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Li-ion batteries have three primary charging stages. First, a precharge current is applied if the battery is deeply discharged (usually below 2.8 to 3 V). This stage prevents overheating of a deeply discharged battery.
The precharge current is usually set to 10% of the fast-charge current. Once the battery voltage is greater than 2.8 V, the fast-charge current is applied; this is the second charge stage. During this period, the battery is charged with a constant current of 1C or less, "C" being the rated capacity of the Li-ion cell. Thus, during both the first and second stages, the charger is operating in the constant-current (CC) mode.
The rising battery voltage reaches a target of 4.1 or 4.2 V. Here stage 3 begins--a transition from CC mode to constant-voltage (CV) mode. At this point, the battery is approximately 70% replenished and the current should fall while the voltage remains constant...
