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.
Need spare parts for your laptop?
If you are looking for spare parts for your laptop you can find brand new and used parts here. Just search by the part name and laptop model.
January 27th, 2010 at 2:36 pm
Lux,
I’m not sure if disconnecting the CMOS battery will help to get rid of the BIOS password. Maybe you can find this info in the service manual for Compaq Presario R3000 notebooks here: http://h10032.www1.hp.com/ctg/Manual/c00212209.pdf
January 26th, 2010 at 1:34 pm
Where can i find the Cmos on the compaq presario r3000?
I need to bypass a bios password.
Is there an other backdoor to compaq notebooks other than “compaq”?
Thanx in advance
January 19th, 2010 at 11:38 pm
the lost ranger,
If the BIOS password was set by the laptop, call Toshiba and explain what’s going on. Most likely they will clear the password at no charge.
The password is cleared by shorting two contacts on the motherboard. I cannot tell you more. If it’s your laptop and registered on your name, call Toshiba and most likely they will help you.
January 19th, 2010 at 11:29 pm
Michael,
I believe it’s Sanyo ML1220 rechargeable battery. Used in many newer Toshiba laptops.
January 15th, 2010 at 11:50 am
Whats the Number of this Bios Battery??
http://www.laptopparts101.com/wp-content/uploads/2008/12/cmos-battery-under-top-cover-560×420.jpg
January 15th, 2010 at 12:24 am
Is there a way to reset the bios password on the satellite a215-s5825? ( I do not see this model listed on http://www.insidemylaptop.com/my-toshiba-laptop-displays-password-prompt-on-startup/ ) And i saw mentioned “By the way, as I mentioned in the previous comment, removing the CMOS battery in this model will not help you to clear the BIOS password.”
If removing the battery does not reset it, is there a method i can use to do it? any jumper settings that may get it done? or anything?
thanks in advance
January 4th, 2010 at 4:24 pm
HI there Thanks for the info man! I was looking long time the location of it…it’s really hidden hehe
Next week i’ll do it and post results then.
January 3rd, 2010 at 5:59 am
Hi Pedro, the CMOS battery of the Extensa 600 is located behind the Parallel Port Connector. It’s on the front side of the mainboar. You have to completely disassemble the laptop. The battery is NimH 3.6 V /40mAh and is soldered on the mainboard (1cell). It can’t be seen because it’s hidden under the metal frame of the laptop. Basically
on the right of the Display connector … search for the service manual of the Acer Extensa 600 named ‘extensa 600 650.PDF’ for easier disassebmly. Good luck ! … it took us 2 days to find it … finally found it this night … it was a nightmare
December 22nd, 2009 at 12:12 pm
Hi
Anyone knows were’s located the onboard soldered cmos battery on Texas Intruments EXTENSA 600? It’s a really old laptop from 98…would be great if i could revive it. Dead cmos message on the startup screen.
Thanks
November 4th, 2009 at 8:19 am
Hey there RM, thanks for the sites they sure helped me out!
The battery definitely was drained through the short as ‘LOW CMOS BATTERY’ was displayed in BIOS upon successful boot and BIOS was altered to its default state.
November 4th, 2009 at 6:05 am
notuagain,
Thank you for your comment. I wasn’t aware that shoring the CMOS battery this way will fix the problem. I’ll try that when I have a chance.
Also, thanks for finding my guide for taking apart a Satellite L45 laptop.
November 4th, 2009 at 1:59 am
Toshiba PRO L40
Update
I was under the impression that since after WinXP BIOS update, the laptop had initially rebooted, passed BIOS, entered WinXP and after the second reboot and attempted save of BIOS, a simple BIOS reset to default was necessary.
(Summary WINXP_BIOS_UPDATE > REBOOT > WINXP > RESTART > ENTER_BIOS_TO_SAVE_DEFAULT > REBOOT > BLACK_SCREEN)
1.The laptop was dismantled with help from the excellent guide:
http://www.irisvista.com/tech/laptops/Toshiba-Satellite-L45/laptop-disassembly-instructions-1.htm
2. The CMOS battery was located and a small piece of solder was placed across battery wires connected to motherboard and left for fifteen minutes, in order to create a short and therefore force a BIOS default reset.
This was a success.
The BIOS was AMI based.
I hope someone, somewhere finds this information helpful, as it took me a full day to resolve this problem.
Regards.
Thanks.
November 3rd, 2009 at 10:41 pm
Toshiba PRO L40
Hi there,
After a bios update computer rebooted fine into windows. I then restarted and went into bios to save settings.
Computer performed reboot but gave black screen syndrome. I think a bios reset is required. The laptop has been stripped to locate the soldered CMOS battery and now I was hoping for information to perform a manual reset.
Thanks in advance.