Reviving this old thread again.
I can confirm the battery looks soldered in place (or, even worse, welded) and also confirm that the battery doesn't hold any meaningful charge after a few years.
I will try to measure the size of it, hopefully this will indicate the model from battery sizes list on Wiki. But judging by the fact that the tiniest batteries are from the Silver-oxide/Alkaline category, I'm guessing the nominal voltage should be 1.5V
https://en.wikipedia.org/wiki/List_of_battery_sizes

Repeating Jgbreezer's request, if anyone has the PDF mentioned on page 1 of the thread, please share.

As Theox mentioned, it might be dangerous to try to unsolder the battery with temperature. What I'm guessing might work, is to solder (or somehow fixate) the new battery in parallel to the used one. It would be very helpful to remove the old one though, since otherwise it would just permanently discharge the new battery. I'm planning to unsolder the battery holder altogether and get a new battery+holder soldered back.

Soldering batteries isn't just dangerous, it's also very difficult since the battery itself drains heat from the soldering point. Said heat dissipates externally via the surface and also damages the internal chemicals.