Summary: A battery management system using an active balancing method is introduced, which can accurately balance the voltage and accurately estimate the charging capacity to ensure that the system can make full use of the stored electricity in the battery pack. Safer and more reliable, longer life, suitable for hybrid and all-electric vehicles.
Key words: battery management; passive balancing; active balancing; power storage; compensation
Hybrid and all-electric vehicles are popular in the market, and their growth rates have been soaring, indicating that the development of vehicle electrification is about to enter another new stage.
For electric vehicles, the battery pack is the most expensive and most questionable component in the car. A high-performance battery management system can provide an ideal solution for hybrid and all-electric vehicles to solve the problem of insufficient battery power. As the Chevy Volt’s design team states: “Our engineers discovered during development that using a battery management system can significantly extend battery life and ensure that the battery can perform to its full potential. In other words, the battery management system is the key to solving battery problems.”
Passive Balancing Solutions and Related Issues
The design of the battery management system faces many problems, such as the charging of a large number of small batteries in series and how to ensure that each small battery in the battery pack is not overcharged. Lithium-ion batteries are extremely sensitive to overvoltage conditions, which can degrade battery performance, or even severely damage the battery, rendering it unusable. Different batteries have different performance parameters, so the performance is also different. In addition, different batteries have different residual charges before each recharge, so some batteries will be fully charged more quickly, and these batteries will be severely damaged due to overvoltage and cannot be used again.
There is currently a method to ensure that all the small cells in a battery pack are fully charged by shunting the current to a shunt resistor, known as the passive balancing method. This method shunts unwanted charging currents to the resistors, allowing the resistors to dissipate these currents to avoid overcharging the battery. This power dissipation feature limits the current shunted from the battery to a certain range.
Passive balancing methods do not work when the battery is discharged, so other solutions must be sought.
Even with a high degree of balance between the different small cells within a battery pack, their power storage varies. This phenomenon is called energy storage imbalance. Even if the storage capacity of different small batteries is exactly the same at the beginning, due to the large internal loss of some small batteries, their actual storage capacity will be different later. In addition, the small batteries produced by the same manufacturer have different performance parameters, so the manufacturer usually strictly selects the small batteries with the smallest parameter difference and puts them in the same battery pack. However, the whole testing process takes a lot of time, and unqualified small batteries will be screened out, which will increase the cost burden of manufacturers. As the battery ages, its power storage will decrease accordingly, further widening the gap between the parameters of each small battery. In addition, different small cells in the battery pack have different temperature gradients, so the aging degrees of small cells are also different. Thermal management technology can play a key role in cell balancing, but introducing such technology can add significant cost.
Smaller batteries that actually hold less power and are “weak” are subject to the greatest discharge stress and therefore drain the fastest, allowing them to charge less than other powerful batteries. After a period of use, such “weak” small batteries will age faster, and the power storage will decrease significantly. In other words, the lifespan of these small cells will be shorter, and the lifespan of the entire battery pack will be shortened as a result.
Actively balanced solutions
Active balancing methods can solve the problems faced by lithium batteries. Rather than shunt and then dissipate battery current, an active balancing system has the advantage of delivering charge to the small cells within the pack through a DC/DC converter. Charges can be transferred whether the small cells are charging, discharging, or idling, and the small cells are constantly balanced. Since the charge transfer efficiency of the active balancing method is very high, it can provide a higher balancing current, so that the small cells in the battery pack can be balanced faster, and the charging speed is also higher, which cannot be achieved by the passive balancing method. of.
Idle cells also leak, and even if the cells are fully equilibrated, due to temperature gradients, the internal leakage rates of the cells vary, resulting in different rates of charge leakage. For every 10°C increase in battery temperature, the leakage rate doubles. Active balancing ensures that small idle cells are constantly rebalanced. There must be constant balance between the different small cells to fully utilize all the stored power in the battery pack.
Figure 1 shows the advantages of active balancing methods over passive balancing methods. Since each battery has a different storage capacity, with passive balancing, the total storage capacity of the battery pack is equal to the difference between the maximum and minimum storage capacity at the time of charging.
The entire battery pack can be continuously discharged until the storage capacity of a small battery has dropped to its minimum level. At this time, other small batteries have unused residual power, so the actual storage capacity (charging capacity) of the battery pack will be reduced.
Since the active balancing method transfers the charge during charging using a high-efficiency power converter, small cells with different storage capacities can be fully charged and power loss can be minimized. With passive balancing, some of the charge is dissipated, but active balancing transfers this charge to a smaller battery with a larger storage capacity. When discharging, the situation is roughly the same. Since the electric energy of the large-capacity battery can be redistributed to the smaller-capacity batteries, all the small batteries can be fully discharged, and there will be no residual electric energy left in the battery pack. A battery pack with active balancing has a larger actual power reserve than a passively balanced battery.
