# Homework Help: What Dynamo To use?

1. Jun 26, 2013

### R Power

Hello

I am working on a project to charge a small battery say of 3.5 Volts and 1000 Milli Amp through cranking/wind up but I am not able to figure out what dynamo is required and how much time I would have to crank in order to draw the required power(3.5 Watts) in order to charge the battery completely.

2. Jun 26, 2013

### Basic_Physics

A starting point would be to look into the specs or measuring the current and voltage of a typical charger for your specific battery. Keep in mind that it usually takes quite long to charge a battery. A dynamo is actually a DC motor in reverse. So you can use such a motor to accomplish the task.

3. Jun 26, 2013

### R Power

I checked it. It is a 5 Volt and 350 mAmp charger. Now, how do I find a dynamo which can charge it and how much time I would have to crank in order to charge it fully????

4. Jun 26, 2013

### CWatters

This paper might help. It has info on the max power that you can comfortably crank out and at what rpm. If I remember correctly 60rpm was considered comfortable.

http://www.designsociety.org/publication/23972/human_power_comfortable_one-hand_cranking

That should help you figure out what gearbox you need to match the hand to any particular dynamo.

A dynamo approximates to a variable voltage source. You will need some electronics to regulate the voltage to keep it within say 5V+/- 10% for the existing charger you plan to use. Lets say that the regulator needs 3V headroom. You might then aim for a dynamo that generates say:

8V at 30rpm
16V at 60rpm
32V at 120rpm

Then cranking at any speed between 30 and 120rpm would work. Perhaps design the regulator to turn 8-32V into 5V. Beware though because the power dissipated in the regulator could be as high as (32-5)*0.35 = 9W.

If the cells are 1000mAh and the charger delivers 350mA into them then charging will take upto 3 hours. I think the paper says that hand cranking can deliver 50W but your charger only draws 5 * 0.350 = 1.75W. You could consider using a fast(er) charger and suitable cells but the charger and other electronics would be more complicated.

No doubt redesigning the charger and regulator into one unit that matches the dynamo to the batteries would give a better solution.

5. Jun 26, 2013

### R Power

Ok, now I have decided to use a different charger.

5 VOLTS and 1000 Milli Amp and cells as well say 3.7 V and 1500 MAmp.

how do you calculate how much time a charger will take to charge a battery? and how much cranking is needed for same.
Secondly, if the cells accept 3.5 volts then how a 5 volt charger charges them?

And I want to mention that the dynamo can be no bigger than the ones used in clockwork flashlights.

6. Jun 27, 2013

### CWatters

The charger probably converts 5V DC to a constant current of 1500mA. However best post a link to the charger so we can check.

The cells are not "1000 Milli Amp" they will be "1000 Milli Amp Hour" or "1000 mAH". So to calculate how many hours divide the "Milli Amp Hour" rating of the cells by the charging current in Milli Amps. In this case..

1000 / 1500 = 0.67 hours

0.67 * 60 = 40 mins.

If you want to charge them in under 1 hours the cells should be of a type designed for fast charging. Not all cells are.

One problem with this proposed set up is that the charger may not like being switched on and off during charging. For example if the person cranking keeps stopping and starting it may confuse the charger. Depends how sophisticated the charger is. Many have microprocessors in them because it's not always trivial to detect the fully charged state and that's important when fast charging.

This is probably a more complex exercise than many people think.

Is this just to demonstrate the principle or long term use?

7. Jun 27, 2013

### R Power

It is for long term use. Actually, it is a cell phone battery. even if I am able to charge 10% of the battery I am fine. I am looking for something like say crank for 3 minutes and talk for 5 minutes or more. Will it be possible??

8. Jun 27, 2013

### CWatters

You should have said. It's way easier to buy one. Google can find several wind up phone chargers.