[ubuntu] HP laptop boots on battery to blank screen

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http://ubuntuforums.org – Hi, got a laptop booting OK on mains power but to blank, dark screen on battery. I have to open a terminal with Ctrl+alt+Fkeys and shut down, then boot again on power. Once booted, shutting off power has no effect, machine works OK. How should I troubleshoot this? HP tx1219us, AMD Turion 64, Nvidia (driver updated). (Hardware)