CMOS battery also know as RTC battery
Any laptop computer has a CMOS battery also known as RTC battery. The CMOS battery connects directly to the laptop system board and helps to retain important BIOS settings such as system time, date, BIOS configuration while the laptop is turned off or even when the main battery is removed.
The CMOS battery is rechargeable and it’s getting charged when the laptop is plugged into the mains.
CMOS batteries come in different shapes ans sizes.
On the picture below you see a basic coin cell CMOS battery. This type of batteries usually found in older laptops. This battery is removable and replaceable.
Here’s another type of CMOS battery. Basically, it’s two coin cell batteries but they are bundled together and have a cable which plugs into the system board. This battery is removable and replaceable.
On the next picture you see a CMOS battery which is soldered to the system board. In order to replace this type of battery, you’ll have to unsolder it from the system board.
CMOS BATTERY LOCATION IN A LAPTOP.
In some laptops the CMOS battery could be easily accessed from the bottom, as it shown on the picture below. As an example I took a Dell Inspiron 1720.
In some laptops the CMOS battery is hidden under the keyboard. As an example I took a HP Compaq nc6400 laptop.
Here’s the worst case scenario. The CMOS battery is hidden under the laptop cover. In order to access and replace the battery you’ll have to disassemble the whole laptop. As an example I took a Toshiba Satellite A305 laptop.
CMOS BATTERY RELATED PROBLEMS.
Here’s the most common problem related to the CMOS battery.
Each time you turn off the laptop it resets date and time back to factory defaults. When you turn the laptop back on it asks you to set date and time. If that’s the case, most likely your CMOS battery is old and has to be replaced.
By the way, removing the CMOS battery on most newer laptops will not clear the BIOS password.
April 14th, 2010 at 7:11 pm
nevermind… i see that I did not push the card in good enough… the screen loads now but still not able to get past the fact i dont have a boot password
April 14th, 2010 at 3:57 pm
Hey
This is coming from a fellow computer repair person. I dont repair to many laptops and I think this is why…
I have here a Compaq 2510c. The Startup/Bootup password is unknown to the user. I used the documentation from https://computinghardware.web.cern.ch/ComputingHardware/DOC/HP/HP-2510p/c01095444.pdf that shows how to fix it on page 39. It says basically to remove batter and then RTC battery for a few minutes and then replace it. I’ve done this very carefully. Now, the screen wont show anything. It seems as though the HDD works fine since all lights are coming up correctly. Any suggestions? I’ve reseated stuff just to double check likely 5 times. Looking for any tips here
March 29th, 2010 at 3:25 pm
Wes,
I don’t know, never tried it myself. I don’t think that a 3 volt battery will damage anything.
March 13th, 2010 at 9:38 pm
Whats the worst thing that can happen if I stick a 3volt battery in a 1.8v socket? I know the trace on the Motherboard can hold alot more than 3 volts. But as far as the bios chip?
I know and heard of toshiba and other companys making “Backup RTC Batterys that you just plug in somewhere on the motherboard”. Not sure where but i dont think it would hurt to put a 3v battery in a 1.8 socket. worse case scenario once it ran for awhile it would never fully charge up because the A/C adapter would only trickle it to a certain voltage. It would be stronger but im sure a bios chip would hold it. I can see 2 volts max hurting anything.
Just for kicks im sure it would remember wierd things like what you ate yesterday and it would be stored in your bios. You would load up windows and it would ask you.
…Press any key to boot from Cheeseburger….. LOL!