HEV的电池管理系统

SAE TECHNICAL PAPER SERIES 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org 2002-01-1918 A Modular Battery Management System for HEVs Thomas Stuart and Fang Fang The University of Toledo Xiaopeng Wang Visteon Co. Cyrus Ashtiani DaimlerChrysler Ahmad Pesaran National Renewable Energy Laboratory Reprinted From: Proceedings of the 2002 Future Car Congress on CD-ROM (FCC2002CD) 2002 Future Car Congress Arlington, Virginia June 3-5, 2002 www.51greencar.cn/bbs Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department. To request permission to reprint a technical paper or permission to use copyrighted SAE publications in other works, contact the SAE Publications Group. ISSN 0148-7191 Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published in SAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group. Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300 word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE. Printed in USA All SAE papers, standards, and selected books are abstracted and indexed in the Global Mobility Database www.51greencar.cn/bbs ABSTRACT Proper electric and thermal management of an HEV battery pack, consisting of many modules of cells, is imperative. During operation, voltage and temperature differences in the modules/cells can lead to electrical imbalances from module to module and decrease pack performance by as much as 25%. An active battery management system (BMS) is a must to monitor, control, and balance the pack. The University of Toledo, with funding from the U.S. Department of Energy and in collaboration with DaimlerChrysler and the National Renewable Energy Laboratory has developed a modular battery management system for HEVs. This modular unit is a 2nd generation system, as compared to a previous 1st generation centralized system. This 2nd generation prototype can balance a battery pack based on cell-to-cell measurements and active equalization. The system was designed to work with several battery types, including lithium ion, NiMH, or lead acid. Surface mount technology is used throughout to reduce volume, mass, and cost. The weight and volume of the 2nd generation are estimated to be 70% and 87%, respectively, less than the 1st generation. INTRODUCTION Series connected battery packs in electric vehicles (EVs) and hybrid electric vehicles (HEVs) require monitoring equipment that is capable of measuring the voltages of individual segments (several modules/cells connected in series) in order to prevent damage and identify defective segments. As discussed in [1, 2], virtually all types of batteries can be damaged by excessively high or low voltages, and in some cases the results can be catastrophic. Lithium ion cells, for example, will ignite if they are overcharged, which equates to a high voltage. Therefore, once high or low voltage segments have been identified, some equalization process also must be used to re-balance the voltages. Imbalances are especially prevalent in EVs and HEVs since the batteries are frequently charged and discharged. Certain problems associated with these voltage measurement(s) themselves also were previously described in [3], along with possible solutions In the following section, the approach and results of a study to investigate a modular battery management system for Evs and HEVs are provided. APPROACH To process the data at a central module (CM), such as that in Figure 1, these measurement circuits must transfer each segment voltage to a common reference level such as the system ground in the CM. This particular diagram shows a typical modularized battery management system that would use one of these transfer circuits in each of the local modules, LM#1–#4. In this system the 4 LMs are used to obtain data from 4 sections of the series battery pack, where each section contains 12 segments that require voltage measurements. This modularized approach drastically reduces the amount of wiring that otherwise would be required between the CM and the batteries. The transfer circuit in each LM is used to shift all 12 measurements to its local ground reference, i.e., G1-G4. These voltages are then multiplexed, fed to an A/D converter, processed by a local microcontroller, and galvanically transmitted to the CM via a serial data bus such as CAN 2.0B. This transfer circuit approach provides several accuracy and cost advantages over previous methods, and it can be used for almost any number of battery segments. This is an important advantage since each LM can now 2002-01-1918 A Modular Battery Management System for HEVs Thomas Stuart and Fang Fang The University of Toledo Xiaopeng Wang Visteon Co. Cyrus Ashtiani DaimlerChrysler Ahmad Pesaran National Renewable Energy Laboratory www.51greencar.cn/bbs 2 measure a large number of segments to reduce the total number of LMs. For example, a 48-cell pack with 4 LMs each measuring 12 cells can be used in place of perhaps 12 LMs each measuring 4 cells. This represents a large cost savings since, as shown in Figure 1, each LM typically contains a multiplexer, A/D converter, microcontroller, and two galvanically isolated buffers for the serial data bus. Figure 1. Modularized battery management system To provide the greatest flexibility, this BMS was designed to have the necessary features required for Li-Ion, NiMH or lead acid batteries. Some of these include, 1. Modularization is used to avoid a large wiring harness, and each local module can service up to 12 cells. This provides a large reduction in the number of modules and the cost. 2. New types of voltage and current measurement circuits are used to provide higher accuracy. This includes an automated calibration procedure that stores correction factors in flash memory. 3. Battery data can be stored in flash memory in the CM and on disk in a PC. 4. All modules use CAN 2.0B serial communication for convenience and higher reliability. 5. When the battery is not in use, the BMS enters a low power sleep mode and measures the elapsed time in this mode. It also awakes periodically to perform various functions and then goes back to sleep. 