They don't, they're usually quite crappy and break easily when left too long out in the sun. If you plan on taking a hike with your digital compass watch, (makes sense, huh?) my advice is, DON'T.
If you have read How Pendulum Clocks Work, you know that all clocks (regardless of technology) have a few required components: A source of power to run the clock In a pendulum clock, the weights or...