How to charge LiFePO4 battery correctly

The full name of LiFePO4 battery is LiFePO4 lithium ion battery. Because its performance is particularly suitable for power applications, the word "power" is added to the name, that is, LiFePO4 power battery. Some people call it "lithium-iron power battery", and you all know the charging skills of LiFePO4? Here are some charging tips for LiFePO4 batteries.

1. Before solving the problem, we first need to understand the structure and working principle of LiFePO4 battery. LiFePO4 is used as the positive electrode of the battery. The aluminum foil is connected to the positive electrode of the battery. The polymer separator is in the middle. It separates the positive electrode from the negative electrode. Ions can pass but electrons cannot pass. On the right is the battery negative electrode composed of carbon (graphite), and the copper foil is connected to the battery negative electrode. The battery's electrolyte is between the upper and lower ends of the battery, and the battery is hermetically sealed by a metal case. During charging of the LiFePO4 battery, lithium ions in the positive electrode migrate to the negative electrode through the polymer separator; during discharge, lithium ions in the negative electrode migrate to the positive electrode through the separator. Lithium-ion batteries are named because lithium ions migrate back and forth during charge and discharge.

2. When the battery is charged, lithium ions migrate from the surface of the LiFePO4 crystal to the crystal surface. Under the action of the electric field force, it enters the electrolyte, passes through the separator, and then migrates to the surface of the graphite crystal through the electrolyte, and then embeds the graphite crystal. Grid. At the same time, electrons flow through the conductor to the aluminum foil collector of the positive electrode, and then to the copper foil current collector of the negative electrode through the tab, battery pole, external circuit, negative pole, and negative electrode, and then flow through the conductor to the graphite negative electrode. The negative charge is in equilibrium. After lithium ions are deintercalated from LiFePO4, LiFePO4 is converted into iron phosphate.

3. When the battery is discharged, lithium ions are de-intercalated from the graphite crystal, enter the electrolyte, pass through the separator, and then migrate to the surface of the LiFePO4 crystal through the electrolyte, and then re-embedded into the crystal lattice of LiFePO4. . At the same time, the battery flows through the conductor to the copper foil collector of the negative electrode, flows to the aluminum foil current collector of the battery through the tab, the battery negative pole, the external circuit, the positive pole, and the positive pole, and then flows to the iron phosphate through the conductor. Lithium positive electrode to balance the charge of the positive electrode.

Note: Once again remind the LiFePO4 battery to pay attention to waterproof and dustproof when used, so there must be no water in the storage place, which will affect the battery performance and service life.

Correct charging method of LiFePO4 battery pack

It is recommended to use the CCCV charging method for charging the LiFePO4 battery pack, that is, constant current first and then constant voltage. Constant current is recommended to be 0.3C. Constant voltage recommendation 3.65. That is, 0.3C current charging during constant current. When the battery voltage reaches 3.65V, use 3.65V constant voltage charging. Stop charging when the charging current is lower than 0.1C (or 0.05C), that is, the battery has been charged. full. When you charge with a constant voltage power supply, you should also look at the charging current. It is recommended not to charge with too high voltage. After adjusting the voltage, ensure that the charging current is below 0.5C, which is good for the battery.

Generally, the upper limit voltage of LiFePO4 battery is 3.7 ~ 4V, and the lower limit voltage of discharge is 2 ~ 2.5V. Considering the five aspects of discharge capacity, median discharge voltage, charging time, percentage of constant current capacity, and safety, constant current and constant voltage are used. For the charging solution of LiFePO4 battery pack, it is reasonable to set the charging limit voltage at 3.55 ~ 3.70V, the recommended value is 3.60 ~ 3.65V, and the lower limit discharge voltage is 2.2V ~ 2.5V.

The charger of the LiFePO4 battery pack is different from ordinary lithium batteries. The maximum termination charge voltage of a lithium battery is 4.2 volts; the LiFePO4 battery pack is 3.65 volts. When the LiFePO4 battery pack is charged, it is a balanced cable connected to the charging board. Generally, the battery is directly charged in series from both ends, and the charger voltage is greater than the battery pack voltage. The cable detects the voltage of each single cell, which is equivalent to connecting a voltage regulator tube in parallel, and the charging voltage of the single cell does not exceed the regulated value, while the other battery cells continue to charge by bypassing the voltage regulator tube.

Because the power of each cell is almost full at this time, it is only balancing each cell, so the charging current is small, and each cell is fully balanced. The charger can only protect the terminal voltage of the entire battery pack. The balanced charging board ensures that each cell is overcharged and that each cell is fully charged. It cannot stop charging the entire lithium battery pack because one cell is full.

Charging method of LiFePO4 battery pack

(1) Constant voltage charging method: During the charging process, the output voltage of the charging power source remains constant. With the change of the state of charge of the LiFePO4 battery pack, the charging current is automatically adjusted. If the specified constant voltage value is appropriate, it can not only ensure the full charging of the power battery, but also minimize outgassing and water loss. This charging method only considers the change of the single state of the battery voltage and cannot effectively reflect the overall charging status of the battery. Its initial charging current is too large, which often causes damage to the power battery. In view of this disadvantage, constant voltage charging is rarely used.

(2) Constant current charging method: During the entire charging process, the charging current is kept constant by adjusting the output voltage. Keeping the charging current constant, its charging rate is relatively low. The constant current charging control method is simple, but because the acceptable current capacity of the lithium battery pack gradually decreases with the progress of the charging process, in the later stage of charging, the power receiving capacity of the power battery decreases, and the utilization rate of the charging current is greatly reduced. The advantages of this method are simple operation, convenience, easy implementation, and easy calculation of the charge capacity.

(3) Constant current and constant voltage charging method: This charging method is a simple combination of the above two. The first stage uses a constant current charging method to avoid excessive charging current at the beginning of constant voltage charging. The second stage uses a constant voltage charging method to avoid overcharging caused by constant current charging. Like any other sealed rechargeable battery, the LiFePO4 battery pack must be controlled to prevent overcharging, otherwise it will easily damage the battery. LiFePO4 battery generally adopts the charging method of constant current first and voltage limiting.

(4) Chopper charging method: Charging is used for charging. In this method, the current of the constant current source does not change, and the switch tube controls it to turn on for a period of time and then turn off for a period of time. This method has the advantage that when the battery is charged through an external circuit, The ion generation inside the battery requires a certain response time. If it is continuously charged, it may reduce its potential for capacity. And after charging for a period of time, adding a turn-off time can allow the ions generated by the battery poles to have a diffusion process, so that the battery has a "digestion" time, which will greatly increase the battery utilization and improve Charging effect.

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