The PRAM battery for a Titanium Powerbook G4 is rechargeable, it might
not need replacing (though will certainly go flat on the shelf).
The cells are rechargeable lithium/vanadium pentoxide 3V coin cells, similar to
Panasonic VL2020, in a plastic (tape?) wrap, with pigtail connection.
Hold down command-option-P-R at startup to reinitialize PRAM contents;