Lithium battery four-stage charging principle and key points analysis
When the lithium battery is used, the voltage is lowered and the chemical activity of the battery is lowered. In order to better protect the performance of the lithium battery, the lithium battery generally requires the charging process to be controlled in four stages: trickle charge (low voltage precharge), constant current charge, constant voltage charge, and charge termination.
Taking a conventional battery ternary lithium battery as an example, the nominal capacity of the INR26650-50A battery is 5000mAh, the nominal voltage is 3.6V (the standard is also 3.7V on the market), the discharge termination voltage is 2.75V, and the charge termination voltage is 4.20. V.
The basic requirements for a lithium battery charger are the specific charging current and charging voltage to ensure safe battery charging. Additional charging assistance functions are added to improve battery life and simplify charger operation, including trickle charging, battery voltage detection, input current limiting, shutdown of the charger after charging is completed, and automatic discharge of the battery after partial discharge. Start charging, etc.
Lithium battery charging mode is limited voltage constant current, which is 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, pre-charge is required first, and the charging current is set. After 1/10 of the current, the voltage rises to 3V and enters the standard charging process.
The standard charging process is: constant current charging with 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 drops to 1/10 of the set charging current, the charging ends.
The four stages of lithium battery charging are explained:
Phase 1: Trickle Charge - Trickle charge is used to precharge (recovery charge) the fully discharged battery unit. Trickle charging is used when the battery voltage is lower than about 3V, and 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).
Phase 2: Constant Current Charging - When the battery voltage rises above the trickle charge threshold, the charging current is increased 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, and the voltage set by a single battery is 3.0-4.2V.
Phase 3: Constant Voltage Charging - When the battery voltage rises to 4.2V, the constant current charging ends and the constant voltage charging phase begins. According to the saturation degree of the battery, the charging current is gradually decreased from the maximum value as the charging process continues, and when it is reduced to 0.01 C, the charging is considered to be terminated. (C is a representation of the battery's nominal capacity versus current, such as the battery is 1000mAh capacity, 1C is the charging current 1000mA.)
Phase 4: Charging terminated.
There are two typical methods of terminating the charge: using the minimum charge current or using a timer (or a combination of both). 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 is timed from the beginning of the constant voltage charging phase, and the charging process is terminated after two hours of continuous charging.
The above four-stage charging method requires about 2.5 to 3 hours to complete charging of the fully discharged battery. Advanced chargers also use more security measures. For example, if the battery temperature exceeds the specified window (usually 0 ° C to 45 ° C), charging will be suspended. After charging is completed, if the battery voltage is detected to be lower than 3.89V, it will be recharged.