Cell phone battery charger motorcycle

[ElectronicsDevil]

Hi Friends,
I am a newbie to electronics and really this electronics is getting interesting and interesting.
I want to build a portable battery charger for my cell phone.
Can anybody suggest how should i proceed, Because bike battery is having great voltage fluctuations. If you have a schematic ready then please please give it to me.
Rating of battery charger: 5V, 0.7A
Battery of motorcycle : 12V

Added after 2 minutes:

I also have participated in electronics project in my school....

 

[IanP]

You may consider switching dc-dc step-down voltage regulator and the LM2575-5 seems to be a good candidate for the job ..
**broken link removed**
It'll compensate for fluctuations in the bike's battery voltage and, what is also important, it limits the output current to max. 1A ..

Rgds,
IanP

 

[ElectronicsDevil]

but the max allowed current is 700mA and it Should be kept at 350mA..
Can you suggest how to suppress this current to 350 mA.

 

[IanP]

You haven’t mentioned 350mA before, I’m not a clairvoyant ..

In that case you can consider a constant current circuit based on the LM317 (mounted on a heatsink) ..
Regardless (almost) of the input voltage will output the current of 350mA .. see attached picture .. and example at:
http://users.telenet.be/davshomepage/current-source.htm

R ≈ 3.6Ω

Rgds,
IanP