The full name of LiFePO4 Battery is lithium iron phosphate lithium ion battery. Because its performance is particularly suitable for power applications, the word “power” is added to the name, that is, lithium iron phosphate power battery. Some people also call it “lithium iron power battery”, and do you know the charging skills of lithium iron phosphate? The following will introduce you to the charging skills of lithium iron phosphate batteries.
The structure and working principle of LiFePO4 Battery
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, which is connected to the positive electrode of the battery by aluminum foil. Lithium ions can pass through but electrons cannot. On the right is the negative electrode of the battery composed of carbon (graphite), which is connected to the negative electrode of the battery by copper foil. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal casing. When the LiFePO4 Battery is charging, the lithium ions in the positive electrode migrate to the negative electrode through the polymer separator; during the discharge process, the lithium ions in the negative electrode migrate to the positive electrode through the separator. Lithium-ion batteries are named after lithium ions migrate back and forth during charging and discharging.
2. When the battery is charging, lithium ions migrate from the surface of the lithium iron phosphate crystal to the surface of the crystal. Under the action of the electric field force, they enter the electrolyte, pass through the diaphragm, and then migrate to the surface of the graphite crystal through the electrolyte, and then embed the graphite crystal. grid. At the same time, electrons flow to the aluminum foil collector of the positive electrode through the conductor, flow to the copper foil collector of the negative electrode through the ear, battery pole, external circuit, negative pole, and negative ear, and then flow to the graphite negative electrode through the conductor, so that The charge on the negative electrode is balanced. After lithium ions are deintercalated from lithium iron phosphate, lithium iron phosphate is converted into iron phosphate.
3. When the battery is discharged, lithium ions are deintercalated from the graphite crystal, enter the electrolyte, pass through the diaphragm, and then migrate to the surface of the lithium iron phosphate crystal through the electrolyte, and then re-intercalate into the lattice of lithium iron phosphate through the surface . At the same time, the battery flows to the copper foil collector of the negative electrode through the conductor, flows to the aluminum foil collector of the positive electrode through the tab, the negative pole of the battery, the external circuit, the positive pole, and the positive tab, and then flows to the iron phosphate through the conductor. The lithium positive electrode balances the charge of the positive electrode.
Precautions: Once again remind the LiFePO4 Battery to pay attention to waterproof and dustproof when using it, so there should be no water in the storage place, which will affect the performance and service life of the battery.
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 recommended 0.3C. Constant voltage recommendation 3.65. That is, 0.3C current charging during the constant current process. When the battery voltage reaches 3.65V, use 3.65V voltage constant voltage charging. When the charging current is lower than 0.1C (or 0.05C), stop charging, that is, the battery has been charged. full. When you use a constant voltage power supply to charge, it also depends on the charging current. It is recommended not to charge with too high a voltage. After adjusting the voltage, ensure that the charging current is below 0.5C, which is good for the battery.
Generally, the charging upper limit voltage of LiFePO4 Battery is 3.7~4V, and the discharging lower limit voltage is 2~2.5V. Considering the five aspects of discharge capacity, discharge median voltage, charging time, constant current capacity percentage, and safety, the constant current and constant voltage are adopted. For the LiFePO4 Battery pack, it is more reasonable to set the charging limit voltage at 3.55~3.70V, the recommended value is 3.60~3.65V, and the discharge lower limit voltage is 2.2V~2.5V.
The charger of LiFePO4 Battery pack is different from ordinary lithium battery. The highest termination charging voltage of lithium battery is 4.2 volts; LiFePO4 Battery pack is 3.65 volts. When the LiFePO4 Battery pack is charged, it is connected to the flat cable of the balance charging board. Generally, it is directly connected in series from both ends for overall charging, and the voltage of the charger is greater than the voltage of the battery pack. The cable detects the voltage of each single cell, which is equivalent to connecting a voltage regulator tube in parallel. The charging voltage of a single cell will not exceed the voltage regulation value, while other single cells continue to charge through the voltage regulator tube bypass charge.
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 battery cell is fully charged. The charger can only protect the terminal voltage of the entire battery pack. The balance charging board is to ensure that each cell is overcharged and 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 supply 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 charge of the power battery, but also minimize gas evolution 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 shortcoming, 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, the 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 advantage of this method is that it is easy to operate, convenient, easy to implement, and easy to calculate the charging power.
(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 adopts the constant voltage charging method, which avoids the phenomenon of overcharging caused by constant current charging. LiFePO4 Battery pack is the same as any other sealed rechargeable battery, the charging should be controlled, and the battery should not be overcharged, otherwise the battery will be easily damaged. Lithium iron phosphate batteries generally adopt the charging method of constant current first and then voltage limiting.
(4) Chopper charging method: use the chopping method to charge. In this method, the current of the constant current source remains unchanged, and the switch tube is controlled so that it is turned on for a period of time and then turned off for a period of time, and the cycle repeats. The advantage of this method is that when the battery is charged through an external circuit, The ion generation inside the battery requires a certain response time, and if it is continuously charged, it may reduce its capacity potential. After charging for a period of time, adding a turn-off time can allow the ions generated at the two poles of the battery to have a diffusion process, so that the battery has a “digestion” time, which will greatly increase the utilization rate of the battery and improve the efficiency of the battery. charging effect.