http://www.stormcam.org/garmin-cigarette/
Did I keep my GPS System charged to much?
It always stays in my car plugged in the cigarette lighter, so it’s always on charge. I since took the GPS out my car. I plug it up inside my house outlet for a few hours. I then take it to my car the next morning, it immediately says low battery and turns itself off. My mom also has a Garmin and it can stay charged without any power cords for more then 2 hours. Did I overcharge it over the years?
If the GPS has a Lithium battery in it and it gets left in the car during the day then it’s possible you have significantly damaged the battery capacity.
Lithium Ion batteries don’t like getting warm, especially when fully charged. The inside of a car can get fairly warm during the day if it is sitting in the sun.
Unfortunately there is nothing that you can do to reverse this process.
Mod Garmin c550 Vehicle Power Cable