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Replacing CMOS Batteries in Old PC’s

Types of CMOS batteries often found in old PC'sYou’ve just fired up your old PC, the screen comes to life with the usual BIOS messages and memory count. All looks fine, until you hear beep beep and get this warning :

CMOS Checksum Error – Defaults loaded.

At least the PC still works , but the last message is a sure sign the CMOS battery has died and needs replacing. Without a working CMOS battery all the BIOS settings including time, date and hard drive parameters will be lost when you turn off the PC. An annoying inconvenience each time you start your PC.

Here is a guide to replacing CMOS batteries in old PC’s.

What does the CMOS battery do?
The CMOS battery is responsible for supplying backup power to the CMOS Real Time Clock (RTC) which stores date, time and hardware settings such floppy drive type, hard drive parameters, memory, cache settings and various other chip-set settings for the BIOS.

Battery types:

PC’s have used a variety of different battery types since their inception in the 1980’s. These are:

  • Real Time Clock chips
  • Rechargeable NiCd & NiMH soldered to motherboard 3.6 volt
  • External wired lithium non rechargeable 3.6 volt
  • Round lithium button batteries 3 volt

NiCd and NiMH batteries:

These batteries are mostly seen on 286, 386 and 486 motherboards from the 1980’s through to the mid 1990’s . They are rechargeable 3.6 volt type batteries which are recharged each time the PC is powered on. They have a life span of 5 to 10 years depending on the amount of use, environmental conditions such as heat and humidity and the quality of battery. Old NiCd and NiMH batteries are notorious for leaking corrosive substance over the motherboard which can ruin it forever.

Many PC motherboards used batteries made by Varta.

Varta NiCd CMOS battery
NiCd CMOS battery: Typical 3.6v NiCd CMOS battery by Varta. This one is leaking badly. Notice how the corrosion is spreading to nearby components and ruining this 386 motherboard. SMD resistors and copper tracks are being eaten alive. Check the hot melt glue between the battery and motherboard.

Varta NiMH 3.6 volt CMOS battery on 486 motherboard
NiMH (Nickel Metal Hydride) rechargeable: Similar to NiCd but without the environmentally unfriendly cadmium. Old NiMH CMOS batteries are also susceptible to leakage and corroding the motherboard.

Removing old NiCd / NiMH batteries

Old NiCd and NiMH batteries should be removed from motherboards as soon as possible. The longer they are attached to the board, the more chance of them leaking and starting an ingress of corrosion…which you don’t want.

Option 1. Snip off old battery with side cutters.

Snipping the old CMOS battery from the motherboard
Probably the easiest method and least likely to cause damage to the motherboard.  Just snip it off with the side cutters. Some batteries are not always that easy to cut off, so they must be de-soldered.

Option 2. De-soldering the old battery.

Desoldering the old CMOS battery from the motherboard
De-soldering the CMOS battery: Use a soldering iron to release one leg at a time. If the battery has hot melt glue holding it to the board, then break the glue bond by rocking the battery back and forwards a few times.

Take caution not to lift the solder pads or copper tracks when de-soldering the battery. Clean up excess solder with de-soldering wick.

CMOS battery removed and ready for cleanup
Battery removed…and to now clean up the board.

Mainboard cleanup:

Clean the board with methylated spirits or isopropyl alcohol using a toothbrush. This will remove any of the soldering flux and residue.

Any light corrosion around the battery area can be cleaned off with methylated spirits also. If the corrosion is more severe, then use a bicarbonate soda / distilled water solution to scrub down the affected area, again using a toothbrush. White vinegar also seems to work quite well as it neutralizes the alkaline battery leakage. After doing this rinse the area with distilled water and soak up any remaining water with tissue paper and then let dry. If you have compressed air available, blast off excess water with an air gun.

Inspection: Check the condition of the copper tracks around the battery and nearby components. Severe corrosion often destroys the copper PCB tracks resulting in a non working board.

Cleaning PC motherboard. Methylated spirits, toothbrush, biccarbonate soda, distilled water
Cleaning: What you need to clean up the the mainboard.

