Why should a series diode be connected between a solar panel and a storage battery that is being charged by the panel?

Solar panels generate electricity when exposed to sunlight, but in the dark, their internal circuitry can act like a resistive load. Without a blocking mechanism, the higher voltage of a fully charged battery will cause current to flow backward into the solar panel at night, which slowly drains the battery. A diode acts as a one-way electrical valve. By placing a series diode in the circuit, current is allowed to flow from the panel to the battery during the day, but is blocked from flowing in the reverse direction when illumination drops. It is important to note that a simple series diode does not regulate charging voltage or prevent overcharging at high illumination levels; a dedicated charge controller is required for those protective functions.