The mainstream charging method for lithium batteries is Constant Current – Constant Voltage (CC/CV). It begins with constant current charging, then switches to constant voltage charging once a certain potential is reached. High-quality chargers can also perform trickle charging based on the battery's voltage state. Some systems further incorporate pulse charging methods and terminate charging based on time settings. Most chargers integrate functions such as current limiting, voltage limiting, overvoltage protection, overcurrent protection, overtemperature protection, and reverse polarity protection. The specific charging curve is illustrated in the figure below.
- Stage 1: Trickle Charging—Trickle charging is used to pre-charge (recovery charge) fully discharged battery cells. When the battery voltage is below approximately 3V, charging begins with a constant current of up to 0.1C.
- Stage 2: Constant Current Charging—When the battery voltage rises above the trickle charging threshold, the charging current is increased to perform constant current charging. The constant current ranges from 0.2C to 1.0C. The current during constant current charging does not need to be extremely precise; a quasi-constant current is acceptable. In linear charger designs, the current often increases as the battery voltage rises to minimize heat dissipation on the power transistors. Constant current charging above 1C does not shorten the overall charging cycle time and is therefore not recommended. When charging at higher currents, the battery voltage rises more rapidly due to overvoltage from electrode reactions and increased voltage drop across the internal resistance. The constant-current phase becomes shorter, but the subsequent constant-voltage phase duration increases accordingly, so the total charge cycle time remains unchanged.
- Stage 3: Constant-Voltage Charging – When the battery voltage reaches 4.2V, constant-current charging ends, and the constant-voltage phase begins. For optimal performance, voltage regulation tolerance should be better than +1%.
- Stage 4: Charge Termination – Unlike nickel batteries, continuous trickle charging is not recommended for lithium-ion batteries. Continuous trickle charging can cause plate plating effects due to metallic lithium formation. This renders the battery unstable and may lead to sudden, rapid self-disintegration.
Chargers typically perform energy balancing in conjunction with PCM or BMS during the battery's constant-voltage charging phase. For standard lithium cobalt oxide batteries, when the battery voltage drops below 3.0V, the charger initiates trickle charging (approximately 0.1C) to prevent battery damage. Upon reaching 3.0V, it switches to constant current charging (around 1C, with current magnitude determined by the system). When the battery voltage reaches 4.1V, it transitions to constant voltage charging. Once the battery current drops to approximately 0.1C, charging concludes, and the charging system and circuit are deactivated. The sequence—trickle, then constant current, finally constant voltage—employs different control technologies based on power requirements: linear power supply solutions for low-power applications and switching power supply solutions for high-power applications.