Replacement batteries:

  • External 3.6v lithium battery pack with leads: Many 286, 386, 486 motherboards have provision to take an external battery. These can be purchased quite easily on eBay HERE.
  • DIY external battery holder using 3 x AAA alkaline batteries:  Make your own external battery using AAA batteries. Attach this to the external battery connector on the motherboard, but you’ll need solder on a 4 pin plug and lead. Plug leads can can be hacked from PC speakers and also some CDROM audio leads also work. The 3 x AAA batteries give a total voltage of 4.5v which is above the original 3.6v, however it still works fine and I’ve never lost a mainboard using this method. Incidentally, some original mainboards came with 4.5v external lithium batteries installed. Beware of long term corrosion of alkaline batteries. These will probably last 3-4  years before corrosion sets in, so mount these battery packs using velco tabs well away from the mainboard. Think of old batteries in remote controls and what happens to those.  3 x AAA battery holders can be picked cheap on eBay HERE.
  • External NiCD / NiMH rechargable battery: Original NiCd / NiMH CMOS batteries can still be bought. But instead of soldering them onto the mainboard as they orginally were, they can be mounted externally, thus reducing the chance of corrosion ruining your valuable mainboard. This is great idea, when the mainboard doesn’t have an external connector for lithium batteries. Find external NiCd batteries for sale HERE.
  • CMOS backup Capacitor: Rather than using a battery, these heavy duty capacitors can be used as an alternative. They would be connected where the original NiCd / NiMH battery goes. I’ve never tried this myself and not sure of the results. Capacitors can be bought on eBay HERE.
  • Using CR2032 button battery: You can also try hooking a up a 3v CR2032 lithium battery and holder connected to the external battery connector pins. I’ve had varying degrees of success with this method. Later 486 motherboard seem to work ok with 3v and retain settings well for a long period. However, older 286 / 386 board really seem to work best in the 3.6v to 4.5v range to retain settings. Some later 486 boards have a button battery stenciled on the circuit board therefore may accept button battery holder soldered on.

External connector CMOS 486 motherboard

External connector CMOS 386 motherboard
External CMOS battery header pins are often not clearly marked, unlike these examples. You can try googling a mainboad manual if you know the model number, otherwise you’ll need to become polarity detective and determine + and – using a multimeter.

DIY external CMOS batteries: 3 x AAA Alkaline batteries and CR2032 lithium battery
DIY external CMOS batteries: AAA battery pack and CR2032 button cell with connector


  • Always make sure polarity is correct with external batteries. Some PC mainboards indicate the (+)  (-) clearly, but some don’t. You may need to trace the correct polarity using a multimeter to establish this. Put one probe on a screw pad (or another part of the PCB that’s clearly ground) and the other probe on either of the external pins. Where there is zero resistance, this is most likely the negative (-) terminal for the battery. Follow it through from the original battery also.
  • Some PC mainboards require a jumper to be moved to switch between an internal or external CMOS battery.

Do’s & Don’ts:

  • Beware, lithium batteries cannot be recharged like the NiCd / NiMH batteries and are not interchangeable. Doing so may damage the mainboard or cause the battery to explode.
  • Dispose of old NiCd, NiMH and lithium batteries correctly to reduce pollution to the environment.

Original External batteries:

External lithium battery on 286 PC
This 286 PC motherboard is nearly 25 years old and in perfect condition due to the original external lithium battery fitted.

If you’re lucky, your retro PC mainboard has been fitted with an original lithium external battery. The board will probably have survived intact without any of the corrosion associated with a typical soldered on type battery… and as a bonus, finding and fitting a replacement external battery is dead easy. Check out eBay for external 3.6v lithium batteries HERE.

Of course you can DIY an external battery as mentioned above.

Types of external CMOS batteries used in PC's
Examples of external CMOS lithium batteries found in classic retro PC’s.

Inside an external CMOS battery. Tadiran 3.6v lithium battery.
Inside the external battery. A 3.6 volt lithium battery with leads attached.

Real Time Clock (RTC) chips:

RTC chips are found in 286, 386, 486 and some early Pentium PC mainboards. They are quite reliable and can have a long life span, often lasting 10 to 15 years before the internal battery finally fails. RTC chips do not leak and PC mainboards equipped with RTC chips are usually in good condition as a result.

The original Dallas DS1287 was one of the most common RTC chips used and can be replaced with the newer DS12887 which can found on eBay HERE. It’s a direct replacement so it should slot straight in without a hassle. Other types of RTC chips are also used in old PC’s some are compatible with the DS1287 / DS12887 and some are not…so you need to check this carefully.

Other RTC chips used on PC mainboards:

  • ODIN OEC12C887A
  • ST Microelectronics MK48T87B
  • ST Microelectronics MK48T86PCI (pin compatible / equivalent to Dallas DS12887 RTC chips)
  • TH6887A
  • Houston Tech 12888A

Socketed RTC chips:
PC mainboards that have the RTC chip socketed are very easy to fix. The old chip can easily be lifted out using a flat screwdriver and replacement pressed  back in. Make sure it goes the right way around…there’s usually a mark to show the correct orientation.

