Great Solution to Charging your watches when the sun isn't an option. Coolfire Solar Watch Charger. Here's my review of the device. Works with G-shock, Seiko, Garmin, Edifice, and much more
The solar panel on the watch needs light to get charged. Products like CoolFire Watch Charger helps but before we get into the question, how to charge a solar watch without sun, it is important that you understand the basic concept of light.
Keep charging the watch every month. Never allow the battery drops to empty .
If the watch stops due to low battery and if it not charged in time then the battery will drop further down to approach OV. In this case, the battery needs very long time to recover.