6. Battery equalization uses a boost process instead of dissipation to improve battery efficiency. SYSTEM REQUIREMENTS Several types of advanced batteries are under study for EVs and HEVs, two of the most promising candidates being Li-Ion and NiMH because of their high energy densities. Some Li-Ion products are rated up to 125 Wh/kg [4], while the energy density of a NiMH battery can reach 80�90 Wh/kg [4]. However, the characteristics of Li-Ion and NiMH are very different, and this means different management requirements. A Li-Ion battery weighs less than a NiMH battery with the same capacity and is more compact [5], but it also has more stringent management requirements because the battery may ignite if overcharged. This problem is especially serious for the big packs in EVs and HEVs where it could cause a fatal accident. To ensure safety, the voltage of each Li-Ion cell must be measured very accurately since this is the best indicator of the SOC [6]. It also means a fairly large number of voltage measurements must be processed every measurement cycle, eg., 48 cells every 2 sec. Balancing the cell voltages (equalization) also is more difficult because the simple method of overcharge with a small current (trickle charge) cannot be used. Instead, the cells must be balanced individually using a circuit such as the boost equalizer in this present system. Abnormal conditions for the cell voltage, battery current and temperature must activate an alarm and be handled promptly. Since the safety of the battery pack is dependent on the management system, the reliability of the management system becomes very critical. Therefore, various hardware and software safety features are required to secure the battery pack in case of malfunction. It is also very important to control the cost when adding additional features and enhancing existing functions. In addition, the size and weight need to be minimized to allow the BMS to fit into compact battery packs. Figure 2. A conventional battery system Since NiMH batteries are not as prone to the overcharge combustion problem as Li-Ion, their voltage measurement requirements are not as stringent. In addition, since voltage is not a useful indicator of the SOC of this battery [7], the voltage measurements do not require the same accuracy as Li-Ion, and individual cell voltage measurements are not necessary. Instead, the measurements can be reduced to a few segments of series connected cells, e.g., 6 segments of 8 cells each in a 48 cell pack. However, the leakage current is much higher than for Li-Ion, and the SOC can decrease considerable while the vehicle is parked for a few days. This high leakage along with the lack of correlation between voltage and SOC makes it much more difficult to determine the SOC. Because of this leakage, the www.51greencar.cn/bbs PX01 矩形 PX01 矩形 3 parking time needs to be measured to predict the energy loss. The block diagram of a typical battery system is shown in Figure 2. This particular version includes an electronic control unit (ECU) that monitors the real-time situation of the battery pack and an equalizer (EQU) that balances the charge levels of the battery segments. The responsibility of the ECU consists of four functions: data collection, data processing, data transmission, and control. The ECU usually measures all segment voltages, a few selected temperatures, and battery current. It then analyzes the data and extracts information for battery protection and state of charge (SOC) determination. Some information is also transmitted to the vehicle data bus. The ECU also controls certain battery maintenance equipment, such as the battery cooler, heater, equalizer and autodisconnect switch (circuit breaker). A more sophisticated ECU will have the capability to control an on-board battery charger. The purpose of the EQU is to minimize the SOC differences among the battery segments since such imbalances will reduce the usable capacity of the battery pack. However, the batteries in EVs and HEVs are frequently charged and discharged and are thus prone to imbalance. An EQU keeps the battery pack in balance by either charging the weak cells or discharging the strong cells. In some batteries, such as Li-Ion and lead acid, SOC differences are directly proportional to voltage differences. For those batteries the equalizer strategy could depend on voltage [8]. For NiMH, cell voltage is not related to SOC [7], and a more complicated strategy is required [9] [10]. A battery string with a fairly high voltage of 100-400VDC is commonly used in EVs and HEVs to supply high power without an overly high current. The string is usually divided into several segments to be managed. Older BMS designs typically take the whole battery pack as one segment. This kind of BMS is cost efficient since it requires significantly fewer components and is less complex in terms of data processing and communication. However, it does not offer the ability to measure and balance individual cells or segments in the pack. For the more volatile Li-Ion battery, each individual cell must be measured and balanced. PRESENT SYSTEMS Battery management technology is quite mature in portable equipment applications such as laptop computers and cellular phones. Companies like Texas Instruments [11], Power Smart [12] and Philips [13] have all developed battery management IC products, and there are systems for portable equipment based on these ICs [14]. However, the application environment for EVs and HEVs is very different from that in portable equipment in regard to items such as battery pack size, charge/discharge current amplitude and frequency spectrum, temperature range and EMI. These differences make the BMS for EVs and HEVs a much more complicated system, and it needs a very different design. A few modular BMS systems for large battery packs are now commercially available [15-17], and some battery manufacturers have designed systems specifically for their own battery packs. A review of these designs indicates that while undoubtedly functional, they do not have certain features demanded by advanced EV/HEV batteries. Some of these features are very critical and require new technologies. Some of these BMSs do not have SOC determination capability, so it will not be possible to hold the SOC in the desired range, and the user will not know the remaining energy in the battery. To determine the SOC, the BMS will require an accurate charge measurement circuit and a sophisticated algorithm that is designed using knowledge of the battery