is it better to use ac power or battery?

good evening,

recentelly got g4 powerbook titanium price , aslo found battery online. i'm wondering if better battery life use powerbook on ac power when available when im @ home or in starbucks , there outlet available or should use on battery alone unless battery needs charged? dont want harm powerbook or battery still want longest life battery.

in advance

apple's rule

 

http://www.apple.com/batteries/

 

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