Meaning
Charging protocols apply a steady flow of electricity until a target voltage is reached, followed by a period where the voltage is held steady while the current gradually decreases. The constant current constant voltage method is the standard approach for charging lithium ion batteries to ensure they reach a full state of charge without exceeding safety limits. This two stage process optimizes the balance between charging speed and cell longevity.
Initial Stage
High power is delivered to the battery during the first phase to replenish the bulk of the energy quickly. During this period of the constant current constant voltage cycle, the charger maintains a fixed amperage while the battery voltage rises. This stage continues until the cell reaches its maximum allowable voltage, usually around four point two volts for many chemistries.
Final Saturation
Tapering of the current occurs in the second phase to safely fill the remaining capacity. Once the charger enters the second half of the constant current constant voltage sequence, it holds the voltage constant to prevent overcharging. The current slowly drops as the internal resistance of the battery increases, and the process stops when the current hits a predefined low threshold.
This saturation phase is essential for balancing the individual cells within a series string, as it allows slower cells to catch up without the faster ones exceeding their voltage limit.
Thermal Management
Heat generation is most intense during the high current phase and requires careful monitoring. Implementing a constant current constant voltage profile helps manage the temperature rise by reducing power as the battery fills. This protection is necessary to prevent the degradation of the electrolyte and the risk of thermal instability.
