The Meaning of the Duration of the Constant Voltage Phase in Lithium Battery Charging

The duration of the constant voltage phase reflects the magnitude of the constant current ratio. From a data perspective, it is the most direct representation of the battery's internal state.

I. What is the Constant Current Ratio?

The most common standard charging technology is constant current/constant voltage charging, i.e., CC (constant current) + CV (constant voltage). The constant current phase is the rapid charging process, where the charger charges with a fixed high current, and the battery voltage continuously rises. When the voltage reaches its upper limit (e.g., 4.3V for ternary lithium cells), it automatically switches to the constant voltage phase. At this time, the voltage remains constant, and the current gradually decreases until it drops to 0.05C-0.1C, at which point charging ends. Constant current ratio = CC / (CC + CV)

II. The Mechanism of the Duration of the Constant Voltage Phase

The duration of the constant voltage phase is fundamentally affected by two factors: changes in internal resistance and polarization.

1. Changes in Internal Resistance
According to Ohm's Law U=IR, under a constant charging current, an increase in the battery's internal resistance will result in a higher ohmic voltage drop. This results in what we commonly call "artificially high" voltage—the actual amount of charge stored inside the battery is far from full, but the externally measured terminal voltage has already reached the charging cutoff voltage, forcing the battery to switch from the constant current stage to the constant voltage stage prematurely.

2. Polarization Influence
Polarization voltage refers to the difference between the actual operating potential of the electrode and the thermodynamic equilibrium potential. We can understand this with a simple formula: Externally measured voltage = Battery true open-circuit voltage + Ohmic voltage drop + Polarization voltage. At the end of constant current charging, when the externally measured voltage reaches the termination voltage, after deducting the ohmic voltage drop and polarization voltage, the battery's true open-circuit voltage is still far below the full-charge voltage.

The essence of the constant voltage stage is to gradually reduce the charging current to simultaneously reduce the ohmic voltage drop and polarization voltage, allowing the battery's true open-circuit voltage to slowly rise to the true full-charge level. When the battery's polarization resistance increases significantly, it means that a longer time and a smaller current are needed to reduce the polarization voltage to a sufficiently low level to complete the charging of the remaining charge.

III. Duration of the Constant Voltage Phase Reflects Battery Characteristics

1. Cell Health Status:

The constant current ratio is highly positively correlated with the battery's State of Health (SOH). The constant current ratio gradually decreases with increasing cycle count.

2. Fast Charging Capability:

The constant current ratio reflects the fast charging capability. Different charging rates (1C, 2C, 3C) result in different constant current ratios. Furthermore, the constant current ratio decreases with increasing charging rate. Therefore, at the same charging rate, a higher constant current ratio indicates better fast charging capability.

3. Safety Performance:

A lower constant current ratio indicates a longer constant voltage phase, suggesting higher internal resistance within the cell. A sharp drop in the constant current ratio with increasing cell cycle count often indicates serious safety risks. High internal resistance leads to excessive heat generation during charging, accelerating battery aging. More dangerously, severely aged batteries, when charged with low current for extended periods at the end of the constant voltage phase, experience deteriorating negative electrode dynamics, easily triggering lithium plating. The deposited lithium metal may puncture the separator, causing internal short circuits or even thermal runaway.

 4. Battery Pack Consistency
In electric vehicle or energy storage battery packs, the difference in constant voltage time among individual cells is a crucial indicator of consistency. If a cell's constant voltage time is significantly longer than others, it indicates more severe aging, higher internal resistance, and will become a "weak link" in the entire battery pack, limiting overall performance and increasing safety risks.

ACEY-AS11S battery sorting machine is an essential tool for professional battery pack manufacturers. It is purpose-built to test and sort cylindrical cells—including LiFePO4 and NCM types—according to their open circuit voltage (OCV) and internal resistance (IR).

Summary: The duration of the constant voltage phase for fresh cells is determined by the charging rate, ambient temperature, chemical system, and charging cutoff voltage.