Solved! Laptop keyboard stops working whenever AC is unplugged

Jul 15, 2018
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The integrated keyboard on my Dell Latitude E6440 stops working as soon as the AC adapter is unplugged. If the AC is reconnected is still doesn't work, though a USB keyboard is fine. After flashing the bios with the bios update from the dell website, it works again. I'm totally confused. Does anyone have any suggestions?
 
Solution
Maybe the Li battery is faulty.

But it should be hardware related, if it doesn´t work with Ubuntu live too, booted from an usb flash drive
Jul 15, 2018
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Yep, I've tried that too. The cmos battery reads 3V on my multimeter and the system clock stays set when the AC and Li battery are both disconnected.
 
Jul 15, 2018
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Yes, I was thinking to try a live distro, although I expect it will be the same in Linux as the bios menu is affected. I did buy a new battery not long ago although it seems to work fine otherwise.