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
July 17th, 2009 at 2:56 pm
Need a Schematic: Toshiba Qosmio F20. Model: PQF20L-00L00H.
Bought while on R&R from Iraq. Went to Thailand! Have had since 7/06 without any problems untill daughter had spilled a coke upon the Keyboard!
Awaiting New Keyboard & CMOS Battery. Plan to Upgrade & could use the schematic!
Thanks,
Bill
July 16th, 2009 at 10:14 pm
Abdul aziz sanin baba,
It worked on some older laptops. Will not work with newer laptops.
Take your laptop to the authorized repair center. It’s not free but most likely they can help (unless it’s an IBM laptop).
July 15th, 2009 at 12:45 pm
On what i new when system was passworded and accidentally the owner forgot the password by removing the CMOS battery you con retrieve the system back.
but with the introduction of the new laptops am were limited to unblock password,
WHAT SHOULD I DO???????????????????????
July 10th, 2009 at 9:02 am
cb,
Most likely you have bad CMOS battery.
I don’t know why it happens. Maybe you are looking for the CMOS battery in the wrong place?
Install the CMOS battery. Set time and save the settings.
July 9th, 2009 at 9:44 pm
have a inspiron 600m laptop and its not keeping the correct year (it is saying 2002). It was keeping the year perfectly before. When I checked the location of where the reserved battery was there was none. I’m a bit confused because the reserved battery (aka CMOS battery) is not there…but the laptop use to keep the year. What must i do? thanx.
July 7th, 2009 at 10:58 am
Need to replace the battery on my packard bell easynote F7305 laptop cannot get it out is there a special way to remove it please
June 16th, 2009 at 1:17 pm
Rebekah,
In a HP Compaq nc6400 the CMOS battery is located right under the keyboard, so you’ll have to replace the keyboard as it explained in this guide: How to remove keyboard and replace memory in HP Compaq nc6400 laptop. You can see the CMOS battery on the 5th picture, right above the memory modules.
1. Find and buy a new CMOS battery.
2. Remove laptop keyboard.
3. Unplug the old battery and replace it with a new one.
June 16th, 2009 at 1:12 pm
I just bought a used HP Compaq NC6400 and it needs to have the CMOS battery replaced. What do I need to do?
June 10th, 2009 at 5:54 pm
hi i have a lenovo laptop and im trying to remove the bios password and cmos password and i was wondering weather u could help me
June 5th, 2009 at 6:18 pm
Chris,
If you have an older Toshiba laptop this parallel plug should clear the BIOS password.
June 5th, 2009 at 6:06 pm
http://forums.pcper.com/showthread.php?t=464536
I’m trying to get rid of this bios password on this unknown toshiba satellite laptop. I first unplugged the power connector for 15 seconds. Booted it up and the password stayed. I then unpluged wall power, then unplugged the bios battery connector for 3 minutes. Now the computer won’t boot? The power button doesn’t power it on? Any ideas? Help would be high appreciated.
May 19th, 2009 at 12:33 pm
I HAVE ACCER TRAVELMATE C300 I have a HP NX9110 that is locked down with a BIOS password. how i can remove passowor howi can enter BOISE PLS HELP ME
May 8th, 2009 at 1:32 pm
Aza,
I think this battery is 15mAh. Check out this specifications for Sanyo Lithium Cell Type ML1220. Hope it helps.
May 8th, 2009 at 1:26 pm
Hi. Please does anyone know the specifications of the RTC/CMOS battery ML1220 of the Toshiba Satellite A135?
I know that is is 3V, but I have no idea about mAh, if 15 or 36mAh or what?
If anyone knows, please let me know.
Thanks.
April 27th, 2009 at 11:46 pm
I’m also a computer technician, and i found your website very informative, it adds up my knowledge about computer repair, most especially laptops
March 18th, 2009 at 1:28 am
Repairman, I’ve read a thread at the toshiba forum about this problem. Some people had the same prob. One guy sent his p100 for fixing and what they did was to remove the rtc for a while and then reincerting it, this did the trick. My warranty has expired and i found a great step-by step instruction on how to get to that little bugger, so i’ll go ahead and try this. BTW. yesterday i experienced that the clock doesn’t even work properly while computer is on; it was reset in bios at 20,25 and after several hours it was showing 20,48, so it “jumps” back to 20,25, but never before 20,25! It works a couple of hours, tops, and then resets, weird… I’ll let you know how it went!!
March 17th, 2009 at 9:52 pm
johan,
I think it should last longer. I see a lot of different laptops every day. Some of them are over 5 years old with a good working original CMOS battery.
