The best smartwatches include sophisticated battery management functionality, that lets you eke out plenty of battery life between charges.
You can expect around a week of battery life from the best smartwatches if you're just using them in smartwatch mode. That will drop significantly once you start to use the GPS chip to track your activities, but even so, they should rival the best cycling computers.
If you're going long, a good cycling smartwatch should include the option to dial down battery-intensive functionality to add days to its runtime. That might include lowering the sensitivity or sample rate for GPS tracking or turning off some connectivity functions or in-built sensors, or just powering up less of the screen.
Garmin has added to battery life by including the option to choose solar charging in some of its smartwatches. We didn't find it added much to battery life in the Fenix 6, but battery life has been significantly extended in its replacement Fenix 7. The Garmin Enduro, also solar-charged, is claimed to run for up to a year between charges in its lowest power mode and 95 days with low-power GPS tracking.
When it does come time to recharge your smartwatch, most use a USB cable, although it might be one with a connector that's specific to your smartwatch.