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I have a source of about 10v and with a resistance load of 0.5 ohm i am getting current of about 200ma.Can I charge a mobile with it?if this current is not enough then please suggest me a way to increase current as i can sacrifice about 4v to get that current because i need 5 v to charge a mobile?my battery is 3.7v Li-ion battery?

Well that of course depends on the mobile device... If it's an iPhone, I've read online from here: **broken link removed** that "An Iphone seems to need a 5V voltage and current of 500mA"

So an iphone needs 2.5 watts to charge effectively(0.5 amps * 5 volts = 2.5 watts)... you would be giving it 1 watt of power (0.2 amps * 5 volts = 1 watt)...

There's only one way to find out, try it! It should work though, but might take you much longer to charge your phone... Kind of like how my android charges slower when hooked up to my computer through USB port, and faster when plugged into the wall...

Check out this guy Ben Heck in this video(fast forwarded to a good part relating to this somewhat): [he's messing with solar panels which might not be the same thing your doing, but he talks about amps and voltage and how they relate to charging devices / and or batteries...]

If you used a switching regulator you'd get your 4v at around 400mA, which might be enough for your charger.
You need to measure what your 'phone draws on charge.

Change the resistor to 30 ohms and see if you have 6 volts from your source. That will tell you if it can supply 200ma at 6 volts.