This invention relates to a battery charger which supplementally charges a secondary battery such as a battery and so forth in an automobile with use of a solar cell in which a diffusion-blocking layer is formed between a semiconductor and a reverse electrode for the purpose of preventing deterioration of a semiconductor layer, and in which electric potential of the secondary battery can be measured and displayed, and in which the solar cell can be securely provided in a compartment of an automobile in such a manner that the solar cell always faces sunbeams, and which can be freely attached and detached.
