Battery management system for hybrid and all-electric vehicles

Hybrid and all-electric vehicles are widely welcomed by the market, and their growth rate has been soaring. Therefore, the development of automotive electrification is about to enter another new stage.

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For electric vehicles, the battery pack is the most expensive and most reliable component in the car. The high-performance battery management system provides an ideal solution for hybrid and all-electric vehicles, solving the problem of low battery power. As Chevy Volt's design team puts it: "Our engineers found that using a battery management system during the development process can significantly extend battery life and ensure that the battery can fully perform its performance. In other words, the battery management system is the key to solving the battery problem."

Passive balancing solutions and related issues

The design of the battery management system faces many problems, such as the charging problem of a large number of small batteries connected in series and how to ensure that each small battery in the battery pack is not overcharged. Lithium batteries are extremely sensitive to overvoltage conditions. Overvoltage can degrade battery performance and can even cause serious damage to the battery and can no longer be used. Different batteries have different performance parameters, so the performance is also different. In addition, different batteries have different residual charges before each recharging, so some batteries will be fully charged, so that these batteries will be severely damaged due to overpressure and cannot be used again.

There is currently a way to ensure that all the small batteries in the battery pack are fully charged, that is, the current is shunted to the shunt resistor, called the passive balancing method. This method shunts the unwanted charging current to the resistor, allowing the resistor to dissipate these currents to avoid overcharging the battery. This power dissipation function limits the current shunted by the battery to a certain range.

The passive balancing method does not work when the battery is discharged, so other methods must be sought.

Even if a small balance is achieved between different small batteries in a battery pack, the amount of stored electricity is not the same. This phenomenon is called the imbalance of stored electricity. 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. In addition, small batteries produced by the same manufacturer each have different performance parameters, so manufacturers usually rigorously select small batteries with the smallest difference in parameters to be placed in the same battery pack. However, the entire test process takes a lot of time, and the unqualified small battery will be screened out, which will increase the cost burden of the manufacturer. As the battery ages, its power storage will also decrease accordingly, making the parameter gap of each small battery further widened. In addition, different small batteries in the battery pack have different temperature gradients, so the age of the small batteries is also different. Thermal management technology can play a key role in battery balancing, but the introduction of this technology will significantly increase costs.

Small batteries that are actually low in storage and are “weak” are subject to the highest discharge pressure, so they consume the fastest electricity, making them charge less than other powerful batteries. After a period of use, such "weak" small batteries will age faster and the storage capacity will drop more. In other words, the life of these small batteries will be shorter and the life of the entire battery pack will be shortened.

Active balancing solution

The active balancing method can solve the problems faced by lithium batteries. The active balancing system eliminates the need to first shunt the battery current and then dissipate the current. The advantage is that the charge can be transferred to the small battery in the battery pack via a DC/DC converter. No matter whether the small battery is in charge, discharge or idle state, the charge can be transferred, and the small batteries can be constantly balanced. Since the active balancing method has a very high charge transfer efficiency, it can provide a high balancing current, so that the small batteries in the battery pack can be balanced faster and the charging speed is higher, which is impossible by the passive balancing method. of.

The idle battery will also leak electricity, and even if different small batteries have reached a perfect balance state, the internal leakage speeds of the small batteries are different due to different temperature gradients, so that the charge leakage rate is also different. For every 10 °C increase in battery temperature, the leakage rate doubles. Active balancing ensures that small, free batteries are constantly re-seeking balance. Different small batteries must be constantly balanced to fully utilize all of the power stored in the battery pack.

Figure 1 shows the advantages of the active balancing method over the passive balancing method. Since the batteries each have different storage capacities, the passive balance method is used, and the total storage capacity of the battery pack is equal to the difference between the highest and lowest storage capacities at the time of charging.

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The entire battery pack can be continuously discharged until the storage capacity of a small battery has dropped to its lowest level. At this time, other small batteries have unused residual power, so the actual storage capacity (charge amount) of the battery pack is reduced.

Since the active balancing method can transfer charge using a highly efficient power converter during charging, a small battery with different power storage can be fully charged and power loss can be minimized. If the passive balancing method is used, some of the charge will be dissipated, but the active balancing method will transfer these charges to a small battery with a large storage capacity. The situation is also the same when discharging. Since the power of the large-capacity battery can be redistributed into the battery with a smaller capacity, all the small batteries can be fully discharged, and no residual electric energy is left in the battery pack. A battery pack with active balancing has a large actual storage capacity, which is superior to a passively balanced battery.

