The Garmin
Edge 200 allows the use of the supplied data usb lead to power the unit from an external battery while using the GPS.
Unfortunately Until Garmin modifies the firmware of the Edge 500 to allow the use of an external power supply to charge and use the GPS at the same
time, you will have to use a special lead. The secret is a resistor that tricks the Garmin into not switching to charge mode, as mentioned by AWEATHERALL. Or to use various other solutions mentioned in this thread.
To power the Edge 500 from an external battery you need a mini-B usb plug with a 47k Ohm resistor connected between pin 5 ground and pin 4, a suitable power lead can be made using one of the mini-B usb plug power tips that are sometimes supplied with external usb battery supplies or chargers, the trick is to look for the spare tips that can be dismantled, as shown in the photo with the circuit diagram. with a little patience a right angled plug can be made by modifying the plug casing and encapsulating the resistor and wiring with Araldite. Or for a straight plug close the casing of the plug after changing the resistor.
After carefully opening the plug you will find a resistor already soldered between pin 5 and 4, this will be the wrong value and must be replaced with a 47k Ohm resistor.
(yellow Purple orange)
Obviously test the plug before opening it to see if it works as it may contain the correct resistor.
When external power is lost the Edge 500 displays this. Press the power button to revert to charging mode only.
Mini-B usb pin connections
1
Red +5 volts
2 White data
3 Green data
4 ID
5 Black Ground
che ne dite ? 
