iBook G4 doesnt turn on

When I connect the adapter to the computer the light goes green so I guess adapter is fine. Pushing power button does nothing - no sound at all.

But when I push the battery button, no light goes on of the four. Probably the battery is dead, but there must be some other issue too because should be able to turn it on with the battery removed and running on adapter as Ive read, but it still doesnt boot up.

Are there any tutorials that could show me how to diagnose if either the dc in-board or logic board is broken. I'm planning to take it apart and would like to check whats going on.

Thanks!