CMOS Backup Battery

Posted April 7, 2001 1:02 am by with 0 comments

The battery in a PC is often one of the most forgotten parts of the computer. It is quite important, too. It is what holds all of your CMOS settings while your computer is off. Without it, you would have to re-program your CMOS each and every time you turned on your PC.

The history of this is quite simple. The early PC’s from IBM used DIP switches to set the system configuration. Later, though, they decided to transfer this information to bit-information stored in some static memory. This memory is low-power, obviously, and uses a form of static RAM called Complementary Metal Oxide Semiconductor (CMOS). Using a small, low-power battery, they could run just enough juice through this memory to get it to hold its content even when the PC is off.

Like any battery, the battery in your PC has a finite lifespan. Eventually, it will become weak and will no longer be able to sustain the contents of your CMOS. When this happens, you will turn your PC on one day and get an error message. Sometimes you will get a checksum error, or it will notify you that the system configuration doesn’t match the CMOS information. Well, that’s because there is no CMOS information anymore because you need to replace your battery. And one problem is that users very seldom record their CMOS information or back it up.

This is usually a simple repair, but some oddball manufacturers actually soldered the batteries in, making this a much tougher job. If your battery is soldered in, you may want to take the whole thing to the shop. If you’re experienced with soldering, then you can tackle it yourself. The good news is that most manufacturers are now using easily removed batteries, the kind about the size of a nickel and can be removed by moving a small prong.

Before you do anything, though, you should record what your computer is supposed to know. If your battery is already dead, there’s nothing you can do. If your configuration is still there, though, record it. Go into CMOS and write down the info (HINT: You can write it real small and tape a copy inside your PC case). Easier yet, just go to those screens that are important and just hit the Print Screen button on your keyboard. There are also utilities out there that can backup your CMOS settings and record it to a diskette. This is quite convenient, especially when you don’t have a setup disk for the PC. After you remove the old battery, your computer will forget everything.

Let’s go through the battery replacement procedure:


  1. Turn off the computer, unplug it, and remove the case.
  2. Remove the old battery. Record which end faced what direction. Each end has a + or — on it. With skill and dexterity, the battery should snap out. Just study it, and you’ll figure out how to get it out. Don’t force it, though. It may be soldered in. If its a new board, you may only need to move a prong to take the battery out. Many newer boards have a small, flat coin-shaped battery which is a lot easier to remove.
  3. Get a replacement battery. Take the old one to the store and match them up. It should be pretty easy to come by. The CR 2032 is a pretty standard size of battery for us on motherboards. Once you find the correct battery model, you may want to write it down somewhere.
  4. Put the new battery in. Make sure the + and — face the same way as before. It should snap in. If your experienced at soldering, and your computer demands this, you may want to do that yourself.
  5. Put the case back on and plug it in. When you turn it on, expect some type of error message like incorrect cmos. Don’t cry. This will happen. You just need to go into the CMOS and plug in all that info that you recorded before you started. If you didn’t do that, you’ll need to break out the manuals and find the info the hard way.
Two thoughts: In some systems, the battery is quite prominent and the above procedure can be done while the system is operating. This, of course, it not the safest thing to do due to risk of shock, but it can and has been done. Also, if you’re really fast, you may be able to replace the battery and not lose any settings. There will be a residual charge left in the CMOS that MAY hold the data for a few minutes. If you’re fast, this will save time, but don’t rely on this. Make sure you have a backup handy.

Good luck!

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