Battery management and state detection of the hott

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Battery management and state detection of electric vehicle battery pack Abstract: the working state of electric vehicle battery pack mainly refers to the changes of terminal voltage, working current and temperature of each battery during operation. There are usually centralized detection methods and distributed detection methods for the detection of the working state of the battery. The idea of "partial" concentration and "overall" distribution is adopted to divide the battery into several groups. Each group is tested intensively and each group is tested distributed. At the same time, the "bridge capacitance" technology is used to solve the problems of reference point selection and isolation between the tested battery and the detection equipment in the single end voltage detection of the battery, A centralized/distributed detection method with complete isolation function is formed. The test shows that the voltage, current and temperature acquisition function of this detection method is normal, and the data is accurate and reliable

key words: electric vehicle; Automotive engineering; Battery; Centralized/distributed detection

0 introduction

battery technology is one of the core technologies of the next generation of vehicles - electric vehicles. Battery is a complex electrochemical system. A lot of research has been done on battery management technology at home and abroad, and many results have been achieved. It is generally believed that the battery management system mainly has the following functions: battery state parameter acquisition (including temperature, voltage, current, etc.); Accurate estimation of battery state of charge (SOC); Early diagnosis of unhealthy batteries; Comprehensively monitor the safe operation of the battery pack, such as preventing overcharge and over discharge of the battery

since the battery pack of electric vehicles is usually composed of dozens (hundreds) of single batteries, the normal working state of each single battery not only reflects the performance of the battery pack, but also affects the capacity and residual energy of the battery pack. The practice shows that if the aging battery is not detected and adjusted in time during the operation of electric vehicles, the capacity of the battery pack will be reduced and the service life will be shortened, affecting the efficient and safe operation of the whole battery pack

the detection of battery working state is completed by the battery management system (BMS), and other functions of the battery management system (including the calculation of residual energy) are based on the detection of battery working state. The research on the detection method of battery working state is of great significance to the development of electric vehicles

1 basic structure of BMS

see Figure 1 for the structural diagram of BMS used in electric vehicle (ev-3) developed by Hunan University. The BMS is composed of battery monitoring system, battery nuclear power status, (SOC) system and data display system. The sensor, battery monitoring system and SOC system constitute the bottom system, the data display system is the upper system, and the systems communicate through the internal can bus

2 detection method of working state of battery pack

electric vehicle battery packs generally work in series. The working current is the same as that of single battery, so it is easier to detect, while the detection of terminal voltage is more troublesome. If you only detect the terminal voltage of the battery pack, the method is very simple. You only need to connect the detection circuit at both ends of the battery pack, but this is not possible, because although you can get the total working voltage, you can't judge the terminal voltage of a specific single battery, and as long as there is a problem with one battery, it will affect the normal operation and performance of the whole battery; In addition, the detection circuit requires high accuracy. The normal working range of the terminal voltage of a single battery is relatively small. For example, the termination voltage of a 12V lead-acid battery is about 10V, and the voltage variation range is between 2 and 3V. The detection circuit can detect the variation of 1V as long as the accuracy of the detection circuit is 10%. If 24 12V lead-acid batteries are connected in series, the rated voltage is 288v, the discharge termination voltage is 240V, and the normal variation range of voltage is 48V. If the terminal voltage of a battery drops to 9V, the total voltage is 285v, which only changes by about 1%. It can be seen that the accuracy of the detection circuit must be at least 1% to detect the change of several volts. However, it is difficult to detect the slow changes of single cells in the detection of the whole group of cells, including the aging of single cells and the cumulative effect caused by the consistency of single cells. The whole set of tests cannot detect the battery and the actual capacity of the battery pack, and it is impossible to screen the aged batteries

the practical method is to detect each single battery. However, for the battery pack formed in series, the main problems encountered in automatically detecting the terminal voltage of each single battery are the selection of measurement reference point and the electrical isolation between the detection circuit and the detected battery pack. The selection of potential reference point not only affects the measurement accuracy as mentioned above, but also puts forward high requirements for the measurement range of the measurement circuit. The isolation between the detected battery pack and the detection circuit not only relates to the safety of the system, but also affects the complexity and realizability of the detection circuit. At present, there are mainly two methods: distributed detection and centralized detection

1) distributed detection method

the so-called distributed isolation detection technology is to modularize and localize the detection of the voltage and temperature of the single battery, and then wipe the surface clean and apply a thin layer of mechanical oil. The data detected by these detection modules are collected through certain communication means, and finally processed uniformly. The purpose of this is to solve the problems of centralized detection methods. See Figure 2 for the schematic diagram

its main advantages are:

(1) the connection is simple, the multiplexer switch is omitted, and the performance is reliable

(2) the measurement accuracy is high, which is more in line with the development trend of CAN bus of automotive electrical appliances

(3) the distributed module solves the reference point problem and solves the isolation problem between the main control computer and the battery pack by using the bus communication mode (adopting new thinking optocoupler devices)

but the application of distributed detection technology must also solve the following problems:

