The % left is an estimation based on battery voltage, and Li Ion batteries have a very shallow discharge curve. If it changes a little as the battery ages, it can lead to big difference in estimated vs…
Here're a few things that might help. Turn back light off, or to a low level and set to turn off after 15 sec. Use only GPS, not Glonass or Galileo. Don't stay on the map page. Use smart recording. Use ANT+ for sensors. Turn of bluetooth and wifi if not using them.
Many have found the battery life to be somewhat better with firmware ver 11.20. It seemed to go down significantly with subsequent firmware versions. You might try reverting to 11.20 and see how that works for you.
I was having what I thought was battery life issues with my Garmin 820. Then something weird happened. I was out on a 80+ mile ride in early July when, at the halfway point, my 820 was almost dead (13%) after only 2 and a half hours and ~40 miles. So I thought for sure it would completely die over the second half of the ride. I turned on Battery saver and turned off the brightness completely.
At 3 hours it dropped to 3%. At 3.5 hours it dropped to 2%. At 4 hours it was don to 1%. Then it stayed on for the remaining hour at 1% and recorded the entire ride successfully. That's odd.
When I contacted Garmin the service rep told me that if I hadn't been charging the 820 using the Garmin supplied cable off of a computer, that the battery life reporting gets confused and becomes inaccurate.
So I tried what he said and that didn't work. I have even reset my device to factory settings (and clearing all data) and that didn't work.
So this past Saturday I'm out for a 2 hours ride with battery saver turned on and it drops to 51% by the end of the ride. Ok, I decide to leave it on post ride to drain the battery down to zero. Saturday night I check the battery status. It's UP to 71%. What?!?
All day Sunday I check to see the battery drain status. It keeps going to sleep, but the battery is still draining, however there are times when the % increases.
Finally, on Monday, it drains to a point where it won't turn on. Good. I have now plugged it into my computer with a Garmin cable and have fully recharged it. Hopefully this issue is now resolved.
How should the Garmin cable be better than the others? It's just a regular USB cable.
Unless I don't know something?
Normally there is no difference. It should be a USB cable with A Male to Micro B connectors. Just make sure it can carry both power and data over the wire. Support will say "Garmin supplied" cable to avoid any issues with some unknown poor quality cable.
That's what the rep told me. Apparently if a different cable is used the device could be confused as to how much charge it is getting. Not all USB cables are the same. Some carry data, some are "Dash" cables for faster charging, etc.
A Garmin Cable is a known variable. It's not that it is special, but other cables may have different properties that can't be accounted for. Essentially using a Garmin USB cable eliminates the cable as a potential source of the issue. Also, I think by charging via computer, it also is a known variable.
That's all I got as a user trying to get this sorted out.
The % left is an estimation based on battery voltage, and Li Ion batteries have a very shallow discharge curve. If it changes a little as the battery ages, it can lead to big difference in estimated vs actual remaining charge. Some devices will re-learn the discharge curve if allowed to discharge until it turns off. It might take doing this several times. IDK if the 820 does in fact re-learn the discharge curve or not. but it's easy enough to try. It sounds like you're trying it now.