Apparently it is known for quite a while (read 6 years) that UPower has
some problem with dual battery systems:
https://bugs.freedesktop.org/show_bug.cgi?id=55278
** Bug watch added: freedesktop.org Bugzilla #55278
https://bugs.freedesktop.org/show_bug.cgi?id=55278
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You received this bug
Joseph, since I am using the laptop as main working device I am very
reluctant to switch from the ubuntu custom kernel to mainline and let
the machine die willingly. I got a pointer via AskUbuntu
(https://askubuntu.com/questions/961278/how-to-prevent-shutdown-when-
one-of-two-batteries-is-empty)
Did this issue start happening after an update/upgrade? Was there a
prior kernel version where you were not having this particular problem?
Would it be possible for you to test the latest upstream kernel? Refer
to https://wiki.ubuntu.com/KernelMainlineBuilds . Please test the latest
v4.16