Dallas DS1287 RTC chip on Pentium motherboard.
Socketed RTC chip. Super easy to replace.

Soldered on RTC chip.
Some RTC chips are soldered directly the PC mainboard which makes them more difficult to replace. They will need to be de-soldered, which can be difficult without proper de-soldering equipment, but it’s also possible to leave them in place and re-power the RTC using an external battery.

ODIN OEC12C887A RTC chip soldered to 486 PC motherboard.
RTC chip soldered directly this 486 motherboard.

Underside of 486 motherboard showing soldered pins of the RTC chip.
The underside of the PCB where RTC chip is soldered on.

De-soldering all these tiny pins can be tricky work and without the right equipment and it’s quite easy to make a mess of things and ruin the board. De-soldering with a standard soldering iron using de-soldering wick or a solder sucker just won’t work and you’ll probably end up destroying the solder pads. A professional de-soldering iron with a vacuum pump is the best way to remove the RTC chip.

Desoldering equipment required to remove RTC chip from motherboard.
Equipment for desoldering the RTC: De-soldering iron, examination glasses, screwdriver and solder.

Desoldering iron being used to remove RTC chip from 486 motherboard.
The desoldering iron makes easy work of removing the RTC chip. It takes skill to get the technique right, so best practice on a scrap board first.

RTC chip being desoldered from 486 motherboard.
First row of pins done.

Loosening the RTC pins after desoldering.
Often a couple of pins always cause trouble where the solder has not been removed properly,  so these will need to be reworked with new solder and sucked clean again. Release and loosen any stuck pins with a screwdriver before attempting to lift the chip out. With a good de-soldering job, the old chip should virtually fall out by itself, but more often it needs a little assistance to be freed from the mainboard.

RTC chip removed from 486 motherboard.
De-soldered RTC chip removed. Solder pads in excellent condition and have not suffered damage.

RTC chip removed from 486 motherboard. Top view
The RTC chip removed from mainboard. Next step, the replacement.

Replacing the chip:
I strongly recommend using a DIP-24 socket before fitting the new RTC chip as this will make future replacement considerably easier. Replace the chip with the same part number where possible, otherwise use the nearest equivalent. Make sure the chip goes in the right way around, there’s usually a marking showing the correct orientation.

Repowering RTC chip with external battery.
There’s a few guides to re-powering old RTC chips with external batteries if you’re keen to try. Check these out:

Button batteries.

Lithium 3 volt button batteries have been used for CMOS back up since the mid 1990’s starting in late model 486 PC mainboards. The most common button battery used is the CR2032 and is still used in modern desktop PC’s today. They have a normal life span of 3 – 5 years. Replacement batteries are cheap and easy to find.

The most common is the 3 volt lithium CR2032, but I have seen larger CR2430 lithium button cells used on some Compaq PC’s.

Sony CR2032 button cell used in Pentium motherboard
The CR2032 button battery has been used since the mid 1990’s t for PC CMOS batteries.

Renata CR2430 button cell used in Compaq PC
CR2430 button batteries are occasionally seen in propriety motherboards such as this Compaq Pentium III.

Replacement: Very easy to replace these. The CR2032 is widely available at supermarkets, department stores, markets & online sellers and only costs a few dollars. The old battery can be removed often without any tools, however some battery holders may require a small flat screwdriver to remove the old battery.

Replacing the CR2032 button cell
Replacing the CR2032. So easy.

Testing voltage on a used CR2032 button cell
Confirming the old battery is dead…and reading 0.12volts, it’s no wonder date & time won’t keep.

CR2032 button cell and holders recovered from used PC motherboards
CR2032 batteries and holders I’ve recovered from scrap PC boards.

Great links to other PC CMOS battery replacement websites:

15 thoughts on “Replacing CMOS Batteries in Old PC’s

  1. Andrew says:

    Hi Jeff

    I recently picked up an old Am386 33MHz retro computer. After opening it I saw that the CMOS battery was in the process of leaking. Remarkably it was still functioning, i.e. keeping the BIOS settings and RTC. It was still losing time after a while though. I wasted no time and removed the battery and cleaned the motherboard where damage was evident. Luckily nothing serious.
    This motherboard has the usual 4 pin ‘external battery’ connection where I can connect an external battery pack (non-rechargeable). I’ve read in some websites that the CMOS chip can accept a voltage range of 3-6V and that some people place a hefty 6V onto it without any problems. I noticed that you prefer 4.5V. What are your thoughts? Will 6V damage the chip or motherboard? Unfortunately I don’t have the original data sheet.


