Hey, I'm wayyyyyyyyyyy outdated in my GPS (Garmin GPS III Plus) and I know that, so as I start to look at this century's (LOL) models is battery "life" an issue. Mine is reliable however a real battery eater!
my garmin oregon 450 is very temperature sensitive. if its warm to hot outside the batteries will last close to their advertised time. and i have found no big difference between lithium and regular good brand alkaline. the colder it gets the less battery life i get. during late fall and winter i keep the thing "inside" my layers of clothing to keep it usable longer.