With a new kernel, /proc/acpi/battery/* was suddenly missing. Turns
out it the interface is is deprecated but can be turned on with a
config option at compile time. The same information is available in a
different form in the /sys filesystem.
I didn't see that coming; almost all programs I use to check the battery state fail. It had been announced, actually:
The correct way is for upstream to announce the change in feature-removal-schedule.txt, which they did 2 years ago.
That's roughly how the Vogon Hyperspace Planning Council operates. Stubborn dev replies:
If the tools are not able to use them, it is their bug. No bug in the kernel.
However, after updating vicious and adjusting some API changes, everything works fine again.