This is spot welding. Such batteries do not like soldering (very much). And the filler metal will not just stick to the surface of such a battery.
Either look for a battery with already welded terminals, or solder the socket (maybe even on the wires, like it's done in some laptops), or weld the battery to the terminals with a spot welder and only then solder it to the board.

You're not supposed to replace just the battery with these, but the whole thing. Desolder it from the motherboard and replace with a vertical holder.

Sorry, I dont follow completely. So you suggest I desolder the entire assamblage (battey, and the holder plates and legs from the board ? And then remount a new battery (I dont have a spot welder) and solder it back to the board ? That still leaves me with the problem to identify the battery (the specs are hidden behind the spot welded plates) So I will still somehow need to disconnect those spot weldings from the battery.

Yes, desolder only the two legs from the board and remove the whole thing. Having the battery in your hands cut most of the welded terminals and reveal the exact battery type, you won't be using the welded terminals again.

Then buy a suitable vertical battery holder + battery, something like this (you might need to adjust the terminals of the holder to match the motherboard holes, maybe cut a third leg if there is one, position the holder diagonally etc). You don't have to match the size though as it depends on the holder, only the type and voltage.

Ok - I did it. I desoldered the whole thing and was also able to rip of the legs.

So the battery is a BR1225 with 3V. I do have the sugested type of Cr2032 holder but it is much too large and does not fit well on the board. On the other hand I can see some similar bateris avalale to buy which already have legs. But the all have slightly diferent specs.

- Panasonic ML1220 Coin Rechargeable Battery ? Is this possible ?
- DBTLAP CMOS Battery ML1220
- Panasonic VL1220 Coin Rechargeable

Would the board echarge them ? Otherwise - these would need desoldering again once they are also dead...

If I'm not mistaken google says that BR1225 is a common non-rechargeable 3V lithium battery. In which case you can just use a 2032 that has the same properties, as long as you can adjust a holder to fit on the board.

Recommend finding somewhere nearby to mount (glue) a 2032 holder and connect it to the board via wires.

Yes - I think that may be an option. Idealy I could then connect that remote holder via cable. So here is another question - are there single pin conectors that I could solder tpo the board (i.e. similar to pin headers for Tubo switch or Led ) So I could use a similar conector to atach the remote Coincell to the board via a cable ?

I attach the board area now with battery removed - there is very little space.

Ok I soldered two single pin headers to the board and remotely connected a CR2032 - Works!
Thanks for the suggestions.