820 is losing battery on short rides. Here is my ride and garmin 820 had 3% left.
You should have my file. I have the latest update 12.70.
Unit ID 3966449429.
Looks like you're trying to contact Garmin Support. This is a users' forum and not Garmin support. Go to the Garmin website for your country and scroll down to the bottom for contact info.
FWIW, the 820 never had great battery life to begin with, and after some years of use, the battery capacity decreases further. You might try leaving it on until it turns off, then charging it. That might help a little. In my case, I connect an external USB power stick for longer rides.
One thing that may help (a little) is to use older firmware 11.20 which is known to provide better battery life. You can find it here for example: https://forums.garmin.com/sports-fitness/cycling/f/edge-820/233243/i-m-searching-for-the-firmware-11-20-file-gcd
Be aware that such downgrade will reset your settings.
And you will then need to block firmware updates (by creating an empty gmapsupp.img file on the device and setting it read-only).
Factory reset may also help in some cases.
Another thing is that your battery may be simply worn out and thus due for replacement. You can either purchase a new one and replace it yourself (there are some YouTube howto videos about that such as www.youtube.com/watch, or simply take it to the Garmin service center and let them do it.