Analysis of battery management architecture of the

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Analysis of battery management architecture of hybrid electric vehicles

battery technology for electric vehicles (EV) and hybrid electric vehicles (HEV) has made significant progress. Not only has the battery energy density been steadily improved, but also the battery can be reliably charged and discharged thousands of times. If design engineers can effectively take advantage of these technological advances, electric vehicles and hybrid electric vehicles have the potential to compete with traditional vehicles in terms of cost, reliability and life span

the specified capacity of a battery refers to the amount of electricity that the battery can provide from 100% state of charge to zero state of charge. Charging to 100% charge state or discharging to zero charge state will quickly shorten the battery life, so the battery should be carefully managed to avoid full charge or full discharge state. Compared with working at 30% to 70% of the charging state (using 40% of the capacity directly discharged by the enterprise), working at 10% to 90% of the charging state (using 80% of the specified capacity) can reduce the total number of battery charging cycles to 1/3 or less

balancing the effective battery capacity and battery life brings challenges to battery system design engineers. Consider the situation of using 40% capacity and 80% capacity mentioned above. If the system limits the battery to only use 40% of its capacity in order to extend the battery life to 3 times the original, the battery size must be doubled to obtain as much usable capacity as 80% capacity. However, this will double the weight and volume of the battery system, thereby increasing costs and reducing efficiency

automobile manufacturers generally require the battery life to exceed 10 years, and have specified the necessary available battery capacity. The challenge facing battery system design engineers is to try our best to achieve the maximum capacity with the smallest battery pack. To achieve this goal, the battery system must use precise electronic circuits to carefully control and monitor the battery

electric vehicle battery pack system

electric vehicle battery pack is composed of multiple batteries in series. A typical battery pack has about 96 batteries. For lithium-ion batteries charged to 4.2V, such a battery pack can produce a total voltage of more than 400V. Although the automotive power system regards the battery pack as a single high-voltage battery, which charges and discharges the entire battery pack every time, the battery control system must consider the situation of each battery independently. If the capacity of one battery in the battery pack is slightly lower than that of other batteries, its charge state will gradually deviate from that of other batteries after multiple charge/discharge cycles. If the charging state of this battery is not periodically balanced with other batteries, it will eventually enter a deep discharge state, resulting in damage and eventually battery pack failure. To prevent this, the voltage of each battery must be monitored to determine the charging state. In addition, there must be a device to charge or discharge the batteries separately to balance the charging state of these batteries

An important consideration of the battery pack monitoring system is the communication interface. For the communication in PC board, the commonly used options include serial peripheral interface (SPI) bus and I2C bus. The communication overhead of each bus is very low, which is suitable for low interference environment. Another option is the controller area network (can) bus, which is widely used in automotive applications. Can bus is very robust, with the characteristics of error detection and fault tolerance, but its communication overhead is very large, and the material cost is also very high. Although the connection from the battery system to the main can bus of the car is worth it, it has advantages to use SPI or I2C communication in the battery pack

linear has introduced a device that enables battery system design engineers to meet these demanding requirements. Ltc6802 is a battery pack monitor IC that can measure the voltage of up to 12 stacked batteries. Ltc6802 also has an internal switch, so that the batteries can be discharged separately, so that they can enter a balanced state with other batteries in the battery pack

to illustrate the battery pack architecture, consider a system with 96 lithium-ion batteries. This will require eight ltc6802's to monitor the entire battery pack, with each device operating at a different voltage. When a 4.2V lithium-ion battery is used, the bottom monitor device will be connected across 12 batteries, and the potential adjustment range is 0V to 50.4v. The voltage range of the next group of batteries is 50.4v to 100.8v, and so on. The communication between these devices on different voltages brings insurmountable challenges. Many methods have been considered, but due to different priorities of automobile manufacturers, each method has advantages and disadvantages

battery monitoring requirements

when making a choice between the architecture of the battery monitoring system, there are at least five main requirements that need to be balanced. Their relative importance depends on the needs and expectations of end customers

(1) accuracy. In order to use the maximum possible battery capacity, the battery monitor needs to be accurate. However, automobile is a kind of noise system, and there is electromagnetic interference in a large frequency range. Any reduction in accuracy will adversely affect the life and performance of the battery pack

