Dead CMOS batteries

Interon · Post by **Interon** » Tue Feb 24, 2004 8:35 pm

As most computer techs, I've encountered PCs with dead CMOS batteries. Trouble is (for most cases), when the battery is replaced, the CMOS chip still can't hold its data and I'm no better off than before I replaced the battery. The replacement battery was inserted correctly.

Are the leads rusted? Or is the motherboard showing its old age? Or some other reason?

johpower · Post by **johpower** » Wed Feb 25, 2004 2:37 am

Battery replacement is and has been one of those sticky ones. A damaged battery area is obvious from acids (a white fuzz, typically) showing. You may be lucky and just have the battery itself starting to leak. If the leads are hurting, they will appear discolored on the board itself. Not good. Remove the bat and attach an external. Alcohol or a solution of water and baking soda on a Qtip will neutralize the acid.

I usually look for the external bat leads (a 4-pin header near the bat or KB connector, typically). With the PC on you can usually attach a good exteral bat (w/3 volts min) to them --BE SURE THE RED WIRE GOES TO PIN ONE-- and shut down. Then swap the on board bat. This saves your settings for next boot. Some systems w/ATX will also save settings just by being plugged in. So you can swap bats with the machine "off".

If the bat has already failed and the sys is having to go into default to boot, it's too late and it's manual mode at the BIOS settings time again.

You can buy an external computer battery at most electronics stores, ~$5. You can make your own easily with a 3 AA/AAA holder and 2 single leads/4 lead header snipped from an derilect case. And finally a good cell phone battery can be used, but the leads will have to be split/cut from the 2 lead header.

Interon · Post by **Interon** » Wed Feb 25, 2004 7:35 am

What about CR2032 batteries (the silver oxide "button" batteries)? That's the ones I had trouble with (Pentium 1).

Unknown_K · Post by **Unknown_K** » Wed Feb 25, 2004 10:17 am

na010894 wrote:What about CR2032 batteries (the silver oxide "button" batteries)? That's the ones I had trouble with (Pentium 1).

The silver batteries can be found alot easier and cheaper then the barrel type thats soldered onto 386/486 boards.

I never seen a silver one leak, the other type will.

Interon · Post by **Interon** » Wed Feb 25, 2004 11:16 am

Yes, I replaced them, but the BIOS doesn't stop complaining about CMOS errors even after I set up the BIOS.

Unknown_K · Post by **Unknown_K** » Wed Feb 25, 2004 11:36 am

There is a jumper that shorts out the cmos so it resets, make sure it is in the correct position. Some motherboards also have a jumper that selects between the built in battery and an external connection, you will need to find your manual or doanload a new one from the company website.

If you cant find the information I can look in my database to tell you what to do (would need manufacturer, motherboard number, and revision)

johpower · Post by **johpower** » Thu Feb 26, 2004 2:18 pm

Some will also require that the PC be unplugged when you do the reset. Usually the ATX boards have this feature. Those who turn off the PC with an external power strip/device don't have this problem.

Interon · Post by **Interon** » Sun Feb 29, 2004 12:10 am

This has nothing to do with jumpers. It's a matter of taking the old battery out and putting the new one in. And the new battery does not help for some reason!

johpower · Post by **johpower** » Sat Mar 06, 2004 6:11 am

Have you actually checked the voltage on the new bat? I've found a few bad ones, new in the package. Can you describe the exact "complaint" message your PC gives?

Non-sequitar: CR2032 and CR2025 bats are interchangable in many cases. There is a slight thickness diff, but the diameter is identical. That hasn't stopped them from working for me if I had to make a swap.

Interon · Post by **Interon** » Sat Mar 06, 2004 11:25 am

The PC simply behaves as though the old battery was still there.

And the batteries I used were spare ones I removed from old worn-out motherboards (but the batteries seemed OK).

johpower · Post by **johpower** » Sun Mar 07, 2004 7:46 am

SEEMED? (she seemed pregnent...) Use a voltmeter and see 3+ volts or not. No "seemed".

OK, back to square 1. Stop fishing. Go to motherboard site and download MB manual and CMOS update, if applicable, as should've been done at the beginning. Follow instrux to letter.

If you can't find maker, MB designation, etc., we'll continue next post with where to look.

Interon · Post by **Interon** » Mon Mar 08, 2004 7:15 pm

Aaah, never mind. It's not worth going through all that trouble to get the CMOS data retained. On start up, all you gotta do is press F1 and presto, Windows 95 starts up. The BIOS automatically uses factory-default settings, and says "CMOS checksum error, Press F1 to continue."

I thought it would be way way simpler than this. Take the old one out, put the other one in. And since I used more than one replacement battery to no avail, I thought the mobo was shagged up. And the old one and the replacements were CR2032. And before the motherboards wore out, the batteries did their job.

Thanks anyway JP. These PCs are at a school I work at, it's not one that I own, so there's no desperate need to fix it. It would be a waste of time to do all that anyway since the computer can still run without a working battery. I didn't know it was a lot more than taking the old one out and putting the new one in.

johpower · Post by **johpower** » Tue Mar 09, 2004 6:18 pm

Usually it's not a biggy. You just appaer to have found a finicky one. Too bad and sorry I wasn't able to help more.

Interon · Post by **Interon** » Thu May 06, 2004 4:07 pm

Actually, later on, I used those batteries that "seemed" OK in other motherboards with failed batteries, and they WORKED!!! I guess it was indeed a motherboard problem.