Heads up: the 100% charge after hours and hours of use is totally normal (in my experience). And then it will precipitously drop to 4% - Low Batt warning, out of nowhere. I carry a Gomadic charger and mount my Garmin in such a way that I can access the port and plug it in while riding.

I have a feeling this has to do with "overcharging" i.e. leaving it plugged in while it's stuck at 99% "Charging". Only when it is very close to dead does it reach 100% and then say "Complete".

As a randonneur who uses GPS for navigation, having my Garmin on and working is super crucial. I get anywhere from 16-18hrs usually on a full battery, and then it can charge back up fairly quickly with the Gomadic. However, when I say "full battery", I mean I drained it to 0% on purpose, and then charged it to 100% Complete the night before. I used to do this regularly, but have gotten a bit lazier with it now that I know I can plug the Gomadic charger while riding.