Stretch target: 10 days of GPS between charging points with minimal additional weight
I'm planning some long on-trail thru-hikes where digital mapping would be much lighter and more practical. But I'll be up to 10 days between charging points in climates where I can't rely on sun. Can this be done?
This is doable for navigation using a phone or a dedicated GPS
Given the many advantages of smartphones over a dedicated GPS, a lot of people have been researching the ins and outs of keeping them going on the trail.
The best source I've found is this comprehensive article by Alan Dixon, the co-founder of Backpacking Light and an actual rocket scientist! Alan and his friends have established protocols that keep a modern phone operational for 5-10 days between charges. To achieve this, you'll be setting up the phone carefully to minimise drain and activating the GPS manually to take bearings as needed. A number of other bloggers confirm that this is doable.
This is similar to the battery life reported by GPS users who only take bearings on an as-needed basis.
For on-trail work I would plan to supplement this with route cards, and by printing out sections of maps for any particularly tricky sections or exit routes. On a trail, this should be fine.
As insurance, most people seem to carry a 6000mAh external battery. This should give you another couple of charges and can be topped up pretty quickly when you hit civilisation, especially if you use one of the fast-charging options. At least one well-known thru-hiker takes a second phone for additional safety, and this would give additional usage time if things go wrong.
For serious off-trail work I personally would be taking maps and using GPS mainly for confirming location. There are some shortcuts that simply aren't worth taking.
For tracking you'll need a dedicated device like an InReach
If you plan to be days between charging points, you can forget about tracking with your phone. Users report battery depletion at around 5% per hour. You're going to have to carry a huge external battery that takes many hours to recharge in every trail town. No-one seems to think that this is practical. Dedicated GPS won't last much longer.
But it should be doable with a dedicated tracker such as an InReach, which is optimised for precisely this requirement. It has a phone-sized battery that lasts up to 100 hours.
If you have the budget, a watch would be a useful supplement
At the time of writing, the top end Garmins and Suuntos claim 40-50 hours of battery life at the slowest update rates, and they only require around 300mAh to recharge. So keeping them going is practical with a small external battery.
Of course they won't give you mapping, so they aren't a stand-alone solution. But they would greatly reduce the need to fire up your big phone or GPS during the typical day. They offer a lightweight altimeter, and their ability to sound an alarm if you drift off route is very attractive. They would be part of my ideal solution.
Hope this is helpful!