Here's a thought if battery life is a concern - a few days before departure put the GPS outdoors where it can receive a signal and turn it on for half an hour or so. Then replace the batteries with the set you intend to use in the field. The idea behind this is to do the slow "cold start" of the GPS ahead of time, on other batteries, so that any field use has a quicker start time since it's a warm start.

(this doesn't apply to some new units that I'm told never need to do a cold start, just the 30 second maximum warm start)