LEL 2013 / London - Edinburgh - London Audax

Thats only 250mi for you.
Point I'm trying to make: If you are using a Garmin 800, don't use it in map mode if you want it to last.

No, I mean on training rides out in the lanes where I am often using turn-by-turn and flipping between maps and stats screens. I remember it ran out before 12 hours once and another time last over 12hrs but had warned me it was low.