ibook G4 - battery isn't charged while computer is off or on standby

Bought myself a new battery for ibook G4 (14 inch, 2004) because it's still working quite alright.

Problem is that even if battery is completely unloaded (3%), it isn't recharged when switched off or when it is on standby. Whereas there is no such problem when ibook is switched on and on duty. Any idea?