Why is it necessary to first use constant current (CC) and then constant voltage (CV) control when charging a car?

Understanding the constant current (CC) and constant voltage (CV) stages is crucial for comprehending how to charge lithium batteries safely, quickly, and healthily.

1. Stage 1: Constant current charging - "Sprint and water filling with full effort"

When the cup is still empty and the water level is low, to fill it up quickly, you tilt the kettle to a fixed, maximum safe angle, allowing a strong and steady flow of water (constant current) to continuously pour into the cup. This is similar to when the battery's state of charge (SOC) is low, its voltage is also low. The charging station applies a "constant high current" (for example, the maximum current negotiated between the vehicle and the charging station, such as 250A). It is evident that:

1) Rapid rise in water level (voltage): As current continues to be injected, the battery voltage steadily and rapidly increases;

2) This is the "main phase" of charging: about 80% of the battery's capacity is quickly charged during this phase, so it is the crucial stage for pursuing charging speed.

      2 Stage 2: Constant voltage charging - "Carefully filling up"

   When the water level (voltage) rises very close to the top of the cup (full-charge voltage), if you still maintain the previous large flow of water, inertia will cause the water to overflow the cup (overcharge), which is absolutely prohibited! At this point, you must immediately adjust your actions: maintain the height of the kettle mouth constant (i.e., keep the "water pressure" at the top of the cup constant), and then gradually and slowly reduce the inclination angle of the kettle (reduce the current).

This is akin to the scenario where, when the battery voltage climbs to the preset charging cut-off voltage (for instance, 4.2V per cell), the charging phase transitions. The charging station shifts to a "constant voltage source" mode, maintaining this voltage unwaveringly. During this period, the following occurs:

1) The voltage is "stuck" at its peak: it no longer rises, preventing damage from overvoltage. 2) The current begins to decay naturally: as the internal chemical potential of the battery becomes closer and closer to the externally applied voltage, the "pushing force" that drives charges into the battery becomes weaker and weaker. Therefore, the charging current gradually decreases like a naturally declining curve, and the BMS continuously monitors the current. 3) This is the "leakage compensation stage" of charging: the battery charge increases from 80% to 100%. The speed becomes slower and slower, just like filling the last few corners. When the current decreases to a very small preset value (for example, 0.05C, or 5% of the maximum current), the BMS considers the battery to be "substantially fully charged" and sends a command to completely stop charging. It's like turning off the faucet when you see the water flow has become a trickle.

   Constant current (CC) and constant voltage (CV) two-stage charging represents an engineering compromise and control artistry between the "nonlinear electrochemical characteristics of batteries" and "linear controllable power supplies" to achieve optimal, safe, and efficient energy transfer. The CC stage utilizes the characteristic of batteries having less polarization in the low/medium SOC range to maximize power and address the pain point of "slow charging"; the CV stage respects the physical reality of batteries having a sharp increase in polarization in the high SOC range, strictly controls the voltage boundary, and addresses the core issues of "overcharging is unsafe" and "overcharging damages battery life".