CMOS Battery for 486 Motherboard

[owensdj]

Someone I know has an older computer with a 486 motherboard. It started getting a "CMOS battery low" error message at boot and won't keep the hard drive settings. After putting in the settings it gets the same error message and won't boot into Windows 95.

I looked at the motherboard, and it doesn't have one of those round CMOS batteries like newer computers use. I saw a rectangular chip with "TH6886A" that I thought might be the CMOS battery. Can anybody tell me how to replace the CMOS battery on this motherboard? Is it even worth getting one? Thanks.

 

[GalvanizedYankee]

I can understand the need to keep an e-machine alive. Generally the clock will slow down
and loose time as the battery weakens.

See if you can solder in a new holder. If you remove the square deal are there leads that
can be soldered to a new holder? What voltage is the battery supposed to be? Only you can
answer these questions.
If you go this route, make sure to have the right voltage and correct polarity.

Go here http://www.allelectronics.com/index.html you will find batteries and battery holders. Basic shipping here is $6. Contact them about USPS uninsured as they might ship
cheaper or you could go to a Radio Shack in your area.

EDIT: Just for fun, I Googled 486 motherboard battery. Some are soldered on and if the
3.6V Lith.battery leaks can damage the board connections. Some just plug onto a board jack.
Some have and aux jack for installing a replacement battery.

...Galvanized

 

[JimPhelpsMI]

Hi, In the days when those were designed there were many battery choices. Most had a clyndrical 3 buttoncell nicad mounted above the MB on two leads. They were automatcally charged. Some used some form of alkaline or lithium battery mounted on the side of the case or PSU and plugged into the MB. The DALLAS Clock looked like a standard IC chip, but about 1/4" thick. It had a rechargable battery built in. Let me know what type you have and I may be able to tell you how to get around it. Luck, Jim

 

[bendixG15]

As I recall, a lot of the 486 mobos did not have a battery. but a "condensor" or "capacitor" that stored up and held the charge. (can't remember what the hell it was exactly)

I remember throwing out the 486 mobos when this "storage thingy got old and green and could no longer hold the charge. No way to replace or stick in a battery.

 

[Rubycon]

Originally posted by: bendixG15
As I recall, a lot of the 486 mobos did not have a battery. but a "condensor" or "capacitor" that stored up and held the charge. (can't remember what the hell it was exactly)

I remember throwing out the 486 mobos when this "storage thingy got old and green and could no longer hold the charge. No way to replace or stick in a battery.

Most of those were actually nicd cells. Some were panasonic gold caps but these were rare. Most of the cells have breached seals and will look like they're covered with salt.

 

[vegetation]

A replacement battery will cost more than what the system is worth. Junk it.

 

[Nessal]

Yeah they are Nicad cells. I had two in my closet that I threw away not too long ago.

 

[ultimatebob]

Why not just let the sucker die? The worst thing that you'll probably lose is the system clock, or perhaps you'll need to manually set the boot order to get the thing started.

 

[McCarthy]

If you have the manual check to see if there are pins for hooking up a replacement battery. In many cases where the original battery was soldered on the manufacturer provided for generic battery replacement this way. If the pins are there it's just a matter of finding someone who still sells replacements that way - or make your own.

One page showing how to build own

This page may be of use to you

Place selling the encapsulated batteries and real time clock modules

 

[JimPhelpsMI]

HI, Don't use that circuit for any size nicad. It will charge at .5 amps continuously. A AA nicad could be as low as 500 MAHs. It will charge in a little over one hour and then soon end it's life. A AA 500MAH nicad should be charged at 50 Ma., the 10 hour rate, for about 14 hours and then removed from the charger unless the charger is automatic and reduces to the 100 hr rate (Trickle Charge) when the bat is fully charged. The circuit shown is definitely NOT Automatic. It could be used for a manually controlled bench charger. Hundred hour rate would be 5 Ma. Hope this helps. Jim

 

[CrispyFried]

There used to be a util that would save and load cmos settings from a floppy disk. you could set the cmos up right, save the settings to floppy and then just boot from floppy to load the settings and then reset or CAD the system to boot windows. the clock would still be off but all the other settings would be reloaded every time you boot with the floppy. save you the bucks on the battery.

wish I could remember the name of the file but that was 386/486 days..

 

[WackyDan]

Originally posted by: Nessal
Yeah they are Nicad cells. I had two in my closet that I threw away not too long ago.

Thanks for F'ing up our environment then.

Seriously...... It's so easy to recycle batteries today. Don't throw them in the trash.

 

[pikachu656]

I had the replace the CMOS battery on one of my 486 machines that I had years ago and luckly for me I was able to take the battery out and I took it with me and it turned out that it was the same size, shape, and voltage as a lithium camera battery. I think the battery cost like 10 bucks or something like that. Bought the camera battery, and popped it into the computer and enterent the new time and info and worked fine for the next couple of years until i junked it.

 

[JimPhelpsMI]

Hi again, The battery can be replaced by 3 Alkaline batts of AAA or AA size. If it is replacing a rechargeable batt then a small diode must be placed in series to keep the charging circuit from destroying the batt. If you will tell me which batt it has now I can give better advice. Jim

 

[owensdj]

Thanks to everyone for their replies, but I'm still very confused about what I need to do. I took some pictures of that 486 motherboard. Could some of you look at them and see if you can tell how I need to fix the CMOS battery low problem? Thanks.

MB Picture 1
MB Picture 2
MB Picture 3
MB Picture 4

 

[JimPhelpsMI]

Hi, The device show in Pic #1 appears to be a DALLAS clock with built in batt. Looks like it has a strap around it so must be in a socket. Maybe you can locate a good one and just plug it in. It contains your time, date and CMOS setup data. Maybe if you can identify it's power pin, maybe the pin at left top, you could lift the lead or cut it and using two diodes add 3 alkaline cells to replace the internal batt which appears to be deceased. Most DALLAS clocks I have seen had DALLAS printed on top, so I could be way off here. If you are interested in going that way let me know and I can send you a circuit diagram.
Luck, Jim

 

[owensdj]

JimPhelpsMI, thanks. I also think that chip labeled TH6887A is what I need to replace. From a Google search I found that it's a Twinhead part that contains the RTC and battery. I wouldn't want to do the 3 alkaline cells thing. I'll probably look to see if I can find a replacement TH6887A module.