Study on the voltage and temperature measurement method of series battery

1. Introduction

In the context of advocating energy conservation and emission reduction, the research on new energy is becoming the focus of public attention, and electric vehicles powered by electricity are one of the research hotspots. Batteries are the source of energy for electric vehicles. In order to ensure the good performance of the battery pack and extend its service life, the battery pack needs to be managed and controlled. The premise is that the existing capacity parameters of the battery must be accurately and reliably obtained. The battery voltage and temperature are two parameters closely related to the battery capacity, so it is very important to accurately collect the voltage and temperature of the single battery.

2. Analysis of Common Measurement Methods

1. Analysis of single cell voltage measurement method

There are many methods for measuring the voltage of a single battery in a series battery pack, the more common ones are mechanical relay isolation detection, differential amplifier isolation detection, voltage divider isolation detection, photoelectric relay method, etc. The mechanical relay method can directly measure the voltage of each single battery, but the mechanical relay has a limited service life and slow action speed, so it is not suitable for long-term rapid inspection. The measurement error of the differential amplifier isolation method is basically determined by the error of the isolation amplifier, but because the measurement cost of each channel is relatively high, it is slightly insufficient in terms of economy. The voltage divider method has a fast response speed and low measurement cost, but its disadvantage is that it cannot adjust the voltage divider ratio well and the measurement accuracy is not satisfactory.

The method has fast response speed, long working life, relatively low measurement cost, no contact on the switch, and can play a role in voltage isolation. If the selected photoelectric relay adopts Photo MOS technology, it can achieve higher measurement accuracy, so the photoelectric relay isolation method is an ideal single cell voltage measurement method. The single cell voltage measurement method in this article is based on the photoelectric relay isolation method.

The on-off control strategy of photoelectric relays is an important issue to be solved by the photoelectric relay isolation method. Common on-off control methods for photoelectric relays include: I/O direct control, decoder control, analog switch control, etc. The I/O direct control method is simple and easy to implement, but it requires a large number of I/O resources. The ideas of decoder control and analog switch control are similar, that is, a small number of I/Os are used to control a large number of photoelectric relays. These two methods reduce the occupation of I/O ports. The use of I/O direct control, decoder control and analog switch control all require the on-off control circuit , A/D conversion circuit and processor to be designed on the same module, namely the sampling module. In this way, the two electrodes of the single cell need to be wired to the sampling module. For the entire battery pack, there will be a large number of wires connected to the sampling module, resulting in cumbersome installation and complex electrical wiring. For the measurement of single cell voltage, three problems should be focused on: using electrical isolation between the field and the measurement system, reducing costs and simplifying design solutions, and improving system accuracy. The three methods of photoelectric relay on-off control, namely I/O direct control, decoder control and analog switch control, are insufficient in terms of design simplicity.

This paper proposes a method for measuring the voltage of a single cell isolated by a photoelectric relay, which is controlled by a shift register array. This method directly designs and installs the photoelectric relay on-off control circuit on the battery, and the wiring between them can be connected in series with a flat cable, which greatly simplifies the design, facilitates installation, and makes the electrical wiring concise and clear.

2. Analysis of single cell battery temperature measurement method

Battery temperature has a significant impact on battery capacity, voltage, internal resistance, charge and discharge efficiency, service life, safety and battery consistency, so the battery must be temperature monitored during use.

At present, the temperature of single cells is generally measured by using thermistors as temperature sensors. The voltage division method is used to read the terminal voltage of the thermistor through A/D sampling, and the temperature value can be calculated based on the resistance-temperature relationship. The thermistor is installed on each battery, and the thermistors on different batteries are connected to the A/D sampling circuit for temperature sampling in a time-sharing manner to realize the inspection of the temperature of the single cell. The temperature is measured by the thermistor, and its measurement accuracy is ±1.0℃, which has a large error. At the same time, due to the manufacturing process, the temperature characteristics of the individual thermistors are not very consistent, which makes it difficult to calibrate the temperature measurement. When performing multi-point temperature inspections, the time-sharing channel selection problem must also be solved, so the design simplicity problem also needs to be considered.

[page] Based on the idea of ​​shift register array to control channel selection, this paper proposes a multi-point temperature sampling method using digital temperature sensors to start and read data simultaneously. This method has high sampling accuracy, fast sampling speed, and simple and convenient installation.

3. Measurement principle and circuit

1. Principle of single cell voltage measurement

The author of this article has designed a battery management system based on the photoelectric relay isolation method. The measurement of the single cell voltage is a time-sharing measurement method. The two ends of each battery in the series battery pack are isolated by a photoelectric relay and then uniformly connected to the detection bus. According to a certain time strategy, the on and off of the photoelectric relay can be controlled to control the single cell to apply voltage to the detection bus separately in different time periods, thereby realizing the time-sharing detection of the single cell voltage. This method has a short inspection cycle and high measurement accuracy. However, controlling the on and off of the photoelectric relay requires a large amount of I/O resources, which limits the number of batteries that can be managed by the battery management system. At the same time, when the battery management system is actually installed, since the two ends of the battery need to be wired to the acquisition module, there will be more wiring, which makes the battery management system inconvenient to install and the complexity of the electrical wiring of the electric vehicle. In order to improve the above shortcomings, this paper proposes a new photoelectric relay control strategy. The connection method of the photoelectric relay and the series battery pack is shown in Figure 1.

