Power source solar battery can be assembled according to the scheme shown in the figure.
Battery GB1 and GB2 are recharged from solar panels GL1. Diode VD1 prevents their discharge through the solar battery when its voltage is below rated as a result of insufficient lighting. Pressing SB1-SB3, control the voltage of the battery, connecting to them via a ballast resistor R1 milliammeter RA1.
The current consumed in the measurement, deliberately selected quite large (about 100 mA). This results in a more precise illumination solar battery and the charge status of the batteries.