Why should a series diode be connected between a solar panel and a storage battery that is being charged by the panel?
Why A solar panel only produces voltage when it is lit. In darkness or deep shade the panel becomes a load, and the battery would push current backward through the cells, slowly draining itself. A series 'blocking' diode conducts only in the panel-to-battery direction, so charge current flows in but reverse current cannot flow out. The tradeoff is a forward voltage drop of roughly 0.3 V for Schottky types or about 0.7 V for silicon.
Watch out Regulating charge voltage or limiting current is the job of a charge controller, not a diode; a plain diode has no way to sense or adjust either one.
Blocking diode = one-way valve: charge flows in at sunrise, nothing leaks back at sunset.