Thanks for the answers, I'm interested in the portable power supplies web site, the prices seem good. Managed to borrow a mates powermonkey but that acts like a charger i.e. When its connected I can't use the unit because the Edge goes into charge mode. Do these units mentioned do the same or is there electronic trickery at play and acts like an external battery.
It's to do with the cable (and/or the battery pack, but the right cable is enough to sort it out). Full on USB data cables may make it go in to 'computer mode' and stop it being usable as a GPS because the unit detects it is connected to something clever (not just a power source) and asks for more power. (I'll explain why below.)
If you use a 'power only' USB cable (that won't do data) it can't detect whether the thing it is connected to is clever or not and so just uses it to top up the battery and/or power the unit whilst still working as a GPS.
AIUI it's to do with negotiating a higher power supply. The USB standard states that a device can only draw 100mA as standard (over a dumb connection). If it wants more (up to 500mA) it needs to ask the host (computer) for this, to do this it needs to speak to the host via the data pins and the USB protocol.
Some smart USB power packs (like the Powermonkey) are able to speak the basic USB protocol so that devices connected can negotiate a higher power draw.
The problem is that the Edge GPSes will automatically go in to 'computer mode' if they detect they're talking to something clever. There's no way to have them negotiate the higher power draw (500mA) but remain working as a GPS.
To get around this, you have to use a 'power only' USB cable so that it doesn't go into 'computer mode' and charges/powers the device at the base 100mA current.
Put it another way:-
My Edge 705 will go in to 'computer mode' when I plug it in to my computer using a full USB data cable (as supplied with the Edge 705). If I plug it in to computer using the cable that came with the PortaPow battery pack it doesn't go in to 'computer mode'.
I've no idea if the PortaPow unit can do the increased power negotiation stuff, I'll try and remember to test it against my Edge 705 with a full data cable to see.