A regular small sized battery powered by sugar.
Inside the battery is a tiny digestive system that slowly burns the
sugar and outputs a stable charge. I suppose a chemical system
would be the best but am not rulling out a tiny mechanical
generator. Part of the battery of course is a regular
battery used
as a buffer and for storage (like in a hybrid car).
The first ones will cost a mint, but the next ones will replace the
current batteries that have a shelf life of 2 years.
Written after my laptop died within 3 minutes out of the charger.
Arrrrrrrgh!!!