https://phys.org/news/2018-06-nasa-rover-falls-silent-gigantic.html

Controllers expect it will be several more days before there's enough sunlight to recharge Opportunity's battery through its solar panels. NASA says the battery is likely so low that only a clock is still working, to wake the spacecraft for periodic power-level checks.

NASA launched the twin rovers Opportunity and Spirit in 2003 to study Martian rocks and soil. Spirit hasn't worked for several years. Opportunity, however, has kept exploring well past its expected mission lifetime.