Yesterday my aging desktop PC decided not to boot, and instead displayed this helpful error "CMOS Checksum Error".
In order to get it to boot into Windows I had to press Delete to go into the BIOS settings and change the configuration from "Halt on all errors" to "halt on no errors". Upon saving the BIOS settings and restarting the error disappeared.
Sometimes though a PC with a dead CMOS battery will boot as normal but forget the date and time on each reboot. This can lead to odd effects. For instance I tried to logging into my webmail only to be told the SSL licence wasn't valid, not because it had expired but because my computer thought it was 2001!
CMOS Battery at Fault
Once I got into Windows I got a few "Windows has found new hardware" messages and my system clock had reverted to a day in 2001.
As soon as I saw my clock had forgotten the time and the date, all evidence pointed towards the CMOS battery being at fault. Its funny how a simple little battery that most people don't even realise existed inside their PC can bring a computer to its knees.
Locating & Changing the CMOS Battery
In a desktop PC the CMOS battery is fairly straight forward to find. They normally look like a large wrist watch battery, with CR2032 lithium batteries being the most common. A simple search on eBay will find you a cheap replacement. Just be careful removing and fitting anything on a motherboard, because any static electricity on your body could fry delicate computer chips.
On a laptop or notebook they are more difficult to find and generally more expensive. On my Dell Inspiron you need to lift out the main battery and pull out a small flap to locate the CMOS battery.
Location of CMOS battery in Dell Inspiron laptop
My Dell just so happens to take a 7.2V 15mAh Ni-MH CMOS battery, which again, performing a quick search on eBay will find you a replacement.
Fitting the new battery in either case is very straight forward.
Read more about Installing a CMOS Battery here.