Analyze the principle and key points of four-stage charging of power battery

As the lithium-ion battery is released during use, the voltage drops, and the chemical activity of the battery also decreases. In order to better protect the performance of lithium-ion batteries, lithium-ion batteries generally require the charging process to be controlled in four stages: trickle charging (low-voltage precharge), constant current charging, constant voltage charging, and charging termination.

Taking a conventional ternary lithium-ion battery as an example, the nominal capacity of the INR26650-50A battery is 5000mAh, the nominal voltage is 3.6V (there is also a standard 3.7V on the market), the discharge termination voltage is 2.75V, and the charging termination voltage is 4.20V.

The basic requirement of Li-ion battery chargers is a specific charging current and charging voltage, so that the battery can be safely charged. Additional charge assist features have been added to improve battery life and simplify charger operation, including trickle charge for overdischarged batteries, battery voltage detection, input current limit, charger shutdown after charging is complete, battery partially discharged Automatically start charging, etc.

The charging method of lithium-ion battery is limited voltage and constant current, which are controlled by IC chip. The typical charging method is: firstly detect the voltage of the battery to be charged. If the voltage is lower than 3V, it must be pre-charged first. The charging current is set to 1/10 of the constant current, after the voltage rises to 3V, it enters the standard charging process.

The standard charging process is: constant current charging with the set current, when the battery voltage rises to 4.20V, it is changed to constant voltage charging, and the charging voltage is kept at 4.20V. At this time, the charging current gradually decreases, and when the current decreases to 1/10 of the set charging current, the charging ends.

Description of the four stages of Li-ion battery charging:

Phase 1: Trickle Charge Trickle charge is used to pre-charge fully discharged cells first (recovery charge). When the battery voltage is lower than about 3V, trickle charging is used. The trickle charging current is one-tenth of the constant current charging current, that is, 0.1c (for example, the constant charging current is 1A, the trickle charging current is 100mA).

Stage 2: Constant current charging When the battery voltage rises above the trickle charging threshold, increase the charging current for constant current charging. The current for constant current charging is between 0.2C and 1.0C. The battery voltage gradually increases with the constant current charging process. Generally, the voltage set by a single battery is 3.0-4.2V.

Stage 3: Constant voltage charging When the battery voltage rises to 4.2V, the constant current charging ends and the constant voltage charging stage begins. According to the saturation degree of the cell, the charging current gradually decreases from the maximum value as the charging process continues. When it decreases to 0.01C, the charging is considered to be terminated. (C is a way of expressing the nominal capacity of the battery against the current. For example, if the battery has a capacity of 1000mAh, 1C is the charging current of 1000mA.)

Stage 4: Charging terminated.

There are two typical methods of charge termination: use the minimum charge current judgment or use a timer (or a combination of the two). The minimum current method monitors the charging current during the constant voltage charging phase and terminates charging when the charging current is less than 0.02C. The second method counts the start of the constant voltage charging phase and terminates the charging process after two hours of continuous charging.

The four-stage charging method described above completes charging of a fully discharged battery in about 2.5 to 3 hours. Premium chargers also incorporate more safety measures. For example, if the battery temperature exceeds a specified window (usually 0°C to 45°C), charging will be suspended. After charging, if the battery voltage is detected to be lower than 3.89V, it will recharge.