Q: powerbook g4 /15" -battery charges but the screen turns off

I just bought a new battery for my powerbook g4 and it charges fine but when I turn on the laptop only with the battery the screen doesn’t turn on but I can hear the fans start. and if I remove the power cable  when the laptop is on  with the battery the screen deems almost off and I can see that there’s something on but I have to look with a light to see the screen but I can also notice a line across the screen. Does anybody know what in the **** is going on? Please help me !!