It's keeps the information in the CMOS from resetting. When you change items in the BIOS (IE: boot order, time, CPU settings, etc.), those have to be saved some how. They're saved on volatile memory, which requires power to hang on to the information. That battery provides the necessary small charge to keep the information stored.
When you reset the CMOS, you're pretty much jumpering around the battery, so the memory goes back to default.
As a quick addendum, a good sign that you need to replace the battery is when your system starts showing strange dates and times when it starts up. Normally it's a sign of a failing battery.
hanging at post is usually due to some conflict or impending failure in a component in the system, not due to the battery. If the battery is bad, the BIOS will just "forget" your settings now and then, and will probably say "CMOS settings wrong, press F1 to continue" or something like that.
On your mobo there should be a place where it says clear cmos or clear bios(in some intel mobos). Read the manual and reset the cmos. That should fix the problem, or try putting your memory in a different memory bank.
probably not the mobo generally when mobo's fail it's an all or nothing thing seldom is it intermitent. Reset the cmos/bios reboot and set your settings to defaults so that you don't have any weird timing issues with your cpu or your ram.
Also check all your connectors a loose ide cable can cause it to hang as well. When it hangs is it hanging before or after it posts ie does it beep and start to display the bios stuff on the screen or is it hanging before that?
sometimes it beeps during startup and sometimes it just hang during startup. the hanging is random, sometimes during loading of windows, sometimes during bootup after bios, sometimes even when i use windows halfway..
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When you reset the CMOS, you're pretty much jumpering around the battery, so the memory goes back to default.
what could be wrong?
Also check all your connectors a loose ide cable can cause it to hang as well. When it hangs is it hanging before or after it posts ie does it beep and start to display the bios stuff on the screen or is it hanging before that?