Not my project, but I have made a few of these and they work well.
https://github.com/necroware/nw12887

I think I may make a similar one in the future, but add a battery header and an on/off switch.

RiP wrote on 2021-07-16, 18:57:

How to check the internal battery connection with multi-meter?

Super good question ! On 1287/12887 versions the internal battery + is hidden inside the case where pin 20 would be.
Here is what I did on a 2020 purchase of some Dallas 12887A.
Note: I have done the battery mods on them so know how to grind w/o damage, DO NOT try this if you have never done the external battery mod, you could kill a working Dallas very easily !
Grind off a bit the side of the missing pin 20 using a very narrow debur or very narrow spherical dremel grinder tip. Go slow and gentle, pausing to look, grind only until you see metal.
Use a Digi voltmeter and measure from the exposed hidden pin to ground pin 12. As you can see the ones from Quest Components show above 3.2v which makes them very Good like new.
Any thing around 3.0v means they have sat around a long while or have been used for a while and may have only a few years left.
Like coin cells they are 3.3v when new but once they get to 2.8v they will stop keeping settings or time or both.

Better pic of the grind to find the metal. In the light when tilted just right it shows as bright silver spot.

Thank you. Is Benchmarq BQ3287MT equal to Dallas DS12887A?

Made one finally.. has a Dallas DS12885T+ onboard (TQFP-32 size), CR1220/CR1225 holder, pins for an EXT-BATT header, and a power enable jumper so you can put the computer in storage with that jumper removed to save the battery life. This is a pretty cheap build, as it just uses a Dallas chip, crystal, battery holder, and pin strips.

KiCAD, BOM, and gerbers attached.

once upon the 90's time of plug n pray motherboards if it didn't have a lithium socket but a rechargeable leaking battery or dead dallas it went into the bin.

RiP wrote on 2021-07-20, 16:40:

Thank you. Is Benchmarq BQ3287MT equal to Dallas DS12887A?

Sort of. It is closer to the DS12C887A as it has "100 year calendar" and the DS12C has the "century Byte" so both are technically Y2K good AFAIK.
The BQ3287 is a replacement for DS1287A which was replaced by the 12887 series....