(2) reliability. No matter what kind of power supply is used, automobile manufacturers must meet extremely high reliability standards. In addition, high energy capacity and the potentially unstable nature of some battery technologies are major safety concerns. Compared with serious battery failure, a fail safe system that performs the shutdown operation under conservative conditions is preferable, although it may unfortunately detain passengers. Therefore, the battery system must be carefully monitored and controlled to ensure the overall control of the whole battery life in the system. In order to minimize false and true faults, a well-designed battery pack system must have robust communication, minimize fault modes and fault detection

(3) manufacturability. Modern cars already contain a large number of electronic products with complex wiring harnesses. In terms of automobile manufacturing, adding complex electronic circuits and wiring to support electric vehicle/hybrid electric vehicle battery system will make the complexity higher. The total number of components and connections must be as small as possible to meet strict size and weight restrictions and ensure that mass production is feasible

(4) cost. The convenience and cheapness of complex electronic control system is an important reason to buy plastic bags. It can be very expensive. Minimizing the number of relatively expensive components such as microcontrollers, interface controllers, current isolators and crystal oscillators can greatly reduce the total cost of the system

(5) power. The battery monitor itself is also the load of the battery. Its low working current can improve the efficiency of the system, and its low standby current can prevent the excessive discharge of the battery after the car stalls

battery monitoring architecture

figures 1 to 4 show four battery monitoring system architectures. Suppose a system consisting of 96 batteries is divided into 8 groups with 12 batteries as a group. Table 1 summarizes the advantages and disadvantages of each architecture in this case. In each case, an ltc6802 monitors a 12 cell battery pack. Each architecture is designed as an independent battery monitoring system, which provides a CAN bus interface to the main can bus of the car, and is galvanically isolated from the rest of the car. Table 1: comparison of battery monitoring architecture

1. parallel independent can modules (Figure 1)

each module composed of 12 batteries contains a circuit board with ltc6802, microcontroller, can interface and current isolation transformer. The large amount of battery monitoring data required by the system will crash the main can bus of the vehicle, so these can modules need to be on the local can sub. The can subsystem is coordinated by the main controller, which also provides the switch to the main can bus of the vehicle

Figure 1: parallel independent can module

2. Parallel module with can off (Figure 2)

each module composed of 12 batteries contains a circuit board with ltc6802 and digital isolator. These modules have independent interface connection with the controller circuit board, which contains microcontroller, can interface and current isolation transformer. The microcontroller coordinates these modules and provides the switch to the main can bus of the vehicle

Figure 2: parallel module with can off

3. single monitoring module with can off (Figure 3)

in this configuration, there is no monitoring and control circuit inside the module composed of 12 batteries, but there are 8 ltc6802 monitor ICs on a single circuit board, and each IC is connected to its battery module. Ltc6802 devices communicate through non isolated SPI compatible serial interface. A single microcontroller controls all battery pack monitors through SPI compatible serial interface and acts as a switch to the main can bus of the car. These, together with can transceiver and current isolation transformer, form a complete battery monitoring system

Figure 3: single monitoring module with can off

4. Serial module with can off (Figure 4)

this architecture is similar to a single monitoring module, except that each ltc6802 is on the circuit board inside the module composed of 12 batteries. These eight modules communicate through the ltc6802 non isolated SPI compatible serial interface, which requires three or four conducting cables to be connected between the battery module pairs. A single microcontroller controls all battery pack monitors through the bottom monitor IC, and also acts as a switch to the main can bus of the car. Can transceiver and current isolation transformer are still needed to form a complete battery monitoring system

Figure 4: serial module with can

battery monitoring architecture selection

because parallel interfaces require a large number of connections and external isolation, the first and second architectures are generally prone to problems. In order to deal with the problem of increasing complexity, the design engineer needs to realize independent communication to each monitor device. The third and fourth architectures are the least restrictive simplification methods. Ltc6802 can meet the requirements of all four configurations. System design engineers can choose two versions of ltc6802, one for serial configuration and the other for parallel configuration

ltc is used for superimposed SPI interface configuration. Multiple LTC devices can be serially connected through an interface that can send data back and forth along the battery pack without an external level shifter or isolator. LTC allows a single device to be used in a parallel architecture. These two versions of devices have the same battery monitoring specifications and functions

electric vehicles have a large demand for battery packs. Car manufacturers want cost-effective battery systems to meet their stringent reliability requirements. Linglilte's latest battery monitor IC gives system design engineers great flexibility in choosing the best battery pack architecture without compromising performance. (end)

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