characteristics. Voltage measurement is another problem for many of these earlier systems. First, the accuracy is not sufficient for a Li-Ion battery, and this might impair safety or reduce the usable capacity of the battery. Furthermore, the time period between each cell voltage sample is so long that the battery current could change significantly during this time period. Since battery voltage changes with the current, this will skew the voltage measurement data. The communication schemes in some of these systems are also questionable. Many systems use an RS232 bus for data communication, which is not designed with strong error management capability and EMI immunity. Possible communication errors or failure may cause serious safety problems, especially with a Li-Ion battery. Dissipative equalizers used in most of the present BMSs [15, 16] dissipate the energy in all the cells until they reach the same the level of the weakest cell in the battery pack. Although the idea is quite simple and the cost is low, it obviously has low energy efficiency. The BMSs in [15-17] have a distributed structure, where local units serve one battery segment and send data to a central unit. Compared to a centralized structure, the distributed structure is more flexible for different types of batteries and numbers of segments. It also dramatically reduces the size and weight of the wiring harness as well as the wiring labor. However, since each of the local units in these systems only measures one battery segment and there is a microcontroller in each local module, the cost of the whole system is quite high. www.51greencar.cn/bbs PX01 矩形 4 PROPOSED SYSTEM The intent of this research is to develop new technologies that will lower the cost and improve the safety and performance of battery systems in order to make them practical for EVs and HEVs. An engineering prototype using these new technologies was designed, built and tested. This prototype also has certain features that enable it to be used with several types of batteries such as Li-Ion, NiMH and lead acid. Compared to previous systems, the new system provides large reductions in cost, size and weight, all of which are of critical importance in EVs and HEVs. Figure 3. New modular ECU/EQU system Figure 3 shows the block diagram of a new modular system designed for a Li-Ion pack. This particular system has four local ECU/EQU modules, one central ECU module and a DC/DC converter module. Each local module serves 12 battery segments because the Li-Ion pack has 12 segments (2VDC�4.2VDC per segment) in each battery module. By changing certain components, it can be redesigned to serve even more segments, e.g., 16 or 24. Compared to earlier centralized systems, this modularized structure reduces component costs greatly. It also simplifies installation and wiring work. Each local module is a combination of a local ECU unit and a local EQU unit, where the ECU controls the EQU. All the local ECU units and the central ECU unit communicate directly via the CAN 2.0B vehicle data bus. Using CAN for both internal and external communication simplifies the system since it offers speed and reliability advantages. When the system is turned off by S2, the Central enters a low current “sleep” mode so it can retain data in its SRAM and measure the off-time. The system can be powered either by the 12V vehicle battery or by the 48 cell battery pack via the DC/DC converter in Figure 3. The controlled switch S1, is closed whenever the Central ECU routes power to the Locals, and the system will be powered by the DC/DC converter as long as the 12V (actually 9VDC�16VDC) battery remains below 15VDC. This arrangement is used so equalization can be done during park without draining the 12V battery. When the system is in sleep mode during park, S1 is open; the DC/DC converter is off; the Central is powered by the 12V battery; and the power to the Locals is turned off by the Central. During sleep mode, the Central only draws about 0.8mA., which is an acceptable drain on the 12V battery. The DC/DC converter is turned off during sleep because even at small loads such as 0.8 mA., its input current is actually several mA. This drain would be excessive for the 48 cell battery pack during a long parking period. All of the microcontroller requirements can be met with the Infineon SAF505CA, which has a CAN 2.0B controller and a 10 bit A/D. The responsibility of local ECU unit includes: � Voltage measurement � Temperature measurement � Communication � EQU control Figure 4. New local ECU/EQU module for 12 cells The responsibility of central ECU unit includes: � Local module on/off control � Voltage measurement synchronization among local modules � Data collection, data processing and communication � Battery charge and current measurement � Battery State of Charge (SOC) determination � Safety features � System monitoring during sleep mode � Battery maintenance equipment control This version uses a selective EQU in each local module, which charges the weakest segment of the local pack with a constant current. Its intelligence is provided by the local ECU, which also controls various safety features. www.51greencar.cn/bbs 5 Since this EQU functions by charging a selected cell, it is important to supply the operating power from the Central ECU instead of from the local cells, i.e., a 4 wire bus should be used instead of a 2 wire bus. The reason for this is that if the Local ECU fails, it might not be able to turn off its EQU. This could result in overcharging for a Li-Ion cell and cause an explosion. With a 4 wire bus, the Central ECU can remove all power to the Locals in the event of a Local ECU failure. Figure 5. New Central ECU module Local Module - Figure 4 is a block diagram of one of the local modules. The ECU components are to the right of the pack and the EQUs are to the left. H1-H12 is a voltage transfer circuit that shifts the 12 cell voltages to a common reference level. By using an operational amplifier based transconductance amplifier, the transfer circuit design is simple and cost effective. It also has a very stable gain over a wide temperature range, e.g., from –30oC to 60oC. This feature is critical for achieving the high accuracy measurements demanded by Li-Ion. An automated calibration technique has been developed to reduce