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Help File: Backup battery on motherboard needs replacing for clock to tell right time

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Washington Post Staff Writer
Saturday, February 26, 2011; 5:23 PM

My computer's clock resets every time I turn it on.

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A: Your computer hasn't run out of time, but a small backup battery on its motherboard has run out of a charge.

That battery - often called the "CMOS battery" after the type of memory it once powered - keeps the computer's real-time clock going. On older models, it also might deliver a trickle of power to keep a trickle of current going to keep a small cache of memory online.

Like any battery asked to do very little, it should run for years without trouble, then surprise you by failing to do so. Or you might not notice this at first if your computer is set to get the current time from the Internet automatically.

(As an aside, the default time you see will depend on your operating system and other factors. A Mac with a dead motherboard battery will reset to Dec. 31, 1969, while a Windows computer in that state may fall back to Jan. 1, 2001.)

Your ability to replace this battery depends on your computer's manufacturer and what type of machine you have.

PC vendors such as Dell and HP often employ standard CR2032 watch batteries and provide reasonably simple instructions on how to replace it in desktop computers. But laptops might not permit easy access to that corner of the motherboard.

Some Macs, however, need a battery about half the size of a AA that can be harder to find, and a few of Apple's desktops require service at its own stores or a qualified third-party shop.

After doing this replacement, you might need to redo some settings besides the clock. Check your computer's manual or its vendor's tech-support site for details.

- Rob Pegoraro


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