Charging requirements of power battery packs and segmented constant current control
1. Charging requirements for power battery pack
If the charging mode of the power battery pack adopts the two-stage charging mode of “current limiting and voltage limiting”, at the beginning of charging, generally the best charging rate (0.3CA for lithium-ion batteries) is used for current-limiting charging. At this stage, due to the low electromotive force of the battery, even if the battery charging voltage is not high, the charging current of the battery will be large, and the charging current must be limited. Therefore, charging at this stage is called “current-limiting” charging, that is, the charging current is kept at the current-limiting value. As the charging continues, the electromotive force of the battery continues to rise, and the charging voltage of the Zhai battery also continues to rise. When the battery voltage rises to the maximum allowable charging voltage, the constant voltage charging is maintained. At this stage, because the electromotive force of the battery is still rising, and the charging voltage remains the same, the charging current of the battery shows a hyperbolic trend and continues to drop until it drops to zero. But in the actual charging process, when the charging current is reduced to 0.015CA, the charging can be stopped. This stage of charging is called “constant voltage” charging, and the charging voltage U=E+I×R=constant voltage value.
The change curve of the charging voltage and current during the charging process of the power battery pack is shown in Figure 1. This is the basic requirement of the lithium-ion battery pack for the charging mode. In addition, the charging system must also have the functions of automatic adjustment of charging parameters, automatic control and automatic protection.
(1) Automatically adjust charging parameters. During the charging process, the charging system must be able to automatically adjust the charging parameters and automatically control the charging according to the status of the battery pack, the information output by the battery management system (BMS) and the information output by the car monitoring.
For example, during charging, especially in the constant voltage charging stage, if the charging voltage of a single cell in the lithium-ion battery pack exceeds the allowable charging voltage (according to the characteristics of different lithium-ion batteries, it is generally set to 3.9-4.3V ), the charging system should automatically reduce the charging voltage and current according to the signal output by the battery management system (BMS), so that the charging voltage of the lithium-ion battery does not exceed the allowable charging voltage, and preventing the lithium-ion battery from overvoltage charging.
For another example, at the beginning of charging, the battery management system (BMS) monitors that the voltage of a certain single battery is too low. The charging system should automatically reduce the initial charging current. After the voltage of the lithium-ion battery is normal, switch to Enter normal charging.
(2) Automatic control and automatic protection functions. During the charging process, when any abnormality occurs in the lithium-ion battery pack and electric car, such as short-circuit, open circuit, high temperature, fire and damage of the lithium-ion battery pack, the charging system should be able to quickly cut off the power supply and stop charging.
(3) Communicate with the entire car CAN bus. During the charging process, the charging system information can communicate with the entire CAN bus.
2. Constant current control of power battery pack charging section
In the process of charging the battery pack, increasing the charging current will increase the amount of active material recovered per unit time on the battery plate, and the charging time can be shortened, but excessive charging current will damage the battery. The acceptable charging current of the battery is limited and will decrease exponentially with the charging time. In the battery charging process, when the charging current curve is above the exponential function curve, it will cause the battery electrolyte to undergo a gassing reaction (overcharge). On the contrary, the charging time cannot be effectively shortened. The idealized battery fast charging process is that the charging current is always maintained at the limit value of the battery charging acceptable current, that is, the charging current curve coincides with the charging acceptable current curve of the battery.
1) Segmented constant current charging control scheme
To realize the automatic control of segmented constant current charging, the parameters for determining the termination of stage constant current charging can select charging time, battery temperature, and battery voltage. A large number of investigation and analysis and battery charging test results show that single parameter control method is difficult to achieve ideal segmented constant current charging control.
(1) The charging time parameter control method is simple, but under the conditions of different battery models, different charging starting states, and different required charging time, if the charging time is used to control the end of the constant current charging stage, it is easy to cause The battery is overcharged or the charging time is extended.
(2) The advantage of the temperature parameter control method is that the battery temperature is too high protection can be realized, but due to the influence of the environment and the response time delay of the sensor, if the battery temperature parameter is only used as the judgment criterion for the stage constant current charging termination, it is easy to cause the battery to overheat. Charge.
(3) Voltage parameter control is considered to be a better stage constant current charge termination control method, and its shortcomings are obvious, such as the inability to identify abnormal rises in the charging voltage caused by the vulcanization of the battery plates and abnormalities in the battery charging process Temperature rise, etc., will lead to prolonged battery charging time or battery damage.
In order to ensure that the actual charging state of the battery can be detected under various conditions, and to achieve a more ideal stepped charging current curve, the three parameters of charging time, battery temperature and termination voltage are used as the basis for determining the termination of constant current charging at each stage, and control The process is shown in Figure 2. In the figure, T is the battery temperature; T0 is the stop charging temperature; I0 is the minimum constant current charging current; t(n) is the set charging time of the nth constant current charging; I(n) is the set nth The current value of the second constant current charging; U(n) is the set termination voltage of the nth constant current charging. After the segmented constant current charging is completed, constant voltage charging is performed for a period of time to ensure that the battery is fully sufficient.
The specific control strategies for the 3 control parameters are as follows:
(1) Time parameter control. According to the battery capacity and charging current, preset a certain period of constant current charging time. When the charging time reaches the set value, a signal is sent through the timer to end the constant current charging at this stage and automatically reduce the charging current, and enter the next step. A period of constant current charging.
(2) Temperature parameter control. Set the highest value of the battery temperature when a certain period of constant current charging reaches the acceptable current limit, and control the charging device according to the battery temperature detected by the temperature sensor. When the external environment temperature is low and the maximum temperature of the battery set is high, the temperature rise control method is adopted. When the temperature rise of the battery reaches the set value, the thermostat stops the charging device until the temperature drops to an appropriate value. Automatically enter the next stage of constant current charging.
(3) Voltage parameter control. The absolute voltage of the battery can reflect the charging status of the battery. Set the voltage at which a certain period of constant current charging reaches or close to the acceptable current limit value of charging. When the voltage reaches the set value, the charging device automatically ends the constant current charging stage and enters The next stage.
2) Intelligent control scheme for segmented constant current charging
The intelligent control block diagram of segmented constant current charging is shown in Figure 3. The circuit adopts CPU control, can detect the charging battery and the temperature of the charging environment, time the battery charging, sample the voltage and current of the battery during the charging process, and control the segmented constant current charging process.
The intelligent control scheme for segmented constant current charging is as follows:
(1) The capacity gradient method is used to determine the stage constant current charging termination standard. The capacity gradient parameter dU/dC is used as the judgment standard for the stage constant current charging termination, and the charging termination capacity gradient parameter is determined according to the constant current charging characteristic curve of this type of battery. During the charging process, the controller samples the charging voltage at a set frequency , Calculate the capacity gradient value under I(n) and compare it with the set charge termination capacity gradient standard, and judge whether to terminate the current stage constant current charging according to the comparison result.
(2) Decrease the descending gradient of the constant current value of each section. The initial constant current value I(1) of this type of pedicle battery is determined through experiments, and the current drop rate of the stage constant current charging is reduced. If after reducing the charging current, the time to reach the end-of-charge capacity gradient value is very short (set a minimum charging time), then appropriately increase the magnitude of the current drop.
(3) Set the battery temperature as the charging safety guarantee control parameter. Set the maximum temperature limit value of the battery. During the charging process, if the battery temperature reaches the limit value, stop charging immediately. When the battery temperature drops to the normal temperature, appropriately reduce the charging current and continue charging until the end of the constant current charging.