I don't think you can cut the end off another cigaretts lighter adaptor, as the Garmin requires 5V (USB) not 12V which apparently fries it
I too have it running on rechargables, but that sorts my capacity issues out with 6-8hrs off one set, but having the unit powered significantly increases the volume, and will move it from 25-30mph up to 35-40 I would guess which would really help with town driving.....