heats not really that big of a deal, just strap a hunk of metal on it and it will run happily with 500ma draw and 12 volts being applied to it ... the trick is to find a hunk of metal on a motorcycle 
(just for clarity a hunk of metal that does not get hot, unlike the exhaust pipe)
celphones also often have a bit of work in the usb wall chargers, like a resistor pulling a data line one way or another ... so it will pull what it wants off of a charger, but I bet if you plugged it into a computer USB port it will draw less than 500ma, but charge slower.