The figure above illustrates the charging topology for lithium-ion cells, with the red line representing constant current and the blue line representing constant voltage.

When the battery is under voltage, constant current is applied to the cell until the voltage reaches 4.2 V. At this point, the system switches from constant current to constant voltage. The cell voltage should not exceed 4.2 V, and the current is gradually reduced until it approaches 0, which means “I” termination. When the cell voltage reaches 4.2 V, the current stops flowing and the cell is considered fully charged.

There are also some considerations to protect against battery abuse or fault conditions. For example, if a cell is shorted or in default state, passing full current through the circuit can cause battery damage. Therefore, at low voltages, only a small test current is applied to wake up the battery in case the battery PCM (Protection Circuit Module) trips.

In the 2 V to 3 V range, a precharge current of approximately 1/10 of the rapid charge current is generally applied. This is called the precharge period, and the battery is charged by slowly flowing current like a dripping water drop.

When the cell is charged to 3V or more, it is judged to be stably charged and switches to rapid charging, and this section is called constant current. Finally, in the case of an actual battery, when the voltage reaches approximately 4.2V, it becomes a constant voltage section and the current gradually decreases.

It is also possible to terminate the charge while maintaining the current at 1/10 of the rapid charge range. At this time, termination detection must be possible at 10mA or less, and accurate sensing is essential.