I have created a known issue when using waze on N900 with custom settings which cause the 20% battery level to always show on start regradless of the real battery level.
This is caused by QtMobility stop recognizing the battery level and always report 0%.

A possible workaround is to disable the battery level check as noted here.
Or in short, add the following section to ~/MyDocs/.waze/data/preferences.ini:
Compatible with all previous Waze-qt releases.