Portable Emergency AA Battery Charger Extender suitable for the Garmin EDGE 500
This comes highly recommended by other randonneurs.
I wonder if it works with the Edge 810?
Andrew Mead says:
I start with the Gomadic plugged in so the Edge draws external power first and doesn’t put the AAs into a high current draw-down needed to recharge the Edge.
The Edge will alert me when it switches to the internal battery so I know when I’m down to my last 15 hours of power. I suppose I could swap in new AAs at that point and continue indefinitely.