I've re-tested with 4.16.6: still happening, the battery life is about
50% down.

If I blacklist the nouveau kernel module, then the module stops crashing
in loop, which improves battery time a little bit, but the difference
with 4.15 is still startling.

Note that blacklisting the nouveau kernel module has the side effect of
preventing the kernel from putting the video card to sleep, which drains
the battery even more.

--
Pierre Neidhardt