Now for a useful answer: The LIR type cells are rechargeable and will be around 3.6V while a CR2032 is non-rechargeable and only has 3V.
The higher voltage may be needed to store settings on this motherboard while the higher voltage charging the CR battery could have already killed it.
I recommend you remove the CR battery before turning the PC on again because it could cause some real damage in the PC from being charged.

original_meusli wrote on 2021-03-09, 14:14:

Hello, […]
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Hello,

I bought a 486 build on eBay for a Christmas present but it has never successfully kept its CMOS settings so every time I freshly boot I need to pop into the CMOS and find my HDD etc. The board is a MV035 486WB.

I thought at the time "it's no problem, it's probably a dead battery", so I ordered some off Amazon and put it in the case, still the same problem. So searching this forum I found an old thread where they said the batteries needed are LIR2032 or LIR2450 and not the DL2032/CR2032 I bought(and the same as I replaced). Could this be the cause of the CMOS settings never saving? (please say yes)

Thanks

again, no.

the Opti 82C602 has an onboard RTC which needs to be supplied by a 3V backup battery.

Check for continuity between pin 55 of the 82C602 and the positive of the battery holder

OK, thanks for the info guys. Most helpful. 😀

Below is a picture of the MB from the Ebay listing (at work so cant get a new pic). I can't see the OPTi chip anywhere, is the write back chip circled the alternative?

That looks more like one of my crappy pcchips m912 boards, actually identical, even has the same version number in the same corner, except for the stickers on the chips.
I've measured on mine, even pulled the label off and, surprise, no chipid engraved, and that chip is different from the opti 82C602.
Neither pin 5 nor 55 are connected to the battery. So it must be another pin. I'll check later.

Black VLB slots and "golden" RAM stick retentions - typical for PC-Chips and their countless sub-brands.

It's indeed a PCChips m912 rev 1.7 to be exact.

http://www.win3x.org/uh19/motherboard/show/573

It has actually a UMC chipset. Yours have fake cache though (and yes the bios reports L2 cache but it lies. Run a cache benchmark and you'll see the truth)

If you're motivated enough, you can get rid of the fake cache chips and install real ones though.

Also it's using a standard cr2032 battery 😀

Deksor wrote on 2021-03-09, 15:41:

It's indeed a PCChips m912 rev 1.7 to be exact. […]
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It's indeed a PCChips m912 rev 1.7 to be exact.

http://www.win3x.org/uh19/motherboard/show/573

It has actually a UMC chipset. Yours have fake cache though (and yes the bios reports L2 cache but it lies. Run a cache benchmark and you'll see the truth)

If you're motivated enough, you can get rid of the fake cache chips and install real ones though.

Also it's using a standard cr2032 battery 😀

Deksor, Any idea on which pin of the UMC... (the small chip) the battery is connected ?

original_meusli you could take JP1 off and put it back on... might be a little crusty and is required for the onboard battery.

Maybe I can do that with my own board but I only have rev 1.4 boards. I hope it's the same.

Also like I said this board uses cr2032 batteries only. If OP used a rechargeable battery, maybe it's just flat because there's no recharging circuit ?

Deksor wrote on 2021-03-09, 15:41:

It has actually a UMC chipset. Yours have fake cache though (and yes the bios reports L2 cache but it lies. Run a cache benchmark and you'll see the truth)

If you're motivated enough, you can get rid of the fake cache chips and install real ones though.

Also it's using a standard cr2032 battery 😀

Never even heard of fake cache chips, so thanks for letting me know. I am motivated to replace them yes. 😀

Do they need fully populating and do you know the type I need?

weedeewee wrote on 2021-03-09, 15:45:

Deksor wrote on 2021-03-09, 15:41:

It's indeed a PCChips m912 rev 1.7 to be exact. […]
Show full quote

It's indeed a PCChips m912 rev 1.7 to be exact.

http://www.win3x.org/uh19/motherboard/show/573

It has actually a UMC chipset. Yours have fake cache though (and yes the bios reports L2 cache but it lies. Run a cache benchmark and you'll see the truth)

If you're motivated enough, you can get rid of the fake cache chips and install real ones though.

Also it's using a standard cr2032 battery 😀

Deksor, Any idea on which pin of the UMC... (the small chip) the battery is connected ?

original_meusli you could take JP1 off and put it back on... might be a little crusty and is required for the onboard battery.

I'll do that as soon as I get home. Its also a standard CR2032 battery in there, none rechargeable. As mentioned it arrived like this and change of battery like for like and this has not fixed it.

There are different brands, speed and sizes, but they're compatible among all the 486 boards. You can take a look at motherboard photos online and pick a cache chip you like. 256Kb and 15-20NS were the most common. You need a set of 9 chips.

I'd recommend you to install sockets in these spots as well and not just solder the cache chips in place.

Installing a new cr2032 should fix your problems. The old one must be discharged since the 1990's ^^

Trouble is it has a brand new Duracell in it, hope it doesn't point to issues I can't fix. Its not the end of the world though, just annoying.

Deksor wrote on 2021-03-09, 15:50:

Maybe I can do that with my own board but I only have rev 1.4 boards. I hope it's the same.

Also like I said this board uses cr2032 batteries only. If OP used a rechargeable battery, maybe it's just flat because there's no recharging circuit ?

Well, I just checked the v1.4 board... and battery is not directly connected to one of the big chips 🙁

Doesn't surprise me for a motherboard with fake cache chips on it. 😒

Should measure about 2.5V between ground and that pin
apparently there's an extra diode that drops the voltage.
if the voltage isn't there, check R51 & D7 (near the battery) should measure 3V and 2.5V between ground

the same location on the v1.4 board.

weedeewee wrote on 2021-03-09, 16:17:

Should measure about 2.5V between ground and that pin apparently there's an extra diode that drops the voltage. if the voltage […]
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Should measure about 2.5V between ground and that pin
apparently there's an extra diode that drops the voltage.
if the voltage isn't there, check R51 & D7 (near the battery) should measure 3V and 2.5V between ground

the same location on the v1.4 board.

I'll try that tonight and get back to you with the results. Again thanks.

Had to cancel last night as my voltage reader was broken, going to try and borrow the one at work and test it tonight.

I can now confirm that there is no charge visible between the pins on that chip or any of the other pins on the same side. I assume at this point that its not going to work, but as mentioned its not the end of the world. Now to think about fake cache chip replacment mentioned and see if I can find any break in the paths on the board to cmos chip while I'm at it.

Thanks again everyone for the help.