For these kind of issues Garmin can typically be talked into exchanging the computer for a refurbished unit at $99 if its out of warranty. You can ask that they send it ahead of returning your unit by paying twice the cost and they'll ship the new one immediately then refund half when your unit arrives at Garmin.

Quick search online turned up this option:

https://www.newpower99.com/...min%20edge%20820.htm

https://www.youtube.com/watch?v=Moy6K4h7AIM

Any external USB battery should work. Not sure how the 820 plays when being plugged/unplugged while recording a ride.

Another option is going full on dynamo on your front wheel.

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Strava

Not a direct reply to your Q... but an observation from my use of a rather old Edge 800.
Ive found mine holds up well enough for 8-10 hr days out, PROVIDING I'm not on the map page constantly and not forever zooming in amd out on the map page - as that puts the screen light on and just generally eats battery power. (It halves in life if I do constantly feck about on the map page).

Are you on the map page ? Or have the backlight on constantly ? Altering those can make a big difference.
Also on a 820 is it constantly connected to a phone with bluetooth ? There's another drain.

Wow. I've got a 510 that's still going over 12 hours between charges.

FYI -
There is a replacement battery offered out on Amazon. I did that, but it wasn't any better. Still wouldn't last 5 hours, and the product is glued together, so taking it apart is a bit risky.

Ok, sound like i should disable few features before i get a new battery... hopefully, it will get me thru summer! (and only change computer next spring)