Battery Juice

Domanda,

If I were to buy a battery for my Powerbook G4 15", a backup, will

a) it work with a 12", 17"
b) work with (an educated guess) the Powerbook G5s

Is it better to

a) Wait until current battery only lasts half an hour, then replace
b) or swap periodically

a) I don't think so (uneducated guess)
b) probably not as the G5 will be a larger revision and will probably need a larger battery capacity since the G5 needs a lot more juice.