For RFID gate controller board I had to build a prototype of a solar powered power supply with 4 Li-ION 3.7V batteries. I utilised for this a CN3791 IC which is capable of delivering 4A charge current - plenty enough for 4 batteries... Another upside of using it was a wide range of input voltages, so finding a proper solar panel (or upgrading to a bigger one) was not a problem. Also thanks to its MPPT capabilities it can utilise the sun a little bit better than the standard PWM controller. Schematic was mostly based on reference design from a manufacturer data-sheet, so nothing special 😬.