How To Charge Lithium Iron Phosphate Battery Correctly?

- May 08, 2020-

The full name of lithium iron phosphate battery is lithium iron phosphate ion battery. Since 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 call it a "lithium-iron power battery," and do you know how to charge lithium iron phosphate? The following is an introduction to the charging skills of lithium iron phosphate batteries.




1.Firstly,we need to understand the structure and working principle of lithium iron phosphate batteries LiFePO4 as the positive pole of the battery, connected to the battery positive electrode by aluminum foil and polymer is among the diaphragm, which separates the positive and negative, but cannot by lithium ionic and electronic, right is composed of carbon (graphite) battery cathode, the copper foil and negative connection of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, which is sealed in a metal case. During charging, the lithium ions in the positive electrode migrate to the negative electrode through the polymer membrane. During the discharge process, the lithium ions in the negative electrode migrate to the positive electrode through the diaphragm. Lithium-ion batteries are named after the fact that lithium ions migrate back and forth between charges and discharges.




2. When the battery is charged, lithium ions migrate from the surface of the lithium iron phosphate crystal to the crystal surface. Under the action of electric field forces, they enter the electrolyte, pass through the diaphragm, and then migrate to the surface of the graphite crystal through the electrolyte, and then embed into the graphite lattice. At the same time, the electrons flow to the positive aluminum foil collector through the conductive body, through the electrode ear, the battery pole column, the external circuit, the negative pole column, the negative electrode ear flow to the negative copper foil collection fluid, and then through the conductive body flow to the graphite negative electrode, so that the negative electrode charge to balance. After the lithium ion is deembedded from the lithium iron phosphate, the lithium iron phosphate is converted to iron phosphate.




3. When the battery is discharged, the lithium ions detonate 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-insert into the crystal lattice of lithium iron phosphate. At the same time, the battery flows to the copper foil collector through the conductive body, through the electrode ear, the battery negative column, the external circuit, the positive pole column, the positive pole ear to the battery positive aluminum foil collection fluid, and then through the conductive body flow to the lithium iron phosphate positive electrode, so that the positive electrode charge to balance.




Note: once again remind the lithium iron phosphate battery in use to pay attention to water and dust, so the storage place can not have water, this will affect the battery performance and service life.


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Correct charging method for lithium iron phosphate battery pack




The charging method of CCCV is recommended for the charging of lithium iron phosphate battery pack, that is, constant current before constant voltage. Constant current is recommended at 0.3c. The constant voltage is suggested to be 3.65. That is, 0.3C current is charged in the constant current process. When the battery voltage reaches 3.65V, the battery is charged at a constant voltage of 3.65V. When you use the balance voltage power when charging, also want to see the charging current, it is recommended not to use too high voltage charge, adjust the voltage, to ensure that the charging current is below 0.5C, so good for the battery.




General upper limit of the lithium iron phosphate battery voltage 3.7 ~ 4 V, discharge threshold voltage of 2 ~ 2.5 V, comprehensive consideration of discharge capacity, discharge voltage, charging time, constant current capacity percentage, security, the five aspects, using constant current constant voltage charging scheme, for the lithium iron phosphate battery pack, charge a reasonable limit set the voltage from 3.55 V to 3.70 V, the recommended value of 3.60 ~ 3.65 V, discharge threshold voltage of 2.2 V to 2.5 V.




The charger of lithium iron phosphate battery pack is different from ordinary lithium battery. The maximum terminal charging voltage of a lithium battery is 4.2 volts. The lithium iron phosphate battery pack is 3.65 volts. When charging iron phosphate lithium battery pack, is the balance of the charging plate connected to the line, generally from the two ends of the direct series overall charging, charger voltage is greater than the battery pack voltage. The voltage of each single cell is detected by the wiring, which is equivalent to parallel voltage regulator tubes. The charging voltage of the single cell will not exceed the voltage regulator value, while other single batteries will continue to be charged by the bypass of the voltage regulator tube.




Because at this time, the charge of each monomer is nearly full, just balancing each monomer, so the charging current is small, and each cell is full of supplementary balance. The charger can only protect the terminal voltage of the entire battery pack, and the balanced charging board can ensure that every single cell is overcharged and every single cell is full, and the charging of the entire lithium battery pack cannot be stopped because of a full battery cell.




Charging method of lithium iron phosphate battery pack




(1) constant voltage charging method: in the charging process, the output voltage of the charging power supply remains constant. With the change of the charging state of the lithium iron phosphate battery pack, the charging current is automatically adjusted. If the specified voltage constant value is appropriate, it can not only guarantee the complete charging of the power battery, but also reduce the gas and water loss as far as possible. This charging method only considers the change of the battery voltage, which is a single state, and cannot effectively reflect the overall charging status of the battery. Its initial charge current is too large, often resulting in power battery damage. In view of this shortcoming, constant voltage charging is rarely used.




(2) constant current charging method: during the whole charging process, the charging current is kept constant by adjusting the output voltage. Keeping the charging current unchanged, the charging rate is relatively low. The constant current charging control method is simple, but because the acceptable current capacity of lithium battery pack decreases gradually with the charging process, in the later period of charging, the power battery's current capacity decreases and the utilization rate of charging current is greatly reduced. The advantages of this method are simple to operate, convenient, easy to implement, easy to calculate the charge.




(3) constant current and constant voltage charging method: this charging method is a simple combination of the above two. In the first stage, constant current charging method is adopted to avoid excessive charging current at the beginning of constant voltage charging. In the second stage, constant voltage charging method is adopted to avoid the phenomenon of overcharging caused by constant current charging. Lithium iron phosphate battery pack like any other sealed rechargeable battery, to charge control, do not overcharge, otherwise easy to damage the battery. Lithium iron phosphate batteries generally adopt the charging method of constant current before voltage limitation.




(4) chopper charging method: the chopper method is adopted for charging. Under this method, the constant current source current, and switch tube to control, to make it open again after a period of time off for a period of time, cycle, the advantage of this approach is that when through an external circuit for the battery, the battery inside the ion needs to have certain response time, if continued to charge of it, may reduce the potential of its capacity. After charging for a period of time, adding a turn-off time can make the ions generated by the battery poles 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 charging effect.