Hi there,
You can draw up to 500ma from the USB port but you are NOT supposed to do this.
The correct protocol is to take up to 100ma, then negotiate the connection to ask
the system for another 100ma, then another, until the max of 500ma is reached.
Things will still work without this negotiation, but the system will not know that
your device is drawing 500ma and that is the whole point as something could go
wrong if another USB device asks for more current or another device like this
is plugged in...it depends on your system though.
As far as getting the 7805 regulator to work (other post), you need to connect
more than 7.5v into the front of the regulator in order to get 5v out. This is
because the regulator drops some voltage too. There are LDO regulators (low
dropout voltage regulators) that can work down to about 5.5v, but you'll
have to look that up on the web. In any case, if you connect cells that total
up to say 9v dc you will surely get your 7805 to work just fine. Remember to
double check the output voltage first though before connecting to the phone.