Battery meter stuck

[rteather]

My Thinkpad X40 battery meter has stopped working.

It is still there in the bottom bar, and it still shows correctly whether it is plugged in or running on battery, but the % battery remaining doesn't change.

When the laptop is turned on or woken from sleep it shows the correct percentage, but then it stays showing that figure whatever happens until it is put to sleep again.

I have tried:
- rebooting;
- turning off and on the "show battery meter" box in the battery management system;
- reinstalling the whole battery management system from the Lenovo website;
- removing the battery for a couple of hours.
Nothing seems to work.

Can anyone please suggest something else that might work?