First, it is important to ensure that the lithium-ion battery charger is compatible with the appliance being charged, including voltage and current matching. Voltage matching is crucial, while the charger's current rating determines the charging time; the higher the current rating, the faster the charging speed. For example, a 3A charger is faster than a 1A charger.
Secondly, a more conventional way to tell is by the color change of the indicator light on the charger or the light on the appliance. It usually changes from red to green, indicating that it is fully charged.
Another more technical point is measuring the voltage of electrical appliances. The terminal voltage of a battery varies depending on its charge level; the lower the charge, the lower the voltage. Therefore, battery voltage has a range, roughly from the lower limit that allows the appliance to operate normally to a full charge. For example, mobile phones use lithium-ion batteries, with an operating voltage range of 3.7~4.2V. Assuming the charge level is divided into three bars:
When the lithium-ion battery voltage is 3.7~3.8V, it is empty and the power alarm sounds; when the battery voltage is 3.8~3.9V, one bar is displayed.
When the lithium-ion battery voltage is measured at 3.9~4.1V, it shows two bars; when the battery voltage is measured at 4.1~4.2V, it shows full bars.
A good quality charger will detect voltage and current to determine if it is fully charged. However, a poor quality charger may not do this and will rely entirely on the lithium-ion battery's own overcurrent and overvoltage protection to charge it. It will only automatically cut off the current when the battery detects an overvoltage and then notify you that charging is complete.
