Thinking of purchasing a Nuvi 550 or one like the Nuvi 2555 and concerned about battery life. I've read that the battery hookup on the back of the unit can be problematic. Wondered about the actual battery life (8 hours on Garmin's site). If less than that in real life use, would a spare battery work? I would rather have the 5" screen on the newer Nuvi's than the 3" on the 550 but worry about the vibration on a mc. TIA Ian, Iowa Edit: I've been using a 2730 that is getting iffy, I assume both these models will do the same tasks as that unit.