How cold are the temps you are riding in up there now? Though the minimum operational temp is about 4°F the device needs to be at least 32°F to charge properly.
I don't know if Garmin has any sophistication in it's firmware or software to check temps when you plug it in to charge. But if it doesn't then charging when very cold might damage the battery capacity permanently.
If it does check for such, and you only plug it in after a ride for a set amount of time, then maybe it never fully charges.
Last edited by Iride01; 12-09-21 at 10:35 AM.