[jan-jaap]

Thank you guys!
Is the yellow battery, let me say, standard? Or is required a specific model.
Just found on ebay a couple of options from China for less than 5 euro

It's this one: https://nl.farnell.com/stmicroelectronic...dp/1467820
Sometimes it's bigger capacity brother (M4T32-BR12SH1) is more easily available.

Farnell is Newark in the US I believe, there's also RS electronics, Mouser, Digikey ...

With China parts there's always the risk you get something that's been on a shelf for a decade, and shipping will take 3 weeks.