I have a 18V solar panel which is connected to a step-down buck converter, which drops the voltage from 18V to 3.5V. When clouds block the sun, the voltage drops to less than 3.1V. I have added a diode on the positive terminal to make sure that the battery doesn't give back any current, but I was wondering if there is any risk in having such a fluctuation. Li-ion is sensible to trickle-voltage, and I am worried that this kind of of fluctuations will destroy the battery. Is this fear motivated by reality?
Right now, I'm only connecting the wires to the battery when it is full sun, because I have a constant voltage more or less. Can I keep this connection all the time or do I need other components to keep the battery safe?