t440s: battery state falls to 5% then back to 75%

[yak_sk]

I'm experiencing a very strange issue when on battery. This started after upgrading from 3.12.2-gentoo kernel to 3.14.2-gentoo kernel,
so I suspect there is some connection. (I compiled it myself and migrated the .config from the previous one)

When the laptop is running on the batteries, everything is fine until the replacable battery drains. Then it starts draining the
internal battery. When at about 75/60%, the battery charge state suddenly drops to 5% in my KDE4 battery monitor and I get urged
to replace the battery / plug the computer to the AC.

When I plug the AC in, just for one second and unplug it, the battery state returns to 75% and everything is fine. I hope that this
is not a hardware error, but it used to run fine on the old kernel.

Any ideas please?