Most laptops have, in addition to the main battery, two other much smaller batteries buried deep within the box: the real-time clock battery and the backup battery.

Real-time clock

The real-time clock, or RTC, maintains the internal clock … in real time. That is how the laptop knows the time as well as the date whenever you turn it on. It also has another purpose: to provide a trickle of power to the computer’s setup information, which is recorded in a special form of memory called CMOS (non-volatile) memory. The little RTC battery is recharged from the main battery and should keep time and hold power for at least a month all by itself (without power from the AC adapter or from the main battery).

If your laptop is going to sit unused for several months or more, you should top off the main battery every month or so. Or you can keep the laptop plugged into an AC source.

An RTC battery should last three to five years or more; however, on nearly every modern laptop, the RTC isn’t something a user can replace. If the battery fails (you may see an error message at startup, or find that the clock or the setup configuration becomes flaky, corrupted, or otherwise odd), you have to arrange for factory service.

Backup battery

This rechargeable battery, common on many laptop designs, provides enough power to keep the computer’s volatile memory (RAM) alive when the computer is in Standby mode.

Also charged by the main battery or the AC adapter, the backup battery usually holds data and settings in computer memory for several days after the main battery has been fully discharged. Like the RTC battery, the backup battery should be good for three to five years or more, and the laptop has to go to the repair shop if it fails.