Design of electric vehicle battery management system based on active equalization technology and transformer

In the new energy vehicle system, whether it is a hybrid (HEV) or an electric vehicle (EV), it is inseparable from the power battery as an energy storage medium. At present, lithium-ion batteries have occupied the leading position of automotive power batteries, in order to achieve longer battery life Mileage usually requires multiple batteries to be used in series/parallel to form a battery pack. Considering the energy, power and environmental requirements of the car, the safe and reliable use of large lithium-ion battery packs is definitely not a simple task. Therefore, it is necessary to adopt an appropriate battery management system to fully utilize the advantages of the new lithium battery.

1.1 The necessity of active balancing technology in electric vehicle battery management

1.1.1 Electric vehicle battery pack system architecture

Lithium batteries have strong energy storage capacity, but the voltage and current of a single battery are too low to meet the needs of hybrid motors. To increase the current, multiple batteries must be connected in parallel, and to obtain a higher voltage, multiple batteries must be connected in series. The voltage of a single lithium battery is generally between 3.3V and 3.6V. For example, connecting up to 12 batteries in series to form a battery block has an output voltage between 30V and 45V, while a hybrid electric vehicle requires a DC power supply voltage of about 336V, so 8-10 battery blocks are usually required. ) Connecting in series means that the battery pack of an electric vehicle is composed of a large number of single cells (above 100 cells).

Figure 1.1.1: Electric vehicle battery pack system architecture.

1.1.2 The need for balance

The single cells in the battery pack have different manufacturing and use conditions, and their characteristics are different. However, these differences, if they are not properly controlled during the charging and discharging process, will be further increased. Accumulation over time may significantly reduce the performance of the entire battery pack, leading to overcharging and overdischarging of some batteries, resulting in battery capacity. And the Sharp decline in life, reducing the mileage of the vehicle and even the damage of the battery pack. Statistically, the average value of the normal distribution of the capacity of a single battery cell in the battery pack shifts to the left, and the kurtosis gradually decreases, as shown in Figure 1.1.2. Shown. After a period of use, there will be a small portion of battery cells whose effective capacity is close to zero, leading to failure. Therefore, in order to improve the life of the entire battery pack, how to balance these rapidly aging battery cells is also an important issue for battery management system designers to consider.

1.1.3 Operating voltage range of the battery

Once the battery voltage exceeds the allowable range, the lithium battery is easily damaged (see Figure 1.1.3). If the upper and lower voltage limits are exceeded (for example, the upper and lower voltage limits of nanophosphate lithium batteries are 3.6V and 2V, respectively), the battery may be irreversibly damaged, and at least the self-discharge rate of the battery will increase. In a fairly wide range of state of charge, the output voltage can remain stable, so the possibility of exceeding the safe range under normal circumstances is relatively small. However, in the area close to the upper and lower limits of the safety range, the change curve is very steep. As a precaution, it is necessary to carefully monitor the voltage level.

When the battery voltage approaches the critical value, the discharging or charging must be stopped immediately. The function of the balance Circuit is to adjust the voltage of the corresponding battery to keep it in a safe area. In order to achieve this goal, when the voltage of any battery in the battery pack is different from that of the other batteries, energy must be transferred between the batteries.

1.2 Advantages of using transformer-based active equalization solutions

1.2.1 Passive equilibrium method

In a traditional passively balanced battery management system, each battery cell is connected to a load resistor through a switch. This passive circuit can discharge individual selected cells. This method is only suitable for suppressing the voltage rise of the strongest battery cell in the charging mode. The advantage of the passive equalization method is that the circuit structure is simple and the cost is lower. But its shortcomings are also obvious, it can only do charge equalization. At the same time, during the charge equalization process, the excess energy is released as heat, which makes the overall system low in efficiency and high in power consumption. In order to limit the power consumption in some occasions, the circuit generally only allows discharge with a small current of about 100mA, which leads to a charge balance that can take up to several hours.

1.2.2 Active balancing method based on transformer:

There are many active balancing methods in related materials, all of which require a storage element for energy transfer. If capacitors are used as storage elements, connecting them to all battery cells requires a huge array of switches.

A more effective method is to use an active balancing circuit based on inductance design. The key element is a transformer, whose function is to realize energy transfer between single cells. The circuit is constructed according to the principle of a flyback transformer. The following connections are made on both sides of the transformer:

a. The primary coil is connected to the entire battery pack

b. The secondary coil is connected to each battery cell

This solution can completely realize the real-time balance during charging and discharging, and give full play to the potential of each battery. Ensure that each battery can be fully charged when charging, and each battery can be discharged to the lowest limit when discharging, and each battery can maintain the same voltage during charging and discharging, so that the capacity of each battery in the battery pack can be maximized. Give full play to.

