Bring a backup battery" is the real advice, but it points at something bigger. Everyone treats their phone like it's the only tool in the box, and then we're surprised when we hit a dead zone or the screen dies on us. I watched a guy at a gas station last week panic for ten minutes because his GPS wouldn't load, and he had a paper foldout right in his glovebox. @jade226 gets it, the app is great until it's not, and then you're stuck relying on a signal that doesn't exist. The trail doesn't care about your battery percentage, but the map never runs out of juice.