This is fine. Believe it or not, there are two possibilities. Neither of which are giving you any less battery life than windoze.
Your battery isn't new and batteries over time will not fully charge and loose their charge. Not all systems catch this. I had an old PIII that when windoze 98 was on it, the OS said the battery was full, but in linux it only showed a 5% charge. Linux was correct. This battery was soo old in fact that it barely held a charge.
Linux or the KDE battery app (KDE has doesn't have perfect power managment, sometimes says my battery is empty but it is full then tries to put lappy in hibernation. Has gotten better since KDE 4.4.1) is incorrectly reading your battery's state.
Believe it or not, the OS doesn't control how or when the battery is or gets charged. So Ur OK
and good to go.
Fixes like these are just a patch away. I'm not sure who you'd report this bug to, because it could be a kernel bug,
or a KDE bug, or gnome bug, etc. But I'm positive, your battery isn't charged any less than would be on Windoze.