Summer camp project questions - bike charger

Question

For a summer camp, I am building a bike-generator. Well I have found a few articles about that on the internet, but I have put together a list of questions and I would be really appreciated if someone could answer them (see below)

The goal: To have a bike, and by pedalling on it generate electricity, which will charge a battery. The battery will be then used to power multiple USB outputs (smartphone charging)

My setup: I have connected a dynamo from old car (14V, 42 amps) to a bike. Then I have a battery (12V, 4 Ah, SLA). My setup will look like this:

^{simulate this circuit – Schematic created using CircuitLab}

And I have a few questions about this:

1. Is this form of "charging" suitable? How do I limit the current, so when pedalling faster (-> more current), I wont destroy the battery?
2. Is it possible to use the voltmeter VM1 to monitor the charge level of the battery? Or how to do it?
3. Can I plug smartphone into the USB ports? Is it safe? Can I use PolySwitch fuses?
4. Will this even work? :) Or any other comments/suggestions/etc. appreciated

Notes:

• this a followup to this
• sorry for any mistakes in my English
• my level of understanding electronics is somewhere near medium-low, I learned it all myself, so I hope this isnt a total nonsense :)

Thanks in advance!

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Ok then, I will divide it into multiple part with specific questions. To my question with Polyswitch fuses, I never worked with them before, so I wanted to know if there isn´t something I overlooked on the Internet. And to Jim, I know about solar cells, but this has more of fun/educational use :) For larger loads than charging smartphones I would use electrogenerator. Thanks.

Answer 1

I suspect the the car dynamo is far to big to be driven efficiently by a human. A child might be able to produce 50 watts of continuous power but the dynamo here is rated at 600 watts. Turning it with 50 watts input power may not produce any significant output voltage. In addition, most cars use alternators rather than dynamos so produce AC rather than DC. I would consider instead clip on bicycle dynamos (or multiple dynamos). You can get them for less than $5. (although some of these are alternators too so those will need a rectifier). Or you could use a permanent magnet DC brush motor as your dynamo. A typical permanent magnet DC brush motor about the size of a coke can could generate 50 watts. Depending on the voltage rating of the motor, you may need to use a voltage regulator (buck) rather than a DC-DC boost converter to stabilize the voltage to the battery.
You should limit the charge current to a quarter of the battery AH rating so in your case 1 AMP. A 1 AMP polyswitch fuse should take care of that. VM1 will tell you the voltage at the battery and is an indication of charge level. Power specs for USB ports can be found here. https://en.wikipedia.org/wiki/USB For fuses in the USB port I would be inclined to use 2 Amp fast blow wire fuses as I suspect PS fuses can be slow to react. Whilst this setup may be OK for occasional charging, some devices have current limiters built into their chargers to extend battery life. These are my personal amateur suggestions - I am not an electrical engineer. The best solution when going to summer camp however is to leave the devices at home.