Two options:
- klaptopdaemon (ironically, this daemon eats lots of battery power)
- ksysguard, which can be embedded in your panel (but doesn't show intrusive warnings when your battery goes low)
However, the last option requires tweaking configuration files. I did some little work for you (see attachment). Place this file in ~/.kde/share/apps/ksysguard (or create the necessary folders if they don't exist already).
Now load the applet by opening a panel menu and choose Panel Configuration -> Add Applet and choose for KSysguard. The battery status will be added to you panel.
