Battery broken!?!

i am getting mad.. i got a powerbook g4... it has a wlan card... but ok, i bought it second hand... his battery leasted about 2 hours with Wlan... but now the battery jsut makes 30 minutes... whats wrong with it? my book NEVER made Probelms and now that... the battery has ab bug i think, someone has an advice? where to send it, repair, buy etc... ???

thanks to all