The circuit can be made automatic by adding a 1k resistor and diode:
The oscillator will turn off when the output from the solar panel is above 1.3v and although the circuit does not shut down to zero current, it consumes about 3 mA, while the shut-off circuit takes about 1mA.
On a bright day, the solar panel delivers 20mA to the battery, so the overall net charging current is about 15mA max.
This means any data logging circuit or transmitter connected to the supply will only work at night.
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