Just to add, when you plug this adaptor in the Garmin lights up, so I have to turn the back light off again. It behaves as an external power source. The plug for it projects quite a way out the back of the unit, at least as much as the computer USB plug. So, if you can't get your computer lead plugged in when the 705 is mounted on the bike, you'll have to change the mount to be able to use this battery pack.
You can get other TipExchange converters, I have one for the iPod/iPhone too, so the battery pack will get me out of a problem with a flat phone too.