A steady white light on the Dell laptop's power supply indicator generally means the power adapter is connected and the battery has more than 5% charge, or that the battery is in charge mode with the AC adapter present. If the white light has been on for a lengthy period of time you may need to replace the battery.

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Thanks, but the light on the supply cable is on always, if connected or not. 
battery is dead and disconnected since a half year, I work just with cable. 
I searched a little and it seams that the cmos battery could be responsible..

what do you think?

@V-Dell​ go ahead and change the cmos battery to see if that will resolve the issue and let us know.