1.2.3 Advantages of using transformer-based active equalization scheme

1) The bottom balance can be achieved

Relatively passive balance, not only provides low top balance, but also can achieve bottom balance. When the voltage of a certain battery is too low, the energy of the battery can be transferred to the battery through the winding connected to the battery, which improves the system energy Utilization

2) High system efficiency and low loss

When the control system is not charging and discharging, the quiescent current is less than 2μA. The system equalization circuit is automatically turned on during charging or discharging, and the total power consumption of the control part is less than 1 W. The effective value of the equalizing current reaches more than 5 A, and the peak value reaches 20 A. According to the actual balanced power test during the discharging and charging process, the utilization efficiency of the transfer energy of this scheme has reached more than 85%. The remaining 15% of the energy, except for the supply circuit (microcontroller, power chip, etc.), only a small part is consumed in the transformer, MOSFET and internal resistance of the circuit.

Figure 1.2.3: Comparison of several different equalization methods.

2 Balance method

Using a flyback transformer as the core, through the conversion of the magnetic field and the electric field, the energy is transferred in both directions between a single battery cell and the entire battery pack. When the voltage of a battery is too high, the excess energy can be transferred to the entire battery pack through the windings connected to the battery. This process is called the top equalization method. When the voltage of a certain battery is too low, the energy of the battery can be transferred to the battery through the winding connected to the battery. This process is called the bottom equalization method.

2.1 Top equalization

If the voltage of a battery cell is higher than other cells, then the energy in it needs to be exported, which is especially necessary in the charging mode. If equalization is not carried out, the charging process will have to stop immediately after the first battery cell is fully charged. Equalization can keep the voltages of all battery cells equal and avoid prematurely stopping charging. Figure 2.1 shows the energy flow in the top balance mode. After the voltage scan, it is found that battery cell 5 is the cell with the highest voltage in the entire battery pack. At this time, switch sec5 is closed, and current flows from the battery to the transformer. After the switch sec5 is opened, the main switch is closed. At this time, the transformer enters the energy output mode from the energy storage mode. Energy is sent to the entire battery pack through the primary coil.

2.2 Bottom balance

The current and timing conditions in the bottom equalization method are very similar to the top equalization method, except that the sequence and the direction of the current are opposite to the top equalization method. The scan found that battery cell 2 is the weakest cell and must be recharged. At this time, the main switch “(prim”) is closed, and the battery pack starts to charge the transformer. After the main switch is disconnected, the energy stored in the transformer can be transferred to the selected battery cell. The corresponding secondary “(sec”) switch in this example is that after switch sec2 is closed, energy transfer begins. Especially when the voltage of a battery cell has reached the lower limit of the SoC, the bottom balance method can help extend the working time of the entire battery pack. As long as the current provided by the battery pack is lower than the average balance current, it can continue to discharge until the last battery cell is also depleted.

Figure 2.2: Principle of top equalization.

2. 3 Balance method between battery packs

As shown in Figure 2.3, closing the Electronic switches SP1 and SP2 of one of the battery packs can charge the primary of the leftmost winding, and then closing SP1 and SP2, you can put energy into the total battery pack. In this way, more battery cells can be connected in series.

2.4 Voltage detection

In order to manage the state of charge of each battery, the voltage of each battery must be measured. Since only the No. 1 battery is within the analog-to-digital conversion range of the microcontroller, it is not possible to directly measure the voltages of other batteries in the battery block. One possible solution is to use a differential amplifier array, but this requires maintaining the voltage level of the entire battery block.

The following proposes a method that can detect all battery voltages by adding a small amount of hardware. The main function of the transformer is charge balance, but at the same time we can also use it as a multiplexer. In the voltage detection mode, the flyback mode of the transformer is not used. When one of the S1 to SN switches is closed, the voltage of the connected battery is transmitted to all windings of the transformer. After a simple pre-processing with a discrete filter, the detection signal is input to the microcontroller ADC input pin. The duration of the detection pulse generated when any switch of S1 to SN is closed is very short, and the actual on-time may only be 4 μs, so the energy stored in the transformer is not much. When the switch is turned off, the energy stored in the magnetic field will be fed back to the entire battery block through the main transistor, so the energy of the battery block is not affected. After scanning all the batteries, one scanning cycle ends and the system returns to the initial state. It also reads the voltage signal of each battery in the battery pack.

3 Design plan

3.1 Hardware part

The battery management system uses an independent internal CAN bus for data and command transmission. Each internal CAN bus sub-node circuit is connected to a maximum of 12 battery cells in series to form a battery pack. Each battery pack is connected in series to form the battery assembly required by the electric vehicle. The main node uses the automotive-grade 16-bit single-chip XC2267 to connect to the internal CAN network, and at the same time connects to the public CAN network on the electric vehicle to send and receive related instructions and data. See Figure 3.1.1

Each child node can monitor the SOC of the 12 batteries in the battery pack and perform battery balancing functions.

The sub-node transformer adopts a design scheme of 1 primary coil and 12 secondary coils. The primary coil is connected in series with automotive-grade MOSFET (ie Sp1) to the positive and negative poles of the battery pack. Each secondary coil is connected in series with automotive-grade MOSFETs (ie S1, S2-Sn) to the positive and negative poles of each battery cell. This constitutes a two-way power supply in flyback mode for energy transfer.

