Charging and discharging characteristics of power battery pack
In the battery pack of the power battery, the difference between the monomers always exists. Taking the capacity as an example, the difference will not tend to disappear, but gradually deteriorate. The same current flows in the battery pack. Relatively speaking, the one with large capacity is always at low current, shallow charge and shallow discharge, which tends to slow capacity decay and life extension, while the one with small capacity is always at high current, overcharge and overdischarge. Due to the acceleration of capacity decay and shortening of service life, the difference in performance parameters between the two is getting larger and larger, forming a positive feedback characteristic, and the small capacity fails early and the life of the power battery pack is shortened.
1.Charging
At present, the charging of power battery packs mainly adopts the voltage-limiting current-limiting method. The initial constant current (CC) charging has the strongest battery acceptance capacity, mainly endothermic reaction. However, when the temperature is too low, the material activity decreases and may enter the constant current Therefore, in the cold winter in the north, warming up the battery before charging can improve the charging effect. As the charging process continues, the polarization is strengthened and the temperature rises intensified. With gassing, the electrode over-potential increases and the voltage rises. When the charge reaches 70%~80%, the voltage reaches the maximum charging limit voltage, and it switches to constant voltage. (CV) stage. Theoretically, there is no objective overcharge voltage threshold. If it is understood as gas evolution and temperature increase, it means overcharge. At the end of the constant current phase, different degrees of overcharge always occur, and the temperature rise reaches 40℃~50℃. Body deformation is easy to sense, and some of the escaped gases can be recombined, while others are as a result of irreversible reactions and lose capacity. This can be regarded as the current exceeding the battery’s ability to accept. In the constant voltage stage, also known as trickle charging, it takes about 30% of the time to charge 10% of the electricity, the current decreases, the gas evolution, the temperature rise no longer increases, and changes in the opposite direction.
2.Overcharge
The above charging process considers the total voltage or average voltage control of the power battery pack. In fact, there are always those with higher cell voltages, which have entered the overcharge stage relative to other batteries in the power battery pack. When overcharging occurs in the constant current stage, the voltage, temperature rise, and internal pressure continue to rise due to the large current. Take a 4V lithium ion battery as an example. When the voltage reaches 4.5V, the temperature rises by 40℃ and the plastic case becomes hard. , The temperature rise can reach 60℃ at 4.6V, and the shell deformation is obvious and cannot be recovered. If the overcharge continues, the air valve opens, the temperature rise continues to rise, and the irreversible reaction intensifies.
In the constant voltage stage, the current is small, and the overcharge symptoms are not as pronounced as in the constant current stage. As long as the temperature rises and the internal pressure is too high, the battery capacity will be reduced with side reactions, and the side reactions have inertia and develop to a certain extent, which may burn the contents of the battery during charging or in a short period of time after the end of charging. , Causing the battery to be scrapped, overcharging will attenuate the capacity of the accelerator battery, resulting in the failure of the Xiao battery.
3.Discharge
When the power battery pack is discharged at a constant current, the voltage drops abruptly, which is mainly caused by the ohmic resistance. The resistance includes the resistance of the wire connecting the single electrode and the resistance of the contact. The voltage continues to drop. After a period of time, it reaches a new electrochemical balance and enters the discharge plateau period. The voltage change is not obvious. The battery has a high temperature rise. The discharge voltage curve is similar to the single discharge curve, continuous discharge, the voltage curve enters the ponytail declining stage, the polarization impedance increases, the output efficiency decreases, and the heat consumption increases, and the discharge stops when it is close to the termination voltage.
4.Over discharge
The voltage of the single battery in the power battery pack is close to the ponytail curve at the later stage of discharge. The capacity of the single battery in the power battery pack is distributed normally, and the voltage distribution is very complicated. The voltage drop of the single battery with the smallest capacity is the earliest and fastest. At this time, if the voltage drop of other single batteries is not obvious, the voltage drop of the small-capacity single-cell battery is covered up, and the single-cell battery with a smaller capacity has been over-discharged at this time.
After the single battery enters the horsetail curve, if the current continues to be large, the voltage drops rapidly, and quickly reverses, the battery is charged in the opposite direction, or passively discharged, and the active material structure is destroyed. After a period of time, the battery active material approaches All loss is equivalent to a passive resistance, the voltage is negative, and the value is equal to the voltage drop generated by the reverse charging current on the equivalent resistance. After the discharge is stopped, the original battery electromotive force disappears and the voltage cannot be restored. Therefore, a reverse charge is enough to make the battery useless. The over-discharge of the single battery in the power battery pack is prone to occur and is not easy to control. The voltage and current limiting methods of the motor controller are not effective. The ohmic and polarized voltage fluctuations caused by the change of battery output power are sufficient to drown the single voltage drop signal. Make the voltage monitoring of the power battery group meaningless.
5.Economic speed and driving range
Traditional cars consume the most fuel when traveling at economic speeds. When evaluating fuel consumption per hundred kilometers, economic speed is determined by engine efficiency, power transmission efficiency and friction. Electric cars also have economic speed, which is determined by the efficiency of the power battery pack, the efficiency of the electric motor and the controller, and the frictional resistance. The economic speed is directly related to the internal resistance of the power battery pack and changes within a certain range. Driving at economical speeds, electric cars can reach the maximum driving range. The driving range can be used to investigate the energy supply capacity of the power battery pack. The economic speed reflects the power supply capability of the power battery pack. Electric cars hope that the power battery pack can provide large capacity and high power.
6.Accelerate and climb
Electric cars have high output power when accelerating and climbing, the power battery pack has a large discharge current, and the voltage drop is also large, the output efficiency decreases, and the ohmic loss increases. On the other hand, the voltage drop will also cause the motor efficiency to decrease and the working conditions are bad. , Over-discharge may occur, that is, the current output capacity of the power battery pack is exceeded, and the power battery pack is in overload use. The measures to avoid overloading are:
①Use a power battery pack with higher power;
②Limiting voltage, current, power or a combination of limiting methods to limit the output power of the power battery pack;
③ Stable driving, limiting the acceleration of electric cars.
7.Braking and inverter
As long as the acceleration is negative, the transmission mechanism can drive the generator to generate electricity, and the feedback electric energy can charge the power battery pack, convert mechanical kinetic energy into chemical energy and store it. The instantaneous inverter power and output power are of the same order of magnitude, depending on the generator reverse Variable efficiency, over-intensity discharge during acceleration, over-intensity charge during inverter.
Overcharge and overdischarge are fatal to the power battery pack. The only difference is that overcharging produces a large amount of gas, which is easy to spontaneously ignite and explode. The appearance is severe, and the appearance of overdischarge changes slowly, but the failure rate is extremely fast. It should be strictly avoided in the normal use of power battery packs.
The performance parameters of the same raw material and the same batch of battery cells, such as capacity, internal resistance, and life span, conform to the normal distribution and the degree of dispersion is limited. Under the same current excitation conditions, the consistency of the voltage change process of the single battery gradually approaches others. The consistency of performance parameters, of which the most important parameter is the degree of charge. The battery is not easy to fail prematurely during its life span that has not been overcharged or overdischarged. It can be inferred that if the cell voltage in the power battery pack is balanced by the method of energy conversion during the charging and discharging process, If the voltage of the single battery tends to be the same, the relative charge degree of the single battery also tends to be the same. It can be fully charged and discharged at the same time, and the life of the power battery pack should be close to the average life of the single battery.