A tip for cmos battery replacement
 

Hello all from Dave. I bought a 486 computer and when I fired it up I found I had to reset the cmos configuration. Yes you have got it in one, the varta battery was dead. Thankfully it haden't started to leak, so I decided to fit 3 rechargeable aaa cells to make it easier to change the battery without having to remove the motherboard in the future. As it happened I found a 3 cell box for these cells from CPC so by using 2 standoff pillars of the female type each end I was able to screw the box onto a spare slot cover. Countersunk screws are required to fit the box to the pillars to avoid fouling on the cells. Extending the leads from the box to connect to where the original battery was removed completes the job. I now have a fully working computer without having to reset the bios every time I switch on.

Re: A tip for cmos battery replacement
 

The only real drawback will be finding NiCd cells to fit. NiMH will work, but have different charging characteristics so may have a short life.

A single LiIon cell will also work, though again there are charging issues.

Re: A tip for cmos battery replacement
 

I have to ask, what sort of battery (chemistry) was the original battery? I seem to recall that in the old 386 / 486 systems the CMOS backup battery was often a rectangular thing with a strip of velcro down one side and I don't think they were rechargeable.

If it was a one-shot (not rechargeable) then rechargeable cells of Nicad / NiMH type will self-discharge by themselves no matter how much current the motherboard does or does not draw from them.

That said, the current versions of NiMH seem a bit better in that respect - they are usually described as 'pre-charged' and don't self disharge in a matter of months the way the old ones used to.

Re: A tip for cmos battery replacement
 

Most 386/486 era PCs used a 3.6V rechargeable battery soldered to the mobo. It was just a stack of 3 NiCd cells. They were trickle charged from the PSU and did indeed go flat after a week or two without power.

Re: A tip for cmos battery replacement
 

Thanks for your replies, the original battery was a Varta NiCd and point taken about the liable to self discharge if the computer is used infrequently. At lest now I can remove the 3 cells easily and recharge them in a charger I have rather than having to remove the motherboard.

Dave