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.
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.
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.
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.
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.
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.
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.
- 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 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.
- 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:
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.
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)
- 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.
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.
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.
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.
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:
- Fixing a Flat Dallas DS1287 Real Time Clock Chip
- Reworking the DS1287 / DS1387 RTC chip
- Modifying a Dallas DS 1287 battery/real-time clock in a Compaq SLT/286 Portable Computer
- IBM Model 25 Clock 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.
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.
Great links to other PC CMOS battery replacement websites: