Design of monitoring system for series connected lithium-ion battery packs

Lithium-ion batteries, which have the advantages of high voltage, high capacity, long cycle life and good safety performance, have broad application prospects in portable electronic devices, electric vehicles, space technology, defense industry and many other fields. Power lithium-ion battery packs composed of several lithium-ion batteries connected in series are currently the most widely used. Due to the inconsistent voltage of each single cell, the battery is not allowed to be overcharged or over-discharged during use, and the battery performance and life are greatly affected by temperature. It is necessary to monitor the series lithium-ion battery pack to ensure that the lithium-ion battery is in good condition during use, or to immediately alarm when there is a problem with the battery during use, so that the power management system immediately takes protective measures and reminds relevant personnel to repair it. The single cell voltage and the temperature of the battery pack are the main technical indicators for distinguishing whether the series lithium-ion battery pack is working properly. Reference [1] uses a direct sampling method to store the single cell voltage to be measured on a non-capacitor for measurement. This method has a slow response time, large errors and complex control; Reference [2] uses an operational amplifier and a photocoupler relay to measure the single cell voltage of the series battery pack. This method has high requirements on the linearity of the optocoupler, resulting in high hardware costs. At present, the monitoring system of lithium-ion battery packs in series directly using integrated chips is favored, but the number of batteries in series is fixed in this method, resulting in inflexible application and high hardware cost. In this paper, a power lithium-ion battery pack monitoring system is developed to monitor the single cell voltage and battery pack temperature of the series lithium-ion battery pack online. When the single cell voltage deviates from the specified range, the monitoring system starts the alarm program to sound and light alarm; when the battery pack temperature deviates from the specified range, the monitoring system starts the fan or heating control circuit and stores relevant data to ensure the normal operation of the battery pack. The entire monitoring system has the characteristics of continuous measurement, simple and economical, high precision and high reliability.

1 Technology and solutions

1.1 System structure

The monitoring system for series-connected lithium-ion battery packs includes a core control module using a 51 series single-chip microcomputer, a lithium-ion battery pack status acquisition module, a signal conditioning module, and an alarm and processing system module. The monitoring system can form a distributed monitoring system with a PC through an RS485 interface, enabling one PC to monitor multiple series-connected battery packs. The system structure block diagram is shown in Figure 1.

The status acquisition module collects parameters such as the voltage of the single cell and the temperature of the battery pack, and then processes the measured signal, samples it through the A/D converter and transmits it to the microcontroller for data processing, and transmits the valid data to the local PC through the serial port. The monitoring personnel can understand the working condition of the battery pack by analyzing the status data, and deal with unsafe conditions in a timely manner to ensure its working reliability.

Figure 1 Structure diagram of the monitoring system for series lithium-ion battery packs

1.2 Common grounding issues for series-connected lithium-ion battery packs

There are many methods for measuring the voltage of a series lithium-ion battery pack. The simplest one is the resistor voltage division measurement method. The disadvantage of this method is that the drift error of large-value resistors and the resistor leakage current lead to low measurement accuracy and affect the consistency of the battery pack. Another more commonly used method is to use an isolated operational amplifier for each single cell, but it is large in size and expensive. It is suitable for occasions where high measurement accuracy is required and leakage current and cost are not considered. The design uses INA117 from Texas Instruments to solve the common ground problem of series lithium-ion battery packs [3]. The distortion of INA117 is 0.001%; ​​the minimum common-mode modulation ratio is 86 dB, and the common-mode input voltage range is ±200 V, which is suitable for high-precision measurement.

INA117 has built-in 380 kΩ, 20 kΩ and 21.1 kΩ resistors, so the external circuit does not need precision resistors, reducing the error and system complexity caused by precision resistors. Figure 2 shows the connection method of INA117 outputting the voltage of a battery. The voltage between pin 6 and pin 1 is the voltage difference between the two ends of a battery.

Figure 2 INA117 output voltage is the difference between the two input voltages

The detection system uses 16 INA117 to select the single cell voltage of 16 lithium-ion batteries. If their 1 pins are connected to the same ground, the 16 INA117s can have the same signal ground, and the A/D converter can perform sampling. The common ground is selected at the connection between the negative electrode of the 8th battery and the positive electrode of the 9th battery.