The performance of an active balancing system depends on the ratio between balancing current and battery charge and discharge rates. The higher the unbalance rate of the cell and the greater the charge or discharge rate, the higher the required balancing current. Active battery management systems can provide compensation for the difference in storage capacity between small cells during charging or discharging (assuming a constant balancing current is used), Figure 2 shows the compensation value for this difference.
Balancing method for battery modules
The battery pack of an electric vehicle generally contains as many as several hundred small cells, all divided into a number of different modules. A balance must also be maintained between small batteries and between modules, because different modules have different parameters, different temperatures can also affect performance and cause different degrees of aging of the modules, and the battery pack must be checked regularly and even replaced from time to time old module.
The balancing method described above ignores the problem of transferring charges between modules.
One way to ensure balance between modules is to connect the small cells of each module with one of the small cells in an adjacent module to create a path for the transfer of charge between the two. The disadvantage of this method is that it is less efficient because the charge must be transferred to a small cell before being distributed to other small cells within the module. If the charge is to be transferred to a distant module, the charge needs to be transferred multiple times separately, further reducing the efficiency.
National semiconductor‘s Active Balancing Battery Management System Solution
National Semiconductor’s Active Balance Battery Management System is a comprehensive system solution for large lithium-ion batteries. Basically, it’s a printed circuit board with multiple application-specific integrated circuits (ASICs) configured to provide active cell balancing, high-accuracy data acquisition, protection, and complete battery management.
National Semiconductor’s battery management system uses an optimized isolated Inductor topology to provide high-efficiency and high-current cell balancing, ensuring a high degree of balance within and between modules. Charge can be transferred bi-directionally between any small cells within the same module and between different modules, which minimizes the number of retweets during the transfer and balances the cells and modules simultaneously. The system can also select the most ideal balancing strategy through intelligent control algorithms to optimize system performance. In addition, because the system adopts a modular architecture, the scale of the system has greater flexibility. Since a single circuit board can manage multiple battery modules containing up to 14 small cells, each of the circuit boards responsible for managing these modules can be stacked together to manage high-voltage battery packs. There is basically no limit to the number of modules stacked together, the only condition is that the module voltage cannot exceed the highest rated voltage of the insulating components. The size of the balance current depends on the selected components, and engineers can choose the current size as needed to make the appropriate trade-off between cost and performance, ensuring that both meet the design requirements.
In addition to performing the charge balancing function, the battery management system is also responsible for comprehensively monitoring the operation of the battery pack, ensuring that the system can measure the voltage of each cell in the pack with unprecedented accuracy. National’s analog front-end circuitry is responsible for charge balancing, and has always played a key role in this, but in addition to that, the analog front-end circuitry ensures accurate estimates of battery pack charge and health.
The battery management system completes the voltage measurement of all the small cells in the battery pack in a very short time. In other words, the measurements of all the small cells in the entire battery pack are synchronized.
The battery management system is equipped with multi-layer diagnostic and fault detection circuits, which can detect battery undervoltage, overvoltage, communication failure, sensor circuit open circuit and battery overheating and other fault conditions, and send reports to the main controller. In addition, the system has redundant fault detection circuits, which can report fault conditions through channels other than the main hardware and firmware channels. The parameters are compared with programmable thresholds stored in the firmware, and the independent fault detectors with built-in comparators also monitor the relevant values.
Each board is equipped with a multi-contact isolated CAN bus interface, allowing the board to communicate at high speed with other modules and the host controller. The board can also utilize the CAN bus to perform a number of different diagnostic, programmable and configurable functions.
Because this battery management system accurately balances voltage and accurately estimates charge capacity, it ensures that the system can fully utilize the stored power in the battery pack to further extend the range of miles, and it can also be aided by the advanced and reliable “remaining capacity estimation function”. Drivers can drive with confidence by accurately predicting the remaining “mileage”.
Because National Semiconductor’s battery management system has many advanced functions, whether the battery is in charge, discharge or idle state, this battery management system can accurately control the charge of each small battery in the battery pack, which helps to greatly improve the battery pack. safety and reliability, as well as extending its life cycle.
This battery management system provides an advanced and efficient solution for large batteries for automobiles, power storage systems for vehicle charging stations and smaller battery packs. It is unique in that on the one hand it is suitable for large battery packs, and on the other hand, unlike other battery management system solutions, it does not use the design of the old small battery pack, so the performance is far better than other battery management systems.