I uh...feel kinda bad, I was trying to point out that it exists and you could buy a really high capacity battery for relatively cheap that takes up the same space as carrying around a dedicated gps unit. There are a lot smaller batteries that would do the job as well.

I haven't been taking trips long enough to require an external battery. What I personally do is that I discovered that in the last 2 years or so they suddenly started making phones and batteries that charge much, much faster than they used to - if you have a quickcharger like this one:
Amazon.com: Anker Quick Charge 2.0 18W USB Turbo USB Wall Charger for Samsung Galaxy S6 / S6 Edge, Note 4 / Edge, Nexus 6, HTC M9, Xperia Z3 / Z2, Moto X and More (Black): Cell Phones & Accessories

(That's the one I own, though I also bought an ankr 10 ft charging cable). I have a similar charger for my car.

This page demonstrates the different charges rates (though note that you do need a phone that supports it, if your phone is old it won't help):
https://www.qualcomm.com/news/snapdr...see-difference

It charges the biggest phone you can buy (nexus 6) to 50% in 40 minutes. As long as you keep the screen off, gps tracking doesn't take up **that** much battery on my phone, so I put it on the charger for the drive home.

Of course there are other situations where an external battery is either necessary or just more convenient as well.

Again, I feel kinda bad, sorry if my post was misleading.