Tech Files
I Licked it and my Tongue Didn't Sting
(CMOS Battery Replacement)
After listening to weeks and weeks of nagging, you've finally gotten around to working on your Mom's antiquated computer, trying to fix that irritating configuration error she's been telling you about. You get into the BIOS Setup Utility and glance at the clock.
4:37 a.m.
You're fairly certain that it's not actually 4:37 a.m. since the sun is shining outside and the last time you checked, you weren't living in Fairbanks, Alaska, during the summer. Since the evidence points against you forgetting Daylight Savings time for five years, you decide that the clock is absolutely, drop-dead wrong.
The little voice in the back of your brain says, “If this were a clock on the wall, I would just change the battery, reset the time, and hang the thing back up on the wall.” Well, except for the part about hanging it up on the wall, this may be the very thing you need to do to fix your Mom's computer – taking care of the clock and the nagging configuration error in one broad stroke.
The trouble is that batteries don't actually go “bad” (Try to imagine a “bad” battery… holding up convenience stores and shoplifting from the mall) all that often, so you've probably not had to change one in a computer before. After all, CMOS (Complimentary Metal-Oxide Semiconductor – a legacy term left over from the early days of computing) batteries are supposed to last for five years and who keeps the same system for more than five years? Evidently, your mom is the only one in the world to do this.
Sometimes, the battery that comes with the motherboard is just a dud. If your computer is fairly new and you're sure that it's the CMOS, you really have two options:
- Send your whole system back if it's still under warranty.
- Just get out the little book that came with it and look up how to change the battery.
What is that? You chose door number 2! Surely the info you need is in the manual; that's why the manufacturer gave it to you. Oh, there it is, you found it. The manual says:
Replacing a battery:
Be sure to replace the battery with the same type of battery as was shipped originally with the motherboard.
That certainly wins the Award for Helpfulness, right?
Never fear, changing a battery isn't all that difficult. The hardest part, actually, is trying to read the little letters and numbers on the battery after you get it out of the machine. Remember what the book said: “Replace the battery with the same type of battery” under penalty of death (I guess the manufacturer forgot to include that little section in the manual as well).
Locating the battery
Now begins the great CMOS Battery search. You might want to get the kids, it'll be a fun family event.
You are looking for one of two things: a little coin-shaped disk in a round plastic housing with a level holding it in place (Figure 1) or a little barrel-shaped thing that is soldered to the motherboard (Figure 2).
Figure 1: Disc Battery (click for larger version)
Figure 2: Barrel Battery (click for larger version)
If you find the barrel-shaped thing, then take the following steps:
- Walk out to the front yard.
- Face away from your home.
- Scream at the top of your lungs for no less than 15 seconds.
- Repeat this five times.
Why, you might ask, should I do this? Since the battery you found, the little barrel-shaped thingy, is soldered to the motherboard, you aren't going to be able to replace it. Maybe it's time to consider a new system for Mom for Christmas.
If you're lucky, you'll find the coin-shaped battery and won't have to go out in the front yard and make a fool of yourself in front of the neighbors.
Testing battery voltage
Now that you've found the battery, it's time to identify it as the culprit. You'll want to test the voltage of the battery before going out to buy a replacement – if it's some other thing beside the battery, think of all that time you would waste buying a replacement!
To test the voltage, place the probes of a volt-meter on the two terminals that connect the battery to the motherboard (Figure 3).
Figure 3: Testing the Battery (Click to see what the heck she's doing)
You should replace the battery when:
- A 3-volt coin-shaped battery has less than 2.5 volts
- A 3.6-volt cylinder-shaped battery has less than 3 volts
Replacing the battery
After you've tested the battery, figured out that it's a dud, and bought a replacement, it's time to take the old one out and put the new one in. Remember that computer manual from before? Here's what it should have said:
- Obtain a replacement battery.
- Boot your PC, enter the BIOS, and write down all of the settings
from the various menus (don't count on yourself to remember them, be
sure to write them down.)
- Shut down your PC.
- Gently lift the little lever holding the battery in place. Pop the
old battery out of its housing.
- Slip the new battery in and turn on the PC.
- Enter the System Setup Utility and re-enter all the settings you wrote down before (some settings might already be okay and some might not, you need to check everything before you proceed.
Ideally, you should change the battery before you start having problems or getting error messages. When working on older systems, or if you don't want to risk losing your CMOS or BIOS settings, replacing the battery would be a great idea.
Plus, if you change the battery in your Mom's computer, she'll be so proud of your incredible knowledge and forethought, not to mention her accurate clock, and she might even bake you those cookies she used to make when you were a kid.
Photo snagged from Linani's Cookie Factory (Mom
was busy with her digital camera and refused to take pix of any homemade cookies!)
- Truly Biggs / Jeremy Conn