The performance of an active balancing system depends on the ratio between the balancing current and the battery charging and discharging rate. The higher the imbalance rate of the battery and the higher the charge or discharge rate, the higher the required balancing current. The active battery management system can compensate for the difference in the amount of electricity stored between small batteries during charging or discharging (assuming the equilibrium current is constant). Figure 2 shows the compensation value for this difference.

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Balance method for battery modules

The battery pack of an electric vehicle generally contains up to several hundred small batteries, all of which are divided into a plurality of different modules. Small batteries must also be balanced between each module, because different modules have different parameters, different temperatures will affect performance and different degrees of aging of the module, and the battery pack must be checked regularly or even replaced from time to time. Old module.

The balancing method described above ignores the problem of transferring charges between modules.

There is a way to ensure that the modules are balanced, that is, the small cells of each module are connected to one of the small cells in the adjacent module to establish a path for transferring charge between the two. The disadvantage of this method is that it is less efficient because the charge must first be transferred to a small battery and then distributed to other small batteries in the module. If the charge is to be transferred to a farther module, the charge needs to be transferred multiple times, further reducing efficiency.

National Semiconductor's Active Balanced Battery Management System Solution

National's Active Balanced Battery Management System is a comprehensive system solution for large lithium batteries. Basically, this is a printed circuit board with multiple Application Specific Integrated Circuits (ASICs) that provide active battery balancing, high-accuracy data acquisition, protection, and battery management.

National's battery management system uses an optimized isolated inductor topology that provides high efficiency and high current battery balancing to ensure a high degree of balance between modules and modules. The charge can be transferred bi-directionally between any small cells in the same module and between different modules, which minimizes the number of transfers during transfer and balances between small cells and modules. The system can also select the optimal balancing strategy through intelligent control algorithms to optimize system performance. In addition, because of the modular architecture of the system, the scale of the system is more flexible. Since a single board can manage multiple battery modules with up to 14 small batteries, each board responsible for managing these modules can be stacked to manage high-voltage battery packs. There is basically no limit to the number of modules stacked together, the only condition being that the module voltage must not exceed the highest rated voltage of the insulating component. The amount of balancing current depends on the components selected, and the engineer can select the current as needed to make the appropriate trade-offs between cost and performance to ensure that both meet the design requirements.

In addition to performing charge balancing, the battery management system is responsible for fully monitoring the operation of the battery pack, ensuring that the system measures the voltage of each small battery in the battery pack with unprecedented precision. National's analog front-end circuitry is responsible for balancing charge and has always played a key role in this, but in addition, the analog front-end circuitry ensures accurate estimates of battery pack charge and health.

The battery management system completes the voltage measurement of all small batteries in the battery pack in a very short time. In other words, the measurement of all the small batteries in the entire battery pack is synchronized.

The battery management system has a multi-layer diagnostic and fault detection circuit that detects faults such as battery undervoltage, overvoltage, communication faults, open sensor lines, and battery overheating, and sends reports to the main controller. In addition, the system has redundant fault detection circuitry that can notify fault conditions through other channels than the main hardware and firmware channels. The parameters are compared to the programmable thresholds stored in the firmware, and the stand-alone detector with built-in comparators also monitors the values.

Each board is equipped with a multi-contact insulated CAN bus interface that allows the board to communicate at high speed with other modules and host controllers. The board can also perform a variety of different diagnostic, programmable, and configurable functions using the CAN bus.

Because the battery management system accurately balances the voltage and accurately estimates the amount of charge, it ensures that the system can fully utilize the power storage in the battery pack to further extend the mileage, and can also help with the advanced and reliable “remaining capacity estimation function”. The driver accurately predicts the remaining “stroke miles” and allows them to drive with peace of mind.

Because National Semiconductor's battery management system has many advanced features, this battery management system can accurately control the charge of each small battery in the battery pack, regardless of whether the battery is in charge, discharge or idle state, which helps to greatly increase the battery pack. Safety and reliability as well as extending their life cycle.

This battery management system provides an advanced and effective solution for large automotive batteries, automotive charging station storage systems and smaller battery packs. It is unique in that it is suitable for large battery packs on the one hand, 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 superior to other battery management systems.

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