The BIOS battery is placed near the vibration motor.

Here are some pictures:





And I think you can change it... as a user, never tried it. My clock was auto-updated by network provider.

Don't EVER use a soldering iron around batteries, you can end up hurting yourself. Even if you wear safety goggles you can end up having burns on your hands/face/body. So please don't. That battery is not soldered but welded there... Please be careful, and take it to a local service for replacing.

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.

Is it worth the risk damaging your mobo?
You could have a script to set the time on boot, eg as described here
http://talk.maemo.org/showthread.php?t=37662

This is the thread you need.
http://talk.maemo.org/showthread.php?t=90864