Not impossible, but not that simple either. The Dallas 1287 doesn't accept external supply, you'll have to exchange it with some chip that does, and then populate all the missing parts.
Biostar MB-8500TVX-A with CR2032 & DS287

quicknick wrote on 2021-11-15, 00:14:

Not impossible, but not that simple either. The Dallas 1287 doesn't accept external supply, you'll have to exchange it with some chip that does, and then populate all the missing parts.
Biostar MB-8500TVX-A with CR2032 & DS287

+++ yes !
a 1285 is the 1287 w/o internal battery. It is easier to hack the 1287 to add a external battery versus populate the missing parts and get a 1285 IMHO.
Added: the 12885 and 12887 are pin compatible and have clocks good to 2099 where the 1285/1287 are not Y2K.
Not that critical if the BIOS does not do Y2K but there are software fixes for BIOS if the CMOS (inside the Dallas) is Y2K 😀

There are these ready-made options too

Solution for dead RTC modules Dallas DS1287 / DS1387, Benchmarq BQ3287 / BQ4287 and others Video : https://www.youtube.com/watch?v=svPNxILeQEw
EDIT : I think this one is DIY

and

https://www.tindie.com/products/glitchwrks/gw … acement-module/
EDIT : This one is available to buy pre-built . I purchased and received one which I have not installed yet .

I just bought one of the replacements that has a battery holder on top. (Normal mobo battery size even)

I thought about playing around and soldering a 2032 bracket to the motherboard, and connecting the wires from the soldering points to the modified terminals of the 1287, but I had a better idea, as it is a computer that will be stored most of the time. , I think I'm going to put the battery out of the case, so when I go to use it, I can easily take it out and put in a battery, or even a pair of AA batteries.

https://monotech.fwscart.com/DS12887_RTC_Drop … 4_19810725.aspx

Email these guys, and give them the numbers on your dallas clock they can send you one of these things. It’s what I did.