If a quartz watch suddenly starts moving its seconds hand in two-second, four-second, or irregular multi-second jumps, it may be showing an end-of-life battery warning.
What the signal means
Many quartz movements use an end-of-life indicator to warn that battery voltage is low. The watch may still keep reasonable time, but the battery should be replaced soon.
Do not wait too long
A depleted battery can eventually stop, and an old battery left inside for a long time may leak. Arrange a replacement while the watch is still running.
Not every movement behaves the same
Some quartz watches simply stop without a warning pattern. Solar and specialty movements may use different signals. Check the manual for the exact caliber.
After replacement
If the unusual ticking continues with the correct new battery, the movement may need a reset or inspection. A watch used around water should also have its gasket checked and case pressure tested after opening.
See What Is an End-of-Life Indicator? for more detail.