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.

Coin cell laptop CMOS battery

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.

Removable laptop CMOS battery

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 soldered to motherboard


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.

CMOS battery accessible from bottom of laptop

In some laptops the CMOS battery is hidden under the keyboard. As an example I took a HP Compaq nc6400 laptop.

CMOS battery under keyboard

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 under laptop cover


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.

Posted in Laptop parts
147 comments on “CMOS battery also know as RTC battery
  1. Heather says:

    Any idea where i can find a replacement cmos/rtc battery for a Toshiba Satellite A505-S6965 PSAP3U-01G00C ? I’ve looked everywhere, can’t find specs or anything on it, it’s soldered to the board.


  2. David says:

    pls i hv laptop toshiba dynabook TX/470LSE1 and when i on the laptop it showns me password up at the rightside hand of the screen i think is Bios password, pls how can i remove it from the system board i need your help. thank you

  3. onejohn says:

    hi repairman,
    i have the same battery as in pic3,
    do you know where i can get a new one? if not a simalar type
    thanks john

  4. deim says:

    Hi Mr. Repair-man,
    my laptop was Toshiba Satellite C800D which has RTC battery like picture #4 only it placed near the HDD slot, not near/around RAM slot.
    i had these problem, supervisor password which i don’t know. i had try the software but none of works. so now i try my self to do the hardware way.
    first, i just unplug the cable (black-red) for about 15minutes, and then plug-in. turn on the laptop and enter the bios. the timer was resetted, but not with the supervisor password (the password is still on it).
    so, is it the right thing to remove the RTC from the mainboard along with the cable, and also the other stuff such as battery, RAM and HDD also being removable??
    please help me.

  5. Misterfish says:

    Hi Repairman
    I had the same symptoms as Diztek (number 17) with the Satellite 1130 switched on, a quick whiz of the hard drive, nothing on screen then the little devil powered off.
    Reading your suggestions to Diztek, I first found the two wires in the memory compartment, shorted them to see if the bios changed, nope. But then I removed one memory dimm and lo! the Sat started first time, and every time since. Great stuff, thank you so much.

  6. Philip says:

    Hello. My toshiba a505 s6005 recently started showing the cmos battery failure error message upon start-up. Do you know if this model’s battery is soldered on?

  7. bill says:

    My laptop (hp dm3 entertainment)keep making a beeping noise (not from the speakers)at random times. mostly just a single beep sometimes a triple. could this be the rtc battery dying? i checked the main battery it’s fine i also ran checks on the hard drive and the memory they both passed, and i removed and put back in the memory and hardive as well and it is still doing it.

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