The yellow LED on the Mayfly board indicates when the battery is actively being charged. If the battery is fully charged, then the yellow LED will be off. What does the data card file say the battery voltage was when you experienced the issue of the yellow LED not being lit? The Red and Green LEDs near the battery jack are programmed to blink rapidly back and forth 5 times when you first boot up the board (or if you press the reset button), assuming you’re running one of our MondularSensors example sketches. Once the logger is running, the Green LED will only come on when the logger is awake an taking a sample. There’s a second Red LED down in the far corner of the Mayfly board near the Grove connectors, and it comes on when the sensors are being powered during a sample. When the logger is asleep, you won’t see any Red or Green LEDs, and the yellow LED will only be lit if the battery is lower than 4.15v and the sun is out. If there’s not sufficient sun to charge the battery, or if the battery is already charged, then the Yellow LED will be out, this indicates that the charge regulator circuit is functioning correctly to keep from over-charging the battery once it’s full. So checking your datafile to see what battery voltage was recorded is important for determining if your charger is working properly.