The full name of lithium iron phosphate 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. And do you all know the charging skills of lithium iron phosphate? The following will introduce you to the charging skills of lithium iron phosphate batteries.
1. First of all, we need to understand the structure and working principle of lithium iron phosphate batteries. LiFePO4 is used as the positive electrode of the battery. It is connected to the positive electrode of the battery by aluminum foil. In the middle is a polymer separator, which separates the positive electrode from the negative electrode, but lithium ions can pass through and electrons cannot pass. 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 a lithium iron phosphate battery is charged, 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 move back and forth during charge and discharge.
2. When the battery is charged, lithium ions migrate from the inside of the lithium iron phosphate crystal to the surface of the crystal. Under the action of the electric field, 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 tab, battery pole, external circuit, negative pole, and negative lug, and then flow to the graphite negative electrode through the conductor. The charge on the negative electrode is balanced. After the lithium ions are deintercalated from the lithium iron phosphate, the lithium iron phosphate is converted into iron phosphate.
3. When the battery is discharged, lithium ions are extracted 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-embed into the lithium iron phosphate crystal lattice 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 battery positive electrode through the tab, battery negative pole, external circuit, positive pole, and positive pole tab, and then flows to the iron phosphate through the conductor. Lithium cathode balances the charge of the anode.
Correct charging method for lithium iron phosphate battery pack
It is recommended to use the CCCV charging method for charging the lithium iron phosphate battery pack, that is, first constant current and then constant voltage. The recommended constant current is 0.3C. Constant voltage is recommended to be 3.65V. That is, 0.3C current is charged during the constant current process. When the battery voltage reaches 3.65V, the constant voltage charge is 3.65V. When the charging current is lower than 0.1C (or 0.05C), the charging stops, that is, the battery Already full. When you charge with a constant voltage power supply, you also need to look at the charging current. It is recommended not to charge with a 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 lithium iron phosphate battery charge is 3.7~4V, and the lower limit voltage of discharge is 2~2.5V. The five aspects of discharge capacity, median discharge voltage, charging time, constant current capacity percentage, and safety are considered. Constant current and constant voltage are adopted. For lithium iron phosphate battery packs, 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 voltage of discharge is 2.2V～2.5V.
The charger of lithium iron phosphate battery pack is different from ordinary lithium battery. The highest terminal charging voltage for lithium batteries is 4.2 volts; the lithium iron phosphate battery pack is 3.65 volts. When charging the lithium iron phosphate battery pack, it is the cable connected to the balance charging board. Generally, the whole charge is directly connected in series from both ends. The charger voltage is greater than the battery pack voltage. The cable detection of the voltage of each single cell is equivalent to paralleling a zener tube, the charging voltage of the single cell will not exceed the voltage stabilization value, and other single cells continue to charge through the zener tube bypass charging.
Because the power of each cell is nearly full at this time, it is just balancing each cell, so the charging current is small, and each cell is fully charged to supplement the balance. The charger can only protect the terminal voltage of the entire battery pack. The balance charging board ensures that each cell is not overcharged and each cell is fully charged. It cannot stop the entire lithium battery pack from charging because one cell is fully charged.
Charging method of lithium iron phosphate 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 lithium iron phosphate battery pack, the charging current is automatically adjusted. If the prescribed constant voltage value is suitable, it can not only ensure the full charge of the power battery, but also minimize gassing and water loss. This charging method only considers the change in a single state of the battery voltage, and cannot effectively reflect the overall charging status of the battery. Its initial charging current is too large, often causing 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. Keep 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 the charging, the power battery receiving capacity decreases, and the charging current utilization rate is greatly reduced. The advantages of this method are simple, 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. In the first stage, the constant current charging method is used to avoid excessive charging current at the beginning of constant voltage charging. The second stage adopts the constant voltage charging method to avoid the phenomenon of overcharging during constant current charging. Lithium iron phosphate battery packs are the same as any other sealed rechargeable batteries. The charging must be controlled and not overcharged, otherwise the battery will be easily damaged. Lithium iron phosphate batteries generally adopt a charging method of constant current first and then voltage limiting.
4) Chopping wave 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 to turn it on for a period of time and then turn off for a period of time, cyclically. 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. If it is continuously charged, its capacity potential may be reduced. After charging for a period of time, adding a turn-off time can allow the ions produced at the two poles of the battery to have a diffusion process, giving the battery a time to "digest", which will greatly increase the utilization rate of the battery and improve charging effect.