I made a mistake with the last PCB design. I added a switch to disconnect the battery and solar panel from the rest of the electronics if needed. Turns out that was not a good idea. The switch S1 is connected in series to the LiPo battery (please see the image below). What I did not realize was the LTC4070 sees the internal resistance of the switch (it's supposed to be less than 70 Milliohms) and now only charges the battery to about 3.95 Volts instead of the usual 4 Volts. In hindsight adding a switch to the PCB was a bad idea anyway. It just adds another point of failure to a system that is never supposed to stop running. At the time I just thought it would be nice to have a power switch while developing :-) .