Real-Time Clock Subsystem
A coin-cell battery (CR2032) powers the real-time clock and CMOS memory. When the computer is not plugged into a wall socket, the battery has an estimated life of three years. When the computer is plugged in, the standby current from the power supply extends the life of the battery. The clock is accurate to 13 minutes/year at 25 ºC with 3.3 VSB applied via the power supply 5 V STBY rail.
If the battery and AC power fail, date and time values will be reset and the user will be notified during the POST.
When the voltage drops below a certain level, the BIOS Setup program settings stored in CMOS RAM (for example, the date and time) might not be accurate. Replace the battery with an equivalent one. Figure 4 shows the location of the battery.