Check your fuses, diodes; power circuitry. The fuses (in red) should have continuity, the diode (in blue) when tested with multimeter: red lead on output and black lead on input should be OL; reversing the leads should give 0.165V (Diode test mode). Also check the resistor values, and inspect the solder joints. The fuse for the battery power is located on the action buttons board (in red). You could also try refering to this post on GBA for additional help/info: GBATEMP Apparently replacing the daugtherboard fixed the issue in that case.