CMOS

Alternatively referred to as a Real Time Clock (RTC), Non-Volatile RAM (NVRAM) or CMOS RAM, CMOS is short for Complementary Metal-Oxide Semiconductor. CMOS is an on-board semiconductor chip powered by a CMOS battery inside computers that stores information such as the system time and date and the system hardware settings for your computer. A CMOS is similar to the Apple Macintosh computer's PRAM. To the right is an image of a CMOS battery on a IBM compatible computers motherboard and the most common CMOS battery you're likely to encounter with your computer.

The bottom right picture contains other examples of batteries that may be used in a computers to power the CMOS memory. As mentioned above the most common battery is the coin cell battery (lithium battery), usually about the size of a U.S. Nickel.

The standard lifetime of a CMOS battery is around 10 Years. However, this can vary depending on the use and environment that the computer resides.