Bike dynamo: producing current from rotating wheel (low amperage in device's specs)

[ArminVanBuuren]

Hi everyone,

I am interested in buying a bike dynamo device to produce some electric current in order to charge batteries via USB:

**broken link removed**

The specs say that only 0.5 A will be generated at 12.5 mph. However, the person riding a bike with this:

https://www.amazon.com/gp/product/B000OBWMGK/ref=cm_cd_asin_lnk

Read 1.22 A at only 11 mph (video on that page at 1:02) !!!

I would buy the Amazon product, as it is evidently more effective & also cheaper, however, I would be unable to connect it to some kind of an USB connector.

1) What is the reason that the eBay USB dynamo is so much ineffective compared to the Amazon one?

2) Could such low-current charging actually damage the phone battery? Since it can even consume more current than is actually supplied? I do not know how the battery/phone would react...

Thank you.

 

[cl10greg]

1) What is the reason that the eBay USB dynamo is so much ineffective compared to the Amazon one?

Based on the physics of the generator or rectification. Most the time, for cheap devices like these, they are pretty much just motors that are rotated and the rotating coil induces the output voltage and it is proportional to the rate at which it is rotated.

2) Could such low-current charging actually damage the phone battery? Since it can even consume more current than is actually supplied? I do not know how the battery/phone would react...

Very unlikely. Phones have their own internal protection for charging and would have a under and over protection.

 

[ArminVanBuuren]

And what exact product would you recommend personally?

I though of something having eg. one 18650 rechargeable battery that would charge by these rotations. Then, when stopping, the phone would go on charging from the device's battery; the rotations would actually charge the 18650 rechargeable battery, not the phone battery directly...

 

[cl10greg]

Depends on your system needs. The ebay one is the whole package it looks like where it does the rectification and regulation for you. The amazon one you would have to do that.

Ah, I see the idea now. You basically are charging a portable battery pack to charge the phone anytime you want. I would hope you wouldn't hook up the phone and battery to the charger at the same time. Nothing to crazy should happen but you don't have control over the conduction path except for the internal resistance of the batteries. So you would need any extra step here to run your controller.

Charge phase
Circuit to detect when the rotational voltage is high enough to charge the battery (you can reasearch various locations but you need a certain voltage present to overcome the internal resistance of the battery to charge it)

Discharge phase
When rotational voltage is too low to charge the battery, turn on the discharge path to charge your phone.

You could do a voltage divider and some transistor logic to use a single signal to turn on or off the paths so only one is on at a time.
I hope this gives you a starting point. Let me know if you need anymore help or have anymore questions.

 

[ArminVanBuuren]

Actually I did want to connect the phone & battery at the same time. I meant a device that would have a battery inside, something like a battery pack (inside eg 18650 rechargeable liion battery). The phone would simply charge from that when connected; but when riding (at appropriate speed - so that voltage start charging), the battery in the pack (not that in the phone) would start charging - 2 independent circuits.

Similarly, batteries in car work. When we need some power, it can supply it even when the engine is not working - from what is stored inside from the previous ride. Once the engine starts, the battery is charged again to its full capacity.

But I doubt such a thing exists as a standalone product, does it?

Thanks

 

[cl10greg]

Okay. You just need to control the flow to them correctly and it should work out. You already mentioned the key. Basically a voltage detection circuit to determine when to turn on and off each circuit. I am not sure if it exist but something to look into. Also keep in mind that you need protect against overcharging. There are IC's that have some feedback systems to do this for you. But I think you are on the right track on what you need and how it works.

 

[Audioguru]

The 3.7V Lithium battery in a phone is charged from the charger circuit in the phone. The charger circuit is powered from 5V, not from a bicycle dynamo and not from a 18650 cell.
The phone battery is 4.20V when it is fully charged and its charging current has dropped. When its voltage reaches 4.20V then it is about 70% charged.

A 18650 cell is a 3.7V Lithium battery that can explode or catch on fire if it is not charged from a proper lithium battery charger circuit, not from a bicycle dynamo.
You cannot charge a phone and charge a 18650 cell at the same time from the same source because the voltage requirements are different.

 

[ArminVanBuuren]

And what about this?

**broken link removed**

They say "Battery Capacity: 1000mAh" ... it provides an USB output w/ 5V. Can I use this to charge my phone & would it work as described above? IE similarly to the car battery system? The 1000 mAh battery could supply current whenever, but when riding, it would charge AND supply current, if I get it right.

 

[kam1787]

- so that voltage start charging), the battery in the pack (not that in the phone) would start charging - 2 independent circuits.

It is not clear to me that you have a separate circuit for charging the stand-alone battery [not in the phone]. As AudioGuru stated you need a "proper lithium battery charger circuit" as well.

 

[pebe]

The chain driven unit should suit your needs. The blurb states that the generator is 3phase(AC). There are probably 3 separate windings each rectified and charging one of 3 Li-ion batteries, and the 3 batteries are wired in series to give a nominal 11.1V. Each cell would be a protected type to prevent overcharge and overdischarge.

