I've done a couple solo off-grid floats and just rented a sat phone. These guys were real easy to work with-
https://www.bluecosmo.com/iridium-satellite-phone-daily-rentals.html
$10 per day isn't much if something goes wrong. It allowed me to send a text to my wife every night and morning. I called once or twice as well.
I always carry a USB battery pack (Trond 12500mAh/46.3Wh) with me. Fully charged, it can charge my regular cell phone and the sat phone for 4 days and still have some juice left. You can get 40000mAh ones now for $30 that could charge your phones for a week. Maybe 2 of those?
Btw, if your regular cell phone is fairly new, it will essentially put itself into airplane type mode if it can't find signal for a while. It does this to conserve battery. The older phones would just keep scanning and scanning, burning through the battery in just a few hours. If you're not sure, but know you don't have coverage it's best to just set it to airplane mode. They last a long time on airplane mode. So you can still use your camera and any notes you have.
And any apps that work offline. Also, turn the brightness down at night. The screen is the main consumer of power.
Another cool thing is if you start Google maps (and have it set to satellite image) while you still have coverage and then go into no coverage, some of the features won't work, but it will at least update your location with GPS. As long as you have clear path to satellites...usually to the south.
And if you zoom in/out to the main area before you leave coverage you'll be able to take advantage of the cache function within Google maps app to still be able to use the satellite image map. This can be very helpful when trying to figure out where you are.
Good luck and have fun!
