Hardware Reference
In-Depth Information
a conventional plug-in battery if the original ever fails.
CMOS Battery Troubleshooting
Symptoms that indicate that the battery is about to fail include having to reset the clock on your PC
every time you shut down the system (especially after moving it) and problems during the system's
POST, such as drive-detection difficulties. If you experience problems such as these, you should
make note of your system's CMOS settings and replace the battery as soon as possible.
Caution
When you replace a PC battery, be sure you get the polarity correct; otherwise, you will
damage the RTC/NVRAM (CMOS) chip, which is normally integrated into the motherboard
chipset. Because the chip is soldered onto most motherboards, this can be an expensive
mistake! The coin cell battery holder on the motherboard is normally designed so that the
positive of the battery should be facing up. Older motherboards may use a plug-in battery, the
connections for which are normally keyed.
When you replace a battery, in most cases the existing data stored in the NVRAM is lost. Sometimes,
however, the data remains intact for several minutes (I have observed NVRAM retain information
with no power for an hour or more), so if you make the battery swap quickly, the information in the
NVRAM might be retained. Just to be sure, I recommend that you record all the system configuration
settings stored in the NVRAM by your system Setup program. In most cases, you should run the BIOS
Setup program and copy or print out all the screens showing the various settings. Some Setup
programs offer the capability to save the NVRAM data to a file for later restoration if necessary.
Tip
If your system BIOS is password-protected and you forget the password, one possible way to
bypass the block is to remove the battery for a few minutes and then replace it. This resets the
BIOS to its default settings, removing the password protection.
After replacing a battery, power up the system and use the Setup program to check the date and time
setting and any other data that was stored in the NVRAM.
