From the perspective of electronic technology for the battery: the first phase is called the charging current limiting phase, the second phase is called the high constant voltage phase, and the third phase is called the low constant voltage phase. During the second and third phase transitions, the panel indicators change accordingly. Most of the chargers have a red light in the first and second phases, and a green light in the third phase. The mutual conversion between the second phase and the third phase is determined by the charging current, which is greater than a certain current entering the first second phase, and less than a certain current entering the third phase. This current is called the switching current, also called the turning current.
Early chargers, including chargers for brand-name cars, although they also changed lights, are actually constant-voltage current-limit chargers, not three-stage chargers. Generally, this kind of stable voltage value is about 44.2V, which is still good for the high-specific gravity sulfuric acid battery at that time.