The maximum voltage of each lithium-ion battery is 5 V. As shown in Figure 3, the input potential of the first INA117's 3rd pin is up to 40 V. Similarly, the minimum input potential of the 2nd pin of the 16th INA117 is -40 V. The output voltages of the 1st to 8th INA117 are positive, and the output voltages of the 9th to 16th INA117 are negative, so both the multiple-select analog switch and the A/D converter are required to be able to input positive and negative voltages. The multiple-select analog switch uses MUX16, which is a 16-to-1 analog switch that can input positive and negative voltages. Therefore, only one MUX16 is needed for 16 batteries. However, due to the limited IO ports of the microcontroller, a 74LS154 is used in this article to expand the IO ports. Only the 4 IO ports of the microcontroller can control MUX16 to select a single lithium-ion battery for voltage sampling.

Figure 3. Common ground connection of 16 INA117s

1.3 A/D Converter

Monitoring the battery pack does not require a high sampling rate to sample the voltage of each battery cell. The sampling of the 16 battery cell voltages uses one A/D converter [4]. The measured voltage of each battery cell input is connected to the A/D converter through a multiple-select analog switch MUX16. According to the battery voltage update cycle and voltage requirements, the maximum error of the voltage conversion value transmitted by the A/D converter to the microcontroller is 10 mV. Maxim's MAX1272 is selected.

MAX1272 is a 12-bit serial analog-to-digital converter with fault protection and software-selectable input range. It uses SPI three-wire communication protocol, +5 V power supply, analog input voltage range 0 ~ 10 V, 0 ~ 5 V, ± 10 V, ± 5 V. It has an internal + 4.096 V reference voltage. When the internal + 4.096 V reference voltage is used, the digital output corresponding to the analog voltage input is ideally shown in Table 1.

Table 1 Ideal analog voltage input corresponding to digital output

From Table 1, we can see that the highest bit of the digital quantity output by MAX1272 is the sign bit, and the remaining 11 bits are data. Negative numbers are given in the form of complement code.

When the reference voltage is +4.096 V, 1LSB = 1.2207 mV.

The maximum quantization error of MAX1272, plus the influence of nonlinearity, offset and other errors, the total error is about 5 mV. INA117 has high precision, and under normal circumstances, the error is within 1 mV. Therefore, the combination of INA117 and MAX1272 can meet the requirements of the battery monitoring system of series lithium-ion battery packs with a voltage error of less than 10 mV. If higher voltage accuracy is required, a higher resolution A/D converter needs to be selected.

The circuit connection diagram of MAX1272 is shown in Figure 4.

Figure 4 MAX1272 circuit connection diagram

In Figure 4, the MAX1272 uses an internal reference voltage, and a 2.2 μF tantalum capacitor and a 0.1 μF ceramic capacitor are connected between pin 6 VREF and ground.

During PCB layout, both capacitors must be placed as close to the MAX1272 as possible.

1.4 Temperature monitoring

For series battery packs, traditional temperature measurement methods mostly use analog temperature sensors for measurement. During data collection and transmission, they are easily disturbed by the external environment, resulting in large errors in the measured results. In addition, when there are many measurement points, the wiring is more complicated. This paper uses a single-chip microcomputer and a single-bus digital temperature sensor DS18B20 to solve the above problems [5]. The principle is shown in Figure 5.

Figure 5 Block diagram of temperature patrol detection system

With external 5 V power supply, multiple DS18B20s can be connected to the bus, and accurate temperature conversion can be performed simultaneously without external drive circuits. Temperature measurement range: - 55 ~ + 125 ℃; Temperature measurement accuracy: The accuracy is ± 0.5 ℃ in the range of - 10 ~ + 85 ℃; During the temperature acquisition process, the microcontroller chip needs to send command words to the DS18B20, and also needs to read the temperature collected by the DS18B20. Therefore, the I/O of the microcontroller controller must be set to have bidirectional data transmission capability.

