A very simple solar charger circuit for batteries can be constructed using this circuit diagram .
The nominal voltage of the solar charger circuit module is determined by the number of battery cells to be charged. Because of the typical voltage drop of 0.3 to 0.4 V across Schottky diode D1, the nominal voltage should exceed the charge voltage set on P1 by about 0.3–0.4 V .
The solar panel for this project is a typical solar module that consists of eight series connected solar cells. In sunshine the solar panel will supply about 140 mA -200mA or more( depends of the solar panel used ) at 8 times 0.45 V = 3.6 V.
For a 1400-mAh battery require 12 – 14 hours for full charge at 140mA , so you can use a solar panel with a little high capacity to decrease the charge time .
If you don’t find a zenner diode with this value you can use two normal diodes connected in forward bias ( cathode connected to the ground ) .
Using the P1 potentiometer you can set the final charging voltage at the desired voltage .
The voltage across the batteries is continuously monitored by the circuit around T2.
When the voltage rises above a certain level (full charge ), a power resistor is switched in parallel with the solar panel, which causes output voltage of the solar panel to drop and stops the batteries from being charged .