As Neil mentioned above, the yellow LED that is labeled SOLAR only indicates when a charge current is being applied to a battery, so if a battery if fully charged (~4.2v), the LED will not be on. The charging circuity of the Mayfly v1.1 boards is configured so that the solar panel provides voltage directly to the Mayfly’s main power regulators when the sun is out (and also charges the battery) and it uses the Lipo battery as kind of a “backup” source in case the Mayfly draws more current than the panel can provide. When there’s insufficient power collected by the solar panel (like at night or in really shady locations) then the main power for the Mayfly comes from the battery. If you’re using analog pin A6 to monitor the input voltage, you’ll see the raw battery voltage at night (anywhere from 3.5v to 4.2v), and during the day you’ll usually see something between 4.1v and 4.25v (depending on the amount of sunlight). In the attached graph image below, you can see the raw battery voltage is around 3.85v.
You can actually get useful information from this plot like that because it shows you what days were sunny and what days were cloudy, which could help you understand why the underlying raw battery voltage might increase or decrease over time. For example, in the plot below you can see it was completely sunny on September 9, mostly sunny on Sept 10, totally cloudy on Sept 11 (no charging at all), and mostly sunny on the 12th and 13th. Every station is going to have a different charging pattern based on tree canopy, time of year, weather, obstacles, etc, but having the graph showing the input voltage to the Mayfly can be very useful for monitoring the overall status of a station. Also, if a Mayfly is being powered directly through its USB port (with or without a Lipo also connected to the Lipo socket), then the voltage measured by the analog A6 input will show 4.95v.
If you would rather monitor the raw Lipo battery voltage and not see the swings of the solar panel (or USB) input, then you can follow the directions mentioned below, which come from the Mayfly Jumper Settings page.
SJ27: Analog pin A6 connected to either combination Voltage in (default) or direct Lipo battery socket. The default setting works best so that the Mayfly can sense the input voltage of whichever source is highest. With just a Lipo connected, A6 will see the battery voltage (~3.7v). If a solar panel is connected, then A6 will see approximately 4.2v during full sun. If a USB cable is connected, A6 will see 4.9v. If you want to see only the raw Lipo battery voltage independent of the higher voltage from the solar panel, change the solder jumper to Lipo.