Your cell phone tries to keep you constantly connected to the grid. When you are off the grid your cell phone will try every few minutes to find a tower. This hunting for towers is a large drain on battery life.
If you know you’ll be away from cell towers for a few hours, pop your battery until you reach civilization again. Otherwise it might be dead when you get there.
If you have a phone you can’t remove the battery from it might keep calling home and stick you with a $4800 bill from ATT