Prevent voltage to drop under threshold in solar panel
The 1N4733 zener diode is a 5.1 volt type meaning the maximum voltage you get across it is 5.1V +/-5%. The 1N5819 schottky diode drops typically 600mV at 1A and 400mV at 0.1A.

Either way, the voltage you might be seeing at the terminals is significantly less than 5V and I would estimate it to be: -


4.7V at 100mA charging current
4.5V at 1A charging current


This seems to be at odds with what you are expecting and I'm wondering if you are aware of this?

Assuming you are aware of this limitation, maybe you'd like to offer a new "low-voltage" limit. However, as with most solar cells that I'm aware of, as soon as you disconnect it from a load (via the automatic under-voltage idea) the terminal voltage will undoubtedly rise above the under-voltage threshold and the circuit will re-enable - this might end up in a series of on-offs lasting several minutes and probably isn't what you want?