1. Charging and discharging principle of LiFePO4 battery
The positive and negative electrode materials of LiFePO4 battery are ferrous phosphate and graphite respectively, and the salt bridge in the middle uses a thinner aluminium foil. In the charge/discharge cycle, lithium ions are exchanged through the polymer film between the positive and negative electrolyte.
During charging, lithium ions originally located on the surface of the LiFePO4 crystals of the positive electrode material detach from the crystal surface by absorbing energy provided by the power source, and the crystals are converted to iron phosphate and migrate to the graphite of the negative electrode material through the film. At the same time, the electrons originally located in the positive electrolyte also move to the negative electrode through the electrolyte, which is used to balance the charge around the positive and negative electrodes.
The discharge process of LiFePO4 battery is opposite to its charging process. Lithium ions detach from the surface of the negative electrode material and return to the surface of the iron phosphate crystal through the intermediate diaphragm, re-embedding and entering to form LiFePO4 crystals. At the same time, the electrons return to the positive electrode.
2. What methods can we use to charge the LiFePO4 batteries
2.1 Use solar panels for battery charging
Solar power generation as one of the most green and environmentally friendly way of generating electricity, it can ensure a certain amount of power generation on the basis of reducing emissions. It is feasible to use solar panels to generate electricity to charge LiFePO4 batteries.
In order to prevent overcharging and current backflow damage to the solar panel, it can be equipped with a solar charge controller. The use of solar panel charging can ensure that the user in the off-grid situation for normal charging of LiFePO4 batteries, very suitable for household energy storage in remote areas.
2.2 Charging with AC charger
In order to achieve the use of AC power charging, you can install the use of inverters to convert DC power for charging, at the same time, can be coupled with an AC charger due to charging the battery through the generator and the total power grid. The use of AC power supply for LiFePO4 battery charging has versatility and reliability. It is suitable for grid-connected and standby power systems.
2.3 Charging using an alternator and DC-DC converter
General alternators can also be used to charge LiFePO4 batteries, but because of the small internal resistance of LiFePO4 batteries, charging may occur when the charging current is too high and the generator overheats. Therefore, a DC-DC converter needs to be installed to limit the charging current as a way to avoid the risk of combustion.
An alternator is a generator that converts mechanical energy into electrical energy and generally charges the starter battery that starts the car’s generator. So this type of charging is usually done in by connecting to the mobile vehicle, this method can ensure a stable supply of power in the car.
2.4 Charging with smart chargers
Some intelligent chargers can be adjusted according to the voltage curve of the LiFePO4 battery charging process to maximize the performance and service life of the battery.
3. What are the theoretical charging methods of LiFePO4 battery
3.1 Constant voltage charging method
Also known as parallel charging method. That is, in the battery charging process, always keep the charging voltage constant, and reduce the charging current appropriately when the charging process gradually proceeds to the end. This charging method can ensure that the power battery is fully charged while reducing gas and water outflow during the charging process.
It is worth noting that when using the constant voltage charging method, it is necessary to ensure that the charging current is set at the appropriate threshold. Generally speaking, it should be lower than the maximum charging current of a single battery. When the charging current is greater than this value, it is not possible to ensure that all batteries are fully charged at the same time.
3.2 Constant current charging method
Also known as series charging method. That is, in the charging process, constantly adjust the charging voltage to maintain a constant current. This charging method is simple and easy to realize. When using this charging method, it is necessary to pay attention to balancing each battery at regular intervals.
Secondly, the size of the acceptable current when charging LiFePO4 batteries will decrease with the increase of the charging progress, so there will be a decrease in current utilization in the later stages of charging using the constant current charging method.
3.3 Constant voltage and constant current charging method
Constant voltage and constant current charging method is also known as CCCV charging method, i.e., the constant current charging method is used in the early stage of the battery charging process and the constant voltage charging method is used in the later stage.
This charging method uses constant current charging in the early stage to avoid excessive charging current caused by constant voltage charging, and uses constant voltage charging in the later stage to avoid overcharging caused by constant current charging.
3.4 Chopper Charging Method
This charging method will keep the current constant while constantly switching the charging power supply during the charging process to achieve the purpose of gap charging. About the principle of chopper charging method. When the external current charges the LiFePO4 battery, the detachment and embedding of the particles on the internal crystal of the battery, the movement of lithium ions and electrons, and the chemical reaction takes a certain amount of time to take place.
Therefore, the use of chopper charging method can give the LiFePO4 battery reaction time in the charging gap, improve the charging effect of the battery, to avoid the appearance of battery capacity reduction.
4. In charging LiFePO4 batteries need to pay attention to what problems
4.1 What is a large number of charging and float charging? Does LiFePO4 batteries need to switch to the float state?
Battery in the charging process, the charger will have two different forms of performance, respectively, a large number of charging stage and float charging stage.
First, when charging by CCCV, the charger will steadily increase the charging voltage in the first half to keep providing the maximum charging current to the battery. After reaching a large voltage, the charger will switch to apply a constant voltage, at this time this voltage is the absorption voltage of the battery, when the battery’s in the break in the potential difference between the two poles, also known as the open-circuit voltage, and the absorption voltage is close to the charging current issued by the charger will be gradually reduced to zero, and finally complete the charging. This charging stage is called the bulk charging stage.
However, if the charger is disconnected at this time, the LiFePO4 battery will begin to discharge outward on its own. In order to prevent this discharge loss, the charger will enter the float stage, that is, the charger always maintains a lower voltage to the battery to ensure that the battery is fully charged.
Generally speaking, lead-acid batteries have higher self-discharge rates and need to be switched to the float stage at the end of charging, while the float state of Li-FePO4 batteries is not common, but if your charger is connected to an electrical appliance while charging, and at this time, you need to use a rechargeable battery to supply power to the electrical appliance instead of Li-FePO4 batteries, you can achieve this by setting the float state to maintain the batteries in a specific state of charge.
4.2 Charging temperature
LiFePO4 battery charging temperature range is about zero degrees Celsius to forty degrees Celsius. Thanks to the BMS system installed inside the battery, it can protect the battery from damage caused by the ambient temperature during charging. When the battery overheats during the charging process, the BMS will actively disconnect the charging connection and wait for the battery to cool down before reconnecting it for charging. When the battery is too cold during the charging process, the BMS can control the battery to warm up before reconnecting.
4.3 What is a balanced charging plate, and is it worthwhile to configure a balanced charging plate for LiFePO4 battery charging?
Generally speaking, Li-FePO4 battery packs are connected in series when charging, and connected to a balance charging board for overall charging. The role of the balance charging board is to use its wires to detect the voltage of each battery during charging, to keep the charging voltage of a single battery will not exceed the set threshold, so as to ensure the normal charging of each battery as well as the synchronized charging of the whole battery pack.