    1. Jeff says:

      Hi Andrew,
      I’ve never set the CMOS battery voltage as high as 6v, but I would not be surprised if doesn’t do any harm, but this could vary from motherboard to motherboard. Certainly from my experience 4.5v from 3 x AAA cells has never let me down. If anything, keep the voltage above 3 volts for best results and most importantly make sure polarity is correct. The external ground pin can be checked by using a multimeter to the ground of the board.

      Regards Jeff.

    2. Jeff says:

      Thanks Andrew… and a lot more to write, time permitting.

  2. Tim Verweij says:

    Thanks for the information. Any idea where I might find that AAA battery pack with connector? That looks very useful. I need a four-pin wide connector for my 486 motherboard, but every battery pack I that I find has a different connector or no connector at all.

    1. Jeff says:

      Hi Tim, I’ve got bought these from eBay.

  3. Youri says:

    Hi Jeff,

    Very interesting, your repairing guide!
    I have a CMOS battery problem with my old HP-DC5000, with motherboard P4SD: the CR2032 batteries, where the original one had a lifetime of more than 7 years, are now empty within one or two months (I already replaced the battery 4 times), unless the power cord is never unplugged. So I suspect a defective component on the motherboard draws too much current when either the PC is unplugged or when there is a power outage in the house. Not easy to find and repair this kind of defect. My first idea was to replace the motherboard, but I realized that an appropriate MB is not easy to find, nor cheap. Later I found a cheaper solution, “Build A Simple Rechargeable CMOS Battery” as described here: http://www.rason.org/Projects/cmosbatt/cmosbatt.htm (battery-pack + 1N4001 diode + 10 Ohm Resistor). My problem now is: I cannot find an external CMOS battery connector nor a related jumper on the MB. What are your thoughts about removing the CR2032 battery and soldering the + wire of the homemade rechargeable battery system directly to the battery socket + connector, and the – wire to any ground connector on the MB ?

    1. Jeff says:

      Hi Youri,

      Probably worth a try doing that… or alternatively a 3 x AAA battery pack may do the job as well.

      Nothing beats the original CR2032 and would be preferable to keep it that way.

      Some things to check:

      1. Confirm the CR2032 replacement batteries are still good..many of these new battery sit of the shelves for years and are old stock. Check the voltage of the battery is 3V or higher for a new battery.

      2. Check the motherboard for dust and cockroach or mouse poo which might be causing a short circuit. Remove and clean motherboard with alcohol and toothbrush if it is.

      3. Confirm the CR2032 battery is properly seated in the holder.

      Hope this helps and good luck.


      1. Youri says:

        Thanks for your advice. As soon as I will have some time, I will try to clean the motherboard (your point 2). I checked already points 1 and 3 each time when I replaced the battery. I rather think there is some short circuit on the motherboard.
        Best regards, Youri.

  4. Tobias Hamann says:

    Hello! I’m writting a bachelorthesis and I would like to use your picture of the Dallas RTC “Socketed RTC chip. Super easy to replace.” I’m describing RTC in general and this picture is just for illustration purpose.

    Greetings from Germany
    Tobias Hamann

    1. Jeff says:

      Permission granted. Good luck with the thesis. Regards Jeff.

  5. Roland says:

    I’ve just had an AES 7460 IBM 286 pc. Removed the bad battery. I don’t have enough information on the battery itself to find a replacement online. All that is writen on it is: “VARTA 5/60DK 038” How can i find a replacement battery?? Thx

    1. Roland says:

      http://www.microbattery.com/microbat/pdf/energyplus/energyplus-cmos-battery-057-2015.pdf : CMOS-5/60DK-PC is a direct replacement, COMP-19-2 does it as well. Thx for your very much informative page!!

  6. Marty says:

    I’ve encountered some newer motherboards that still give a CMOS battery error even with new CR2032 button cells. These don’t have the 4-pin external battery connector, so I’m thinking of connecting a 3x AAA battery holder through the CR2032 holder. Is the higher voltage safe for newer boards?

  7. Malvineous says:

    Another tip is to check the datasheet of the RTC chip itself. I have a board with a Dallas DS12888Q and no battery at all, just a connector for an external battery. Looking at the datasheet, this Dallas chip’s rated voltage range for the external battery is 2.5V-4.0V, so in this case three AA batteries (4.5V) would be operating it past its rated maximum limit. But that means I can be comfortable knowing that two AAs (3.0V) will power it just fine.

  8. Fanyit says:

    Very good and useful is your repairing guide! Thank you we need lot of more guides such this.

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