G4 Powerbook Freezes When Power Is Disconnected



I have a g4 powerbook which works fine when the power is connected.
When the computer is turned on and I disconnect the power supply it freezes completely, the cursor will not move etc.
If I start up from the battery I get to the blue screen with the porgress indicator and it will not go any further than 'waiting for network initialization'.
I have tried repairing the permissions and installing the battery update.
I am pretty sure that the battery is ok, but don't know any way to test it.

Any help would be gratefully received