Also you may wish to try to start the APU (or just leave it running after pushback) and then leave it commanded OFF during the flight until this happens, then try commanding it ON when the problem comes up.  If it's fixed then, but you don't show a DISCH light on battery before turning APU back on, then I don't know... but if APU is in RUN status and you command it ON, and the issue doesn't clear, then I have no idea what is happening.