In Figure 1, E1, E2, ... En represent the battery group, and the double-pole switch K1, K2, ... Kn represents the photoelectric relay group. By turning on K1, K2, ... Kn separately at different times, the voltage measurement of the single battery E1, E2, ... En can be realized. The on and off of the photoelectric relay group is controlled by a shift register array composed of D flip-flops in series. It only needs two I/O ports to provide a clock signal (CLK) and a data signal (D) respectively to work, which greatly reduces the occupation of I/O resources. In actual design, a D flip-flop and a pair of photoelectric relays constitute a gating module. One battery corresponds to one gating module, so the gating module is directly installed on the battery, and the gating modules are connected in series with flat cables to form a gating circuit controlled by a shift register array. The gating circuit and the voltage acquisition circuit are also connected with flat cables, and the number of wires required is very small, so the battery management system is easy to install and the electrical wiring is simple and clear.

The battery temperature is measured using the DS18B20 temperature sensor from DALLAS. DS18B20 uses single bus technology, with a temperature measurement range of -55°C~+125°C, full digital temperature conversion and output, and supports multi-point networking function to achieve multi-point temperature sampling. It should be noted that the DS18B20 multi-point networking function can also be used to achieve single-cell battery temperature sampling, but multi-point sampling requires the identification of each DS18B20's unique ROM code, which affects the sampling speed. At the same time, it is impossible to associate the ROM code with the actual physical location of the device, so the multi-point networking function is not suitable for single-cell battery temperature inspection. Based on the channel selection function of the shift register array composed of D flip-flops, this paper proposes a DS18B20 multi-point temperature sampling method that starts simultaneously and reads data in time. In this method, the sampling start and data reading of DS18B20 are performed by skipping the ROM code check. The connection method of DS18B20 is shown in Figure 2.

In the figure, K1, K2, ... Kn represent photoelectric relays, and their on-off status is also controlled by the shift register array. At the beginning, K1, K2, ... Kn are all closed, and the MCU sends a sampling start command to all DS18B20. After the start command is sent, all photoelectric relays are disconnected, and then K1, K2, ... Kn are closed one by one to read the temperature data of the corresponding sensor to realize time-sharing data reading. The multi-point temperature sampling method of starting time-sharing data reading at the same time is adopted. The time used is only more than the time used for single-point temperature sampling. The time for data reading, so its sampling speed is relatively fast.

[page]3. Shift register array principle

The shift register array is composed of D flip-flops connected in series , which together with the photoelectric relays form a gating circuit . In Figure 3, D1, D2, ... Dn represent D flip-flops. The output Q of each D flip-flop is the data signal of the next D flip-flop, and all D flip-flops are controlled by the same clock signal. The inverse output Q of the D flip-flop is used to control the on and off of the corresponding photoelectric relay. When Q is high, the photoelectric relay is disconnected, and when Q is low, the photoelectric relay is turned on. By controlling the data signal of the first D flip-flop, the Q of D1, D2, ... Dn can output low levels one by one, that is, the shift function, thereby controlling the photoelectric relays K1, K2, ... Kn to be closed one by one in sequence, realizing the channel gating function.

The working timing diagram of the shift register array is shown in Figure 4. Among them, CLK is the clock signal, D is the data signal of the first D flip-flop in the shift register array, and Q1 is the D flip-flop that is triggered by the rising edge. At the first rising edge of the clock signal, D is set to a high level, and the output Q1 of the first D flip-flop is a high level during the time period of the first rising edge and the second rising edge of the clock signal, and Q1 is a low level. Next, D is set to a level, and each time the rising edge of the clock signal arrives, the high level state of the output Q of the D flip-flop will be transmitted to the next D flip-flop in turn, that is, the Q end of the D flip-flop of the shift register array outputs a high level in different time periods in turn, so that Q1, Q2, ... Qn output a low level in turn. Under the control of Q1, Q2, ... Qn, the photoelectric relays K1, K2, ... Kn are closed in turn.

4. Measurement Program Design

The measurement program is written in C language according to the modular method and is divided into three modules: channel selection, A/D sampling and temperature sampling. When designing the battery management system, these three modules can be easily transplanted into the system program of the battery management system to provide the battery management system with voltage and temperature data. The flow chart of the measurement program is shown in Figure 5.

V. Experimental Results and Analysis

Take a power lithium battery of an electric motorcycle for experiment, the capacity of the battery is 40AH. During the experiment, the battery is discharged at 0.15C (6A), and the voltage and temperature of the battery are monitored in real time. In the process of the battery voltage dropping from 3.9V to 2.7V, the measurement value is read every 40mV. At the same time, a five-and-a-half-digit voltmeter and a high-precision thermometer are used to measure the voltage and temperature, and the measured values ​​are regarded as the actual voltage and actual temperature. The experimental results are shown in Table 1. The experimental results show that the average error of voltage measurement is less than 10mV, and the average error of temperature measurement is less than 0.1℃. It can be seen that the method proposed in this paper can accurately measure the voltage and temperature of single cells.

VI. Conclusion

The single cell voltage and temperature measurement method based on shift register array control proposed in this paper can realize the voltage and temperature inspection of the series battery pack, and the number of inspected batteries can be flexibly increased and decreased. Compared with other measurement methods, it has the advantages of simple and clear structure and convenient installation. It can provide accurate technical parameters for battery management and has broad prospects in the field of battery applications.