What Stages Does Lithium-Ion Battery Charging Go Through?

Lithium-ion battery charging is a complex electrochemical process that typically goes through three main stages: constant current charging, constant voltage charging, and trickle charging (or maintenance charging). Each stage has a specific purpose and works together to ensure that the battery is charged safely, efficiently, and to its full capacity.

The first stage is constant current charging. When a lithium-ion battery is in a low-voltage state (below 3.0V for a single cell), it enters the pre-charging phase, which is a low constant current charging process. The purpose of pre-charging is to recover the battery's voltage slowly, preventing damage to the battery caused by a large current when the voltage is too low. Once the battery voltage reaches about 3.0V, it enters the main constant current charging stage. In this stage, the charger provides a constant current (usually 0.5C to 1C, where C is the battery's capacity) to the battery, and the battery voltage rises linearly. During this stage, the electrochemical reaction is intense: lithium ions are deintercalated from the positive electrode, pass through the electrolyte, and intercalated into the negative electrode (graphite layer). The constant current charging stage is the main stage for the battery to gain capacity, accounting for about 70-80% of the total charging capacity. For example, a 2000mAh battery charged at 1C (2000mA) will reach about 1600mAh after the constant current stage.

The second stage is constant voltage charging. When the battery voltage reaches the preset constant voltage value (usually 4.2V for a single lithium cobalt oxide battery, 3.65V for a lithium iron phosphate battery), the charger switches from constant current charging to constant voltage charging. In this stage, the charging voltage remains constant, while the charging current gradually decreases. As the battery is gradually filled, the concentration of lithium ions in the negative electrode increases, and the resistance to lithium ion intercalation increases, resulting in a decrease in current. The purpose of constant voltage charging is to fully charge the battery without overcharging. During this stage, the battery continues to gain capacity, but at a slower rate. When the charging current decreases to a certain threshold (usually 0.05C to 0.1C), the constant voltage charging stage is completed, and the battery is considered fully charged. This stage accounts for about 20-30% of the total charging capacity.

The third stage is trickle charging, also known as maintenance charging. Some chargers enter this stage after the constant voltage stage to compensate for the self-discharge of the battery. In trickle charging, the charger provides a very small current (usually less than 0.05C) to maintain the battery's full charge state. However, modern lithium-ion batteries have low self-discharge rates, and long-term trickle charging may cause overcharging, which is harmful to the battery. Therefore, many advanced chargers do not have a separate trickle charging stage, but instead stop charging after the constant current and constant voltage stages, or enter a standby state.

It should be noted that the charging stages of lithium-ion batteries are controlled by the charger's management system, which monitors the battery's voltage, current, and temperature in real-time to adjust the charging process. Proper charging according to these stages can effectively extend the battery's service life, while improper charging (such as overcharging or using an incompatible charger) can lead to capacity degradation, reduced service life, and even safety hazards.

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