All PRAM built-in batteries in PowerBooks from PB1400 to PBG4Ti operate at 6.0V, except for PB2400, PB3400, and the first PBG3 (Kanga), and the only difference is the number of battery cells used (2, 4, 6 pcs).
The link is now broken, but
Tech Info Library 30580: "PowerBook G3 Series...