I've had a Garmin 829 for a little over a year. I've always had good battery life, even doing a 134 mile ride without losing the battery.
Lately, my battery has gone down very quickly, losing about 25% per hour.
It started when it got cold around here.
I have had some changes in settings & added some sensors.

Before (good battery life)
GLONASS on
Recording: 1-second
ANT++ speed/cadence
Bluetooth on
Messages on
Livetrack on

Varia radar:added latte, didn't seem to affect battery life significantly

Changes (poor battery life)

GLONASS: off
Recording: Smart
ANT++ speed/cadence
Bluetooth on
Messages on
Livetrack on
Varia Radar
Power Meter: Favero Assioma (added after battery life decline)


Any thoughts or suggestions?

Do you use backlighting? That makes quite a difference on the 520. You might want to check out the Garmin forums as well. Sometimes a software update can change power consumption.

I don't put a lot of faith in the battery meters in these things. Kind of like the gas gauges in a lot of the old cars I drive. They'll show full for a long time before they drop.

So be sure you are charging it completely. I just leave mine connected to my PC after my rides and disconnect it before I go to bed. Usually five to eight hours after my ride. Certainly way more than is needed, but I've not had any issue and my edge 500 is around seven years old.

Cold will affect how much capacity you can draw out of any battery, but 25 percent in an hours seems a little much. But again, I consider the battery meters flakey anyway. It's either going to last for the entire ride or not.