My 820 used to ask me if I wanted to switch to battery save mode when the battery was running low (I think it was at 20% battery left). However this no longer happens. I can't find a setting that turns this on or off. Any idea how I can turn this back on?
True. I used to run into the issue on my 820 after ~ 4hrs. I did a ~2hr ride with the 840 running multi mode GPS, map, Nav, PM, HR, Cadence, Speed Sensor and Live Track. and used about 10% of the battery, so even if that particular issue is still there, it'd take ~18 hrs before it'd crop up.
Best thing is to check the DCRainmaker review for details on the dif between solar and non-solar.. That said, IMO, no, it's not worth it. Solar charge rate is extremely slow so adds little run time at the expense of somewhat reduced screen contrast.