solar charge

It charges the normal battery. The amount of charge you get from the solar panel, even in full summer sun, is tiny. It really won't affect the battery life at all.

I would not recommend the extra cost of the 955 Solar if you're outside the tropics. At the latitudes of the UK (50°N) it's pretty much useless most of the year.

Also the solar panel adds a little extra glare to the display.

(Yes, I have a FR955Solar, and I'm in the UK.)

For best battery longevity, don't let the watch discharge to less than 25% and don't regularly charge it beyond 85-90%. If you charge it whilst taking a shower or bath, that 15-20 minutes is enough to take it from 50% to 80%. If you have a long activity planned, or expect you won't be able to charge it for a week or more, an occasional charge to 100% is fine.

Incidentally, the watch hasn't fully charged the battery when it shows 100%. It continues to charge it until it's actually full.

Don't expect the solar feature to actually increase the charge level of your watch.  It more or less offsets your current usage rather than noticeably increasing the state of charge.  You may on occasion see it rise 1 or maybe even 2% if you're outdoors in direct sunlight for quite a while, but the solar charging feature on these watches is very gimicky.... especially on the inefficient newer versions of the firmware.  Newer firmwares are much less efficient than the older ones in regards to battery usage and it seems like the solar charging doesn't do enough to offset what's being used now.  I actually recently downgraded my firmware to version 15.19 and my battery life has been much better and I've actually noticed that i occasionally do get gains from solar charging.  I used up 5% battery last night during a 7 mile run, then another 1% while sleeping overnight, and i actually gained back 5% from my watch solar charging while I was driving into work this morning with my arm in the car window for 20 minutes.  That's by far the most solar charging i've ever seen on this thing dating back to October of 2022 when I first bought it.  The only thing that sucks is that I can't really install any new watch faces or apps since the IQ store wants me to upgrade the watch firmware before doing any of that.  Keep in mind that from an engineering perspective, determining state of charge of a lithium battery is not an exact science, so just because my watch said it gained 5%, it may have only actually gained a small %, but the gain was enough to spike the voltage enough to make the controller think it gained 5%.

How do you downgrade the firmware?

Download the firmware you want from here:

https://drive.google.com/drive/folders/1LnSFxMx3ENj_frUEoq0Y2pjz6Imftypm

Using a personal computer, unzip the file and extract the contents to your computer.  There are 2 files in there GUPDATE.GCD and FORCE.TMP.

Connect your watch by cable to the computer and make sure you have at least 60% charge in the battery.  Copy the 2 files mentioned above to the /Garmin folder on your watch.

When you disconnect watch cable from the computer the watch will ask you if you want to install an update. Click Install Now and wait for it to complete. 

You'll have to setup your watch again after the install and if you use a real old version you may not be able to install watch faces with ConnectIQ (only by sideloading).