(1) because the detection module directly takes power from the tested battery continuously, it is not conducive to energy conservation and safety

(2) when there are many batteries, the number of modules is also large, which increases the cost and complexity, and requires a high carrying capacity of the communication bus

from the perspective of function, the detection module is mainly composed of detection sub module and communication sub module. The detection sub module should complete the task of data acquisition and conditioning, while the communication sub module should communicate with the main control circuit, receive the instructions of the main control circuit, and upload the detection data provided by the detection sub module

because the development direction of automotive electrical appliances is to adopt can bus technology, the communication sub module and the main control circuit should be connected by CAN bus

2) centralized detection method

centralized detection method (see Figure 3) uses a set of detection circuit to detect each single battery time-sharing. The detection technology is relatively intuitive. In order to detect the voltage of each battery, it is necessary to introduce the voltage signal of each battery into the detection equipment (if the battery pack is composed of n single batteries, it is necessary to introduce n+1 detection lines). The multi-channel switching technology is adopted, that is, the voltage signal of multiple single batteries is switched to the same differential amplifier through the switching device (relay). After signal processing, an A/D converter is used for sampling. "Switch switching" dynamically changes the reference point to ensure that each measurement is the terminal voltage of a single battery; The differential input ensures that the battery pack and the detection circuit are not grounded together. Although it is not completely isolated, it is safer than the common ground connection. Generally, digital or analog temperature sensors can be used to detect the battery temperature. Since there is no electrical connection between the temperature measurement process and the battery pack and the technology is relatively mature, this article will not repeat it. In addition, due to the large current required by electric vehicles (tens to hundreds of AMPS), the detection of battery pack charge and discharge current is generally realized by non-contact current sensors or transmitters

the main disadvantage of this method is that there are many signal lines, which increases the difficulty and complexity of wiring, affects the test accuracy and reduces the reliability

3) centralized/distributed detection method

in order to overcome the shortcomings of distributed detection method and centralized detection method, we propose the detection idea of "local concentration" and "overall distribution", that is, all batteries are divided into several groups, each group uses a detection module for "centralized" detection, and the whole system is composed of several detection modules connected through CAN bus. In short, the essence of centralized/distributed detection system is that the detection unit is partially modularized and localized, and the data is transmitted by bus. In addition to the advantages of the first two methods, centralized/distributed detection has the following main advantages: strengthening the flexibility and expansibility of the system; The reliability of the system is increased; It has high cost performance

for lead-acid batteries, due to their high voltage, large volume and small quantity, every 2 or 4 batteries can be equipped with a detection module. For the battery pack with low voltage, small volume but large quantity, such as lithium battery, 2 ~ 8 batteries can be divided into a group. Figure 4 is a block diagram of centralized/distributed detection

in order to reduce the cost, reduce the volume and simplify the circuit, the battery detection module should be completed by the single chip microcomputer. The main conditions for the selection of single chip microcomputer are: there are at least 4 A/D converters with more than 10 bit accuracy (because a battery needs at least 2 voltage and temperature detection); With CAN bus controller

we use picl8 series of microchip for the front-end detection module. The pic18f248/258 and pic18f2585/2680 of this series are 28 pin dual in-line packages. The pic18f248/2587 has 5-way 10 bit a/D converters; Picl8585/26801 has 8-way 10 bit a/D converter and nano watt power management technology, which further reduces the power consumption of the chip. Picl8f2585/2680 also has serial programming technology, which allows the MCU to program after being embedded in the circuit board, providing great convenience for use

in our project, 24 12V lead-acid batteries are used, which are divided into three groups and placed in the car. Six batteries are placed in the luggage compartment on both sides, and 12 batteries are placed in the rear luggage compartment. The two batteries share a detection module, that is, a detection module centrally detects the status of the two batteries. Finally, it should be noted that since the temperature sensor has no electrical contact with the battery, there is no picture in the detection diagram. The signal detected by the load sensor is decomposed into two ways of weight signals through the signal integrated processing device: one way is the DC feedback signal from the temperature detection part, but each detection module has a temperature detection function

in order to overcome the problem of continuous power supply from the battery, we supply power to all modules in a unified way, controlled by a master switch. This involves the isolation between the power supply circuit and the battery. To solve this problem, we use "bridge capacitance" technology. Figure 5 is the internal block diagram of the module. The working principle of "bridge capacitor" is as follows: MCU first closes the double pole single throw switch K1, and then the battery charges the capacitor C1. Because the time constant is very small, the voltage at the capacitor end soon reaches the voltage at the battery end; Then MCU disconnects K1 and closes K2, and the terminal voltage of C1, that is, the terminal voltage of battery, can be collected from a/D; Finally, MCU disconnects K2. So as to complete a collection task. In addition, when the module is not powered on, the two sets of switches are disconnected, and the module will not take power from the battery. It can be seen that this method not only solves the reference point problem, but also completely isolates the battery and the detection circuit. Its main disadvantage is that each module adds two groups of controllable switches. The controllable switch adopts optocoupler relay aqw212. The cost of the module is about 200 yuan

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