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.
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.