I fly sport planes and the battery in the iPad I use to augment the cockpit has a 5 hour life. I definitely have the "charged iPad" on the checklist, I don't make assumptions and I make sure to charge it before flight. I use it for GPS altitude only as I have 2 baro altimeters and an old Garmin GPS, but the iPad's GPS is usually more precise to indicate current height above ground.
A 12 day battery life for a smartwatch I would only wear when flying is more than enough. I would not wear the smart watch on the ground, my regular watch is running without any maintenance for 21 years (Citizen EcoDrive).
I think this likely has to do more with individual habits rather than anything else. But I find that watches that I do not use all the time are more likely not to be ready to be used when I need them.
For example, I have an old Suunto Ambit watch that I used for outdoor activities and/or exercise. I found that because I was not wearing it all the time, it was not charged most of the time that I needed it.
The point was to compare battery life and care for equipment. I cannot mod the plane to add a 12V socket, but I can use an external battery pack. Still, 5 hours vs several days comparison still makes the smart watch good enough.
The issue is certified aircraft. I think the 'certified' USB charger was around $300 for the part when we installed one - in addition to the certified mechanics time. The experimental side is cheap.
The Tecnam P2008 has a 12v socket and it's a rare luxury in an otherwise awful plane. Most single engine piston aircraft don't have charging ports or 12v sockets.
I had one in my '67 182 and two in each of the 58P and then A36 that replaced it. I seem to recall them being present even in the Piper rental airplanes as well.
I would expect them because I would have expected that back in the '50s and '60s people would have started using their plane's cigarette lighter socket for powering things, just like they did with cars, and so people got used to having power available and thus manufacturers would keep including it even when cigarette lighters were no longer standard equipment.
And yes, small planes used to have cigarette lighters.
There's an airworthiness directive from 1979 that required owners of a bunch of Cessna models to either disable the cigarette lighter or add a circuit protection device. There's a copy of that AD here [1]. It includes a big list of Cessna models and serial number ranges, which can give a good idea of how common cigarette lighters were.
My light plane has six 2.1A USB ports for charging use plus several others dedicated to avionics updates that can also charge. In addition I have three 24V cigarette lighter "outlets" which have two dual port USB adapters. So... Plenty of on board charging foe the six seats. Not hard to retrofit. I only wish I had more!
It's trivial to have such things for your plane (talking about small planes with propellers etc used for hobby flying, commercial aviation of course would have specific requirements) -- it's getting them to pass the relevant regulations that's the deal. Eg.
I know very little about planes and think of them as being very like cars, and a 12v socket is a cheap and useful feature in practically every car since the 1950s (more recently USB has replaced them) so I assumed planes might have the same thing.
My C182 had a 24v cigarette lighter plug that was there when the plane came to me and I think was from the factory. Took a while to find a 24V USB charging dongle, but when I did it powered my iPad just fine.
See FAR 1.1. It's (correctly) judged to be a minor alteration in the eyes of most A&Ps and can therefore be signed off by just an A&P with just a logbook entry.
