A clock circuit that barely sips power
A computer does not need to keep its processor running to remember the time. The motherboard has a real-time clock (RTC), a small circuit that counts seconds while the rest of the computer is off. A quartz crystal vibrates at a steady rate; the circuit counts those vibrations and turns them into seconds, minutes, and calendar time.
When the computer is unplugged, the coin cell supplies the tiny amount of electricity the RTC needs. It is not meant to power the computer or keep files in memory. Its job is to preserve the clock—and, on many systems, a small amount of firmware configuration—until normal power returns.
What changes when the cell runs down
Once the battery can no longer sustain the clock, the RTC may lose its time when the computer is shut down or disconnected from power. On the next startup, the date may be wrong or revert to a default. The computer itself can still boot; it simply no longer has a reliable local time to begin with.
Many computers can correct the clock by syncing with an internet time server after startup. Until that happens, incorrect time can make websites or other services reject secure connections, because their security checks depend partly on dates. The small battery’s practical importance is easy to miss: it keeps the computer’s sense of when “now” is—even when everything else is off.
