Citizen advertises Eco-Drive watches as "never needing a battery".

This is not true.

There is a small capacitor / battery inside the watch that will fail after 5-10 years, and it is not replaceable. It is also not covered under warranty.

Basically, you need to throw away your Citizen Eco-Drive watch once it stops holding a charge. At first, a day of sunlight will run the watch for months, but the watch steadily loses power over time, until even weekly 8-hour charges in full sunlight won't keep the watch running for more than a few days.