The simplest solar cell power supply system consists of three parts; a diode, a solar cell panel and a rechargeable battery. The diode prevents the battery being discharged through the solar panel when no or little light fall on to the panel. Although the diode is a type of Schottky type, the forward drop of this type may cause an appreciable loss of energy. The present circuit uses a special low drop diode.
How the solar cell power supply system works?
The opamp draws only a tiny current on average 20µA. Although it can operate at allow voltage of as low as 1.6 V. The gate of T1 requires at least 3.0 V to be switched on. For that reason the nominal battery voltage is specified as 4.0 V.
To align the circuit, replace the solar panel by a variable regulated power supply with its current limiting set to value that is safe for the battery. Set the power supply output to a level that is 0.1 V higher than the current battery voltage. Next, adjust P1 so that the output of IC-1 just becomes logic high. Finally, check with an ammeter that the battery is not being discharged when the power supply is set to below the current battery voltage.