This detection system connects a DS18B20 to the bus for every lithium-ion battery, sets 8 temperature monitoring points, and detects the temperature of 8 points at the same time. In actual application, the microcontroller software determines the temperature value to be displayed: when the temperature is higher than 10 ℃, the highest temperature value among the 8 temperature points is displayed; when the temperature is lower than 10 ℃, the lowest temperature value among the 8 temperature points is displayed, achieving an effective and reasonable temperature monitoring effect.

1.5 Fan and heating control circuit

For the heat dissipation problem of the battery, a fan control circuit is designed to determine whether the fan is turned on or off by judging the measured battery temperature value. When the temperature is too high, the microcontroller will send a signal to turn on the fan.

The circuit is shown in Figure 6. When FAN is at a low level, transistor 9014 is not conducting, and the relay has no action. When FAN is at a high level, transistor 9014 is conducting, causing the relay contacts to close, and the fan starts to work under the power supply of 24 V power supply voltage.

Figure 6 Fan control circuit

For lithium-ion battery packs in series with complex application environments, in addition to considering the situation of too high temperature, the situation of too low temperature must also be considered. Because when the battery is running in an environment with too low temperature, the activity of lithium ions will deteriorate, the ability to embed and escape will decrease, and it will be easy to deposit on the surface of graphite crystals to form lithium metal. The formed lithium metal will react irreversibly with the electrolyte.

If lithium-ion batteries work at low temperatures for a long time, the capacity of the battery will drop significantly. Therefore, a heater control circuit is designed according to the needs, and the principle is the same as the fan control circuit.

2 Monitoring system performance

The actual test proves that the series lithium-ion battery pack monitoring system using INA117, 16-to-1 analog switch MUX16, MAX1272, 51 microcontroller and DS18B20 monitors 16 3.7 V lithium-ion batteries, and the voltage measurement error is completely within 10 mV. In terms of temperature, due to the high accuracy of DS18B20, the temperature error is within 1 ℃. The measurement of voltage and temperature meets the requirements, and the system operates reliably. When the voltage of any battery in the series lithium-ion battery pack is < 2.2 V, the microcontroller calls the mild alarm program to sound and light alarm, and informs the battery with problems.

When the voltage of any battery in the series lithium-ion battery pack is > 5 V, the microcontroller calls the serious alarm program to sound and light alarm. If the temperature value exceeds the allowable range of the preset temperature value, the series lithium-ion battery pack monitoring system will sound and light alarm. Both the fan and heating control circuits can start the control circuit normally according to the set temperature. When the temperature is lower than 5 ℃, the heating control circuit is started; when the temperature is higher than 50 ℃, the fan control circuit is started.

3 Conclusion

The series lithium-ion battery pack detection system uses a high common-mode rejection ratio differential op amp INA117 to solve the common ground problem. The monitoring voltage error is plus or minus 10 mV. If you want to further improve the detection accuracy, you can use a high-bit A/D converter. During detection, the lithium-ion battery is connected in series to the detection module, and the wiring must be correct. According to actual applications, several detection systems can be connected in series to detect more series lithium-ion battery packs, but it is necessary to ensure that the common-mode voltage does not exceed the maximum protection common-mode voltage range of INA117.

References

［1］ Tan Lei. Measurement of cell voltage in multi-cell battery pack［J］. Electrical Measurement and Instrumentation, 1999（11）：17-19.

［2］ Jiang Xinhua, Lei Juan, Feng Yi, et al. A new method for measuring voltage of series battery packs［J］. Chinese Journal of Scientific Instruments, 2007, 28(4): 734-737.

［3］ Sun Xiaozi, Lou Shuntian, Li Xianrui, et al. Principles and Applications of Analog and Mixed-Analog Devices［M］. Beijing: Science Press, 2009.

[4] Peng Mingjie, Zhong Hanshu. Series battery monitoring system [J]. Instrument Technology and Sensor, 2005 (5): 42-44.

[5] Fu Jinjun, Qi Bojin, Wu Hongjie, et al. Research on single bus temperature measurement technology for power battery pack management system [J]. China Testing Technology, 2004, 6 (30): 10-12.