Attaching a new battery onto an existing M48T02

The contents of the (nv)ram are backed up by a 3V lithium battery.
It's located together with a quartz on top of the ram in a kind of
backpack.  The battery is on the side that's opposed to the dot marking
pin 1, next to pin 12:
                         _oscillator
                        /
                       / _battery
                      / /
                   -------
                   | O O | <-- cut here
                   -------
                  /
             Pin 1

At the point marked above, some kind of nose is reaching down from the
backpack over the resin.  Carefully cut through the polyester resin
filling the dimple.  This works best with some kind of mini drill with
a small milling head or a razor knife.  Buried in the resin you'll find
two small diagonal metal connectors.

Be careful not to short-circuit them, or you'll lose the contents of
your nvram (if it was still able to keep them). -That's why you should
save them *before* :)

The connector closest to pin 12 is ground, the other (opposing) one
+3V.  You can now solder some wire to them and connect them to a new 3V
lithium battery.