Hi mtijn,
This LED pattern is appearing as the battery is reaching end of life because performance will gradually degrade as the battery is used. I recommend you replacing the battery and it would resolve the issue for you.
If you are in the USA, you may contact our Spare Parts and Accessories dept on 1-800-357-3355. If you are in a different region, go to http://dell.to/18NmI2D select the country/region from the top, scroll down and click on 'Contact Dell Sales' to get the contact information.
Please reply if you have any questions, I'll be happy to assist.