Hi Carlos,
It has 4 serial connected solar cells protected with diodes, when not all cells are equaly illuminated.
The power passes another schottky diode to prevent backward current and discharge of the battery. As you can see there is no charging circuit. This is not necessary here.
A solar cell has a so called MPP (maximum power point). This is about 85% of the free running voltage the cell produce. More explanation can be found here
My calculation:
Solar cells 4 x 2V (free running voltage)
Battery is a 4Ah industrial type NiCd
Schottky diode 3A / 0.4V
4 x 2V = 8V => MPP = 85% => 6.8V
Protecting schottky diode = 6.8V - 0.4V = 6.4V
The maximum solar cell current is ~350mA which is about 0.1C for this type of battery. The datasheet recommends for 0.1C a max.charging time of 14-16h. Because 350mA is never available during the whole day, no charging circuit is needed. The software disables sleeping when the battery exceeds 6V
The board is powered with this Pololu Step-Up/Step-down Voltage controller
Schemata
Errata:
Sorry, INU219 has a typo. The correct name is INA219 and is made by adafruit
Voltage curve (today)
Current curve (today)
