With my GPSMAP 66 I can extend battery life up to 21 days in Expedition Mode.

Plus I have a backup power bank 10000 mah with a solar window on it so I always have a fully charged in my backpack.

If I need to charge it I just hang it on the back of my backpack and let the the sun do it's thing.

As for using a smartphone doing a hike not so good.

Reasons:

You have it in your hands all the time.
(Not good climbing up on rocks)
(You drop it you break it)
(Or it falls in to the water as you slip on a rock)

As for the handheld I have it hook to my backpack and my hands or free.
If I need to look at the GPS screen I stop unhook it from my backpack or my belt clip.
They can handle a drop or a slip in the water and still work.

The phone stays in my backpack I do not need to call or have someone call me while I am hiking.
So it put in airplane mode.

If there is a emergency it is here to use.....