The MOSFET (ie Sp1) that controls the primary coil uses IPD70N10S3L, with a withstand voltage of 100V and an Rdson of 11.5m? , It can work under the 30~60V voltage state generated by 12 batteries in series, meeting the application requirements of a variety of lithium batteries such as lithium iron phosphate, lithium manganate, and ternary material batteries. For lithium iron phosphate batteries with a lower voltage platform or when the number of cells is small, a MOSFET with a lower withstand voltage but a smaller Rdson can also be used to improve the system efficiency. If the withstand voltage is 75V and Rdson reaches 6.5 m? IPB100N08S2.

The MOSFET (ie S1, S2-Sn) that controls the secondary coil adopts IPG20N04S4L, the withstand voltage is 40V, and the Rdson is 7.6 m? , Which can meet the needs of controlling the energy transfer of a single battery. Another advantage of IPG20N04S4L is that it integrates two independent MOSFETs in a small package, saving more space for circuit board layout. In order to further improve efficiency, you can also choose smaller Rdson products, such as IPD90N03S4L, IPB180N03S4L, etc.

3.2 Software part

What controls the above-mentioned MOSFET to work is the automobile grade 8-bit single-chip XC886CM, this single-chip has 8-channel 10-bit AD, which can conveniently collect the voltage data and temperature data of each battery cell. The package of QFP-48 makes it have enough IO to complete the control work of up to 13 MOSFETs. Abundant communication interfaces such as CAN, SPI, UART, etc. enable the sub-nodes to communicate with the PC in addition to the CAN bus communication function, so as to facilitate the control and demonstration in the test phase. The hardware multiplication and division unit MDU can multiply and divide 16-bit data to achieve fast calculations.

The sub-node software is responsible for the realization of the functions shown in Figure 3.2.1 and executes the state machine.

4 Experimental data

The experiment uses 12 super capacitors (U0~U11) as the balance object. The maximum initial voltage of the tested capacitor is 2.131V (U6) and the minimum is 1.767V (U7). After about 130 seconds of active equalization, the voltages of all 12-cell supercapacitors tend to be concentrated, and the active equalization operation is stopped. The maximum initial voltage of the tested capacitor is 1.962V (U11) and the minimum is 1.939V (U2).

Figure 4.1: Supercapacitor active equalization and passive equalization test.

When passive equalization is used, the capacity of the capacitor is proportional to the voltage of the capacitor. When a fixed resistance is used for passive equalization, the voltage curve is close to a straight line with a fixed slope. The dotted line in the figure is the voltage curve when the passive equalization is simulated. After 18 minutes of passive equalization, the supercapacitor voltage tends to reach 1.76V uniformly.

5 System balance and improvement

Balance between transformer volume and equalization speed: In order to achieve a faster equalization effect, that is, a larger equalization current, a larger-volume transformer is used in this experiment. In the pursuit of a more compact system design, a smaller transformer can be used. But while reducing the volume, it reduces the energy transferred in each equalization and slows down the equalization speed. For some micro-hybrid and medium-hybrid vehicles with small battery capacity, the equalization speed can be reduced to obtain a smaller transformer volume.

The balance between transformer volume and system efficiency: If the switching frequency during active equalization is increased, a smaller-volume transformer can also be used. The problem is that the switching power consumption of the MOSFET is proportional to the switching frequency, and the system power consumption increases due to the increase of the switching frequency, resulting in a decrease in the system power. In a pure electric vehicle system, the battery capacity is relatively high, and the volume of the transformer can be reduced by sacrificing some switching losses to achieve a more compact system.

Improvement of the communication system: Since every 10-12 batteries in series are a sub-node, the internal CAN bus of the entire battery management system has as many as 10-20 nodes, and the reference ground level of each node is different, so an isolated CAN bus solution is required. Can communicate, the product cost is higher. If a dedicated serial bus solution is developed according to the specific application, the overall cost will be greatly reduced.

Improvement of battery voltage detection: This design adopts 10-bit ADC plus software correction method to sample voltage, and the accuracy can reach 5mV. But for systems with very flat voltage platforms such as lithium iron phosphate, the voltage detection accuracy needs to be further improved. If a 12~13 bit ADC and a more complete software correction scheme are used, the voltage accuracy can reach 2mV or even 1mV. This can meet the application needs of various lithium-ion batteries.

6 Conclusion

The active equalization scheme using transformers can not only overcome the various shortcomings of the previous schemes, but also better realize the energy balance and distribution function; greatly reduce the equalization power consumption, help reduce system heat dissipation requirements and increase the vehicle’s cruising range. The large equalization current reduces the equalization time, which has practical significance for electric vehicles using large-capacity batteries. Energy conservation and environmental protection is the current goal and technology development direction of China and the world. Whether it is hybrid power (HEV) or electric vehicle (EV), power batteries as energy storage media are inseparable. The development level of battery technology has become the current new energy source. One of the biggest bottlenecks in the popularity of cars. Choosing a suitable battery management program can greatly increase the service life of the battery and maximize the battery capacity, and give full play to the advantages and huge market potential of the new lithium-ion battery.