A buck converter would then step the 11.1V down to 5V. So you would be safe to plug your phone in while the unit was still charging the batteries.

 

[ArminVanBuuren]

The chain driven unit should suit your needs. The blurb states that the generator is 3phase(AC). There are probably 3 separate windings each rectified and charging one of 3 Li-ion batteries, and the 3 batteries are wired in series to give a nominal 11.1V. Each cell would be a protected type to prevent overcharge and overdischarge.

A buck converter would then step the 11.1V down to 5V. So you would be safe to plug your phone in while the unit was still charging the batteries.

Do you have an idea what they mean by this?

"Output Power: 250mAh"

First, it could be meant that it can supply current of 250 mA (not hours), but it would be EXTREMELY small (would not charge a phone battery at all).

I also do not understand this table:

1-Riding Speed: 5 to 15kg Hour, Electronic Power - 100 to 300mAh
2-Riding Speed: 20 to 30kg Hour, Electronic Power - 400 to 600mAh
3-Riding Speed: Above 30kg Hour, Electronic Power - 700 to 900mAh

kg.hour? Some kind of a new unit? And I also dont know what they mean by "electronic power" in this context.

 

[pebe]

Probably a junior sales person given the task of writing the blurb, but who knows nothing about the technical terms. Like the TV newsreader I heard yesterday, referring to the lost plane near Australia, said “… the naval ship was listening for the electronic signals that the black box sends out from the bottom of the sea…..”.

Obviously ‘15kg Hour’ should be 15km/hour;
‘Output Power and Electronic Power’ should be ‘Charging current’ in mA.

 

[ArminVanBuuren]

So 30 kmh-1 would mean charging by 700 to 900 mA? That is not that bad!

And what does the "Battery Capacity: 1000mAh" figure mean? Do I understand it correctly that it works similarly to car battery? It can supply 1000 mAh when stopped & starts charging again (+ supplying current to phone) when riding? What substance is such a battery made of?

Thanks

 

[pebe]

Battery capacity is the number of hours it can supply power at a given discharge rate, and mAh is the product of the two - ie. amps x hours

For instance a 1000mAh (=1Ah) battery could supply 100mA for 10 hours, or 50mA for 20hours. Or a 7Ah battery could supply 700mA for 10hours.

The batteries in the unit are Lithium-ion polymer (Li-po). Any of the Lithium types must not be overcharged as that would be dangerous. The unit will certainly contain circuitry to charge them properly.

 

[ArminVanBuuren]

I still don't get it completely. So riding at 30 kph would give away up to 900 mA current? But what about the battery function then? I cannot spot when the battery would charge! It would supply current, when stopped, charge the phone from the battery, but when would it actually charge?

Thanks

 

[pebe]

You can liken it to the charging system in a car.

You get in the car with its battery fully charged. While you start the car, the starter takes a lot of current from the battery. As soon as the engine has started, the alternator starts to replace the charge lost from the battery, but with the engine just ticking over, that won’’t be a lot.

It’s night-time and you switch on the headlamps and even more current is taken from the battery. You accelerate and drive away. The alternator is now spinning much faster and starts to charge the battery and supply the current for the headlamps. After driving for about 10minutes the battery is fully charged again and charging stops. The alternator provides the current for the lamps.

If you now stop at traffic signals the alternator slows down and doesn’t have enough power to power the lamps, so they take current from the battery, As you drive away, the alternator starts re-charging the battery until it is fully charged again. And so on….

Does that explain it?

 

[Audioguru]

A lead-acid car battery does not explode or catch on fire when it is over-charged like a Lithium battery does.
The charger circuit in the phone is smart enough to detect when the phone battery is fully charged then disconnect the charger. But you do not have a charger circuit for your extra Lithium battery.

 

[pebe]

Quote "But you do not have a charger circuit for your extra Lithium battery".

If you mean the Li-Po in the cycle unit, then I assume that no-one would be mad enough to manufacture the product without a proper charger.

 

[ArminVanBuuren]

Quote "But you do not have a charger circuit for your extra Lithium battery".

If you mean the Li-Po in the cycle unit, then I assume that no-one would be mad enough to manufacture the product without a proper charger.

I also dont know what they mean by this. As we already mentioned, the charger probably doesnt run an easy-to-explode Li Ion battery for that purpose.

To the previous post: I get it, but the current produced by the car alternator is much higher. You only need to drive for 10 minutes and your battery is full. But here? It is able to supply 900 mA when speeding at 30 kph; so it only charges MY PHONE's battery. But I dont understand when it charges the device's battery (for the case I stop), then.

Thanks

 

[pebe]

My phone's charger is rated at 550mA maximum, so I think it charges at, probably, 400mA. Your phone is probably about the same.

So if your phone and the LiPo both need charging, then 400mA of the available current 900mA will go to charge the phone and 500mA will go to the LiPo. When one or the other is fully charged it will stop charging. When both are charged, the dynamo in the unit will cease to output any current.

So you see it provides charging current for both batteries, as necessary.