying out for a 'simple switcher' at the very least, to avoid wasting so much power and having to dump it as heat when the generator voltage goes up. Personally I'd consider a SEPIC converter, so I can get 5V out regardless of whether the input voltage is higher or lower than this, right down to the point where the generator output falls below the regulator's UVLO point and cuts out.

[midasgossye]

Yeah, a lineair voltage regulator is a bit crusty for this job. On most bikes, it works fine if you're cycling at moderate speed. But indeed, if you're cycling slower it won't charge your cell phone. To fast, and it will warm up quite a bit. In the new version, I'm going to put a switchmode DC-DC converter, but I don't know which type i'm going to use. I knew about modules that can step-up and step-down voltage, but I didn't knew anything about SEPIC converters.