Definitely. Plug in the main battery and charge it overnight. Most likely your CMOS battery will recharge too.
March 17th, 2009 at 7:12 am
Hi! I’ve got a toshiba satellite p100-227. It appears that the rtc battery is the problem. I have updated bios, checked conflicts etc, but to no gain. Everytime i change time/date (f2 during boot), it remains that way after restart. The computer is about 2,5 years old, (is it normal to have to change battery after only two years?) However i did remove the main batteryback for about a month to save it from going bad, then reincerted it(computer was connected to mains all the time). Can this be the reason the rtc has gone sour on me so fast?! Is there any possibility that it will recharge? Do you have a oem partnumber for this battery? Great site btw!
March 15th, 2009 at 1:49 pm
Jase,
I don’t know if it’s possible to clear the BIOS password on this model via jumpers, HP doesn’t share that information. I really doubt that removing the CMOS battery will help you.
I guess the only way to make it work again is replacing the motherboard. You can find a new motherboard by the HP part number. Usually the part number could be found on a sticker by the memory slot.
Also, search for HP BIOS password on ebay. Looks like some people offer BIOS password removal services.
March 14th, 2009 at 5:05 am
Repairman,
I have a HP NX9110 that is locked down with a BIOS password. I have contacted HP in regards to this matter, only to find out that at a considerable cost, they will fix it. The cost was astronomical, it was more beneficial to go and by a new laptop, so I did *Smiles. So I naturally I have a doorstop, that is sitting there, now at this stage I have disassembled the NX9110, and I am wondering if it is salvagable, by either shorting the battery out, or using the jumpers to reconfigure this computer, or even flashing the bios completely. As this is a later model, I am wondering if tinkering with it, will reborn this doorstop again, your guidance on this matter would be greatly appreciated.
March 3rd, 2009 at 11:37 am
Dizteck,
I believe that if you start the laptop with an external monitor already connected to the VGA port, you don’t have to use the function key. The external monitor should be detected automatically. If you cannot get any video on the external monitor, apparently there is no video signal coming from the motherboard.
I guess your assumption is correct, you have a failed motherboard.
March 3rd, 2009 at 11:16 am
I have 2 memory chips, I had taken the laptop apart 2 times, resetting everything that was removed. Like I mentioned, I don’t see the screen light up nor do I see all the lights light up. On a normal boot, I would have seen the screen go to windows start, the light for the WiFi on the front would light up–that’s on a normal boot. But, for some reason, when I turned on the laptop the next day it experienced what I mentioned. Push on button, lights flash normally, HD spins, then stops with no monitor activity. I tried plugging in an external monitor, but with this Toshiba, I would have to press the Function Key and another key to toggle between outputs. My final assumption is that, either the Motherboard has failed but, I understood it was replaced one year ago, included in the warranty.
March 3rd, 2009 at 9:09 am
bruno645,
What exactly it’s doing? Can you see the Toshiba logo on the screen? Will it start loading Windows and then dies?
I believe your Satellite M40X is similar to a Satellite M35X in which the CMOS battery is soldered to the motherboard. I really doubt that replacing the CMOS battery will help to fix the problem.
You can troubleshoot the problem as it explained in this article:
Laptop is dead. How to troubleshoot the problem.
The laptop in that article should be similar to your laptop.
March 3rd, 2009 at 8:54 am
Dizteck,
I don’t think that your problem is related to the CMOS battery.
From my experience I can tell that your laptop should start even if the CMOS battery is completely dead or missing. With a bad CMOS battery you cannot save BIOS settings but it shouldn’t prevent the laptop from booting into Windows. If you set date/time or change settings in the BIOS, they will be reset back to defaults on the next reboot.
Most likely the CMOS battery is soldered to the motherboard, as is done on many newer Toshiba laptops. As an example, take a look at the third picture.
Does it sound like the laptop is working fine except the screen? Can you test the laptop with an external monitor? Can you get video on the external monitor at all?
If both the internal and external monitor do not work, your problem could be related to the memory module. How many memory modules do you have installed?
If you have only one module, try reseating it. Try installing it into the empty slot.
If you have two modules installed, try removing them from the laptop one by one. Test your laptop with each module separately.
March 3rd, 2009 at 7:30 am
My Toshiba satellite M40X stop afer 3 or 4 mn….every time i put it on , it stop again (black screen, no message…) and i need to wait 1 hour then i can start it again, then it stop after 3 mn.
I hav verify ther is no heat, the computer is well cold…
I have oppened it to change the cmos battery, but i didnt find it.
My computer is 5 yrs old.
Thanks