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.
Laptop parts | Repair Man | 
December 26, 2008 3:04 pm
140 Responses to “CMOS battery also know as RTC battery”
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Pages: « 6 5 4 3 2 [1] Show All
March 2nd, 2009 at 5:04 pm
I have a Toshiba L15-104, and I am experiencing no boot up. The HDD spins, a few lights flash, but the screen does not turn on. I recently bought a battery, and charged it. I have been told that the CMOS battery could be dead. I took apart the laptop twice, trying to find anything that resembles a battery or wires attached to a battery. No luck, I did discover, a plug that is not in use that is white with 2 leads that could connect to it.
This is about a 5 yr old system. I took it into repair a year ago, and am wondering if they “neglected” to replace the CMOS battery. I no longer have the warranty on it. Any help, please. Or the part number for this battery.
February 24th, 2009 at 8:24 pm
garry,
In a Toshiba Satellite A210/A215 the CMOS battery is soldered to the motherboard. You’ll find some disassembly instructions at http://www.irisvista.com/tech/
If you cannot find your particular model, search for similar models.
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.
Is it your reason for accessing the battery?
Some newer Toshiba laptop can set the BIOS password on its own. You’ll find more information in this post.
February 24th, 2009 at 8:19 pm
Hoban1962,
What is your reason for replacing the RTC battery?
If you are trying to clear the BIOS password, replacing/resoldering the battery is not going to help.
How exactly do you want me to help you? If the battery is bad, you simply unsolder the old one and replace it with a new one (if you can find it).
February 19th, 2009 at 4:56 pm
hi, im glad to read that im not the only one who cant find the cmos battery, i have tried to dis assemble the case so i can take a look inside but cant get it off, my laptop is a tosh a210-171. where can i find instructions to do this.
many thanks.
ps, already searched tosh`s website
February 19th, 2009 at 11:06 am
I have a soldered-on RTC battery on a toshiba satellite m115 and would appreciate a detailed description of how to replace it.
Please help!
January 13th, 2009 at 2:16 pm
charlotte,
On most newer laptops you cannot erase the password by removing the CMOS battery. I think you’ll have to contact HP customer service and see if they can help you.
January 12th, 2009 at 11:10 pm
I had a password on my HP laptop. I cannot clear the password. I tried everything. I removed the main battery and CMOS battery for 1 day and the password is still there.
Probably have to send it to HP now.
Does anyone know how to remove BIOS password on HP laptops?
January 9th, 2009 at 10:06 pm
alan,
I know that some newer Toshiba laptops can set the BIOS password on its own. Check out this Toshiba support bulletin, it’s related to your problem.
Toshiba will clear the BIOS password on some Satellite A100, A105, A135, A205, L35, M200, P100, P105, P205 laptops at no charge even if your laptop is not under warranty anymore.
January 9th, 2009 at 8:53 am
one day my 1year old toshiba started asking for a paassword but i never had a password. i cannog start windows. can i clean the password? do i have to replace cmos battery?
January 3rd, 2009 at 3:26 pm
Man! Searched everywhere and couldn’t find CMOS battery for my Sony notebook. Finally found it at PCHub. No time and date resets anymore!
January 2nd, 2009 at 3:16 pm
I found the battery!!!
It was under the laptop cover.
January 1st, 2009 at 1:50 pm
Debra T,
Nope, there must be a battery somewhere. I guess it’s buried somewhere inside the laptop and you just cannot find it.
January 1st, 2009 at 12:04 pm
Hi Repair Man. I opened all covers on the laptop and even removed keyboard. I cannot find the battery anywhere. Maybe my laptop doesn’t require one?
December 29th, 2008 at 10:13 am
Serge,
I don’t think so. From my experience I can tell that your laptop should start even if the CMOS battery is removed.
Without CMOS battery you will not be able to save settings (date, time, etc…) but the laptop should start.
Check the AC adapter. It’s possible that your adapter is dead and the battery discharged. You can test your adapter with a voltmeter. Make sure it outputs correct voltage.
December 29th, 2008 at 9:35 am
My laptop will not start at all. When I press the power button nothing happens. Do you think the CMOS battery is dead?