Design of ultra-low power lithium battery management system

　　In order to meet the application of a certain micro-power instrument and improve safety performance, a design scheme for an ultra-low power lithium battery management system is proposed. The scheme adopts a bidirectional high-end micro-current detection circuit, combined with an open-circuit voltage and charge integration algorithm to realize power detection. A button battery is used instead of a DC/DC buck circuit to minimize power consumption. The system realizes basic protection, remaining power detection, fault recording and other functions. The lithium battery management system is verified on the instrument, and the results show that it has good stability and reliability, and the average working current is only 145μA.

　　With the rapid development of electronic technology, the application field of instruments and meters has been continuously broadened, and battery power supply has become an important choice. Battery management system is an effective guarantee for the safety of battery use. At present, most battery management systems are designed for large-capacity battery packs and short battery life applications. The equipment served by this management system has high power consumption, short battery cycle time, and the power consumption of the management system itself is not low. It is not suitable for use in low-power instrument fields. A certain gas remote monitoring instrument has an average system current of only a few milliamperes and is required to run continuously for more than 6 months at low temperatures. In order to meet the application of this project, this paper introduces a design scheme for a low-temperature intelligent lithium battery management system to manage 32 single cells of 20Ah 4 series and 8 parallels. It has basic protection, power metering, charge balancing and fault recording functions. Experiments have verified that the performance of various functions of the system is good and meets the design requirements.

　　1. Overall structure of the system

　　The low-temperature lithium battery management system mainly consists of several parts, including basic protection circuit, fuel meter, balancing circuit, secondary protection, etc., as shown in Figure 1.

　　Figure 1 Low-temperature lithium battery management system structure

　　Considering low power consumption, many low-power devices are used in the design, such as the MSP430FG439 low-power microcontroller as the processor; REF3325 as the voltage reference, which has an extremely low power consumption of only 3.9μA; LT1495 as the op amp with an operating current of only 1.5μA; AD5165 as the digital potentiometer with a quiescent current as low as 50nA, etc. Power management circuits are added to the intermittent working circuits with large working currents to reduce energy consumption.

　　The rated voltage of the low-temperature battery pack is 14.8V. It is composed of 4 groups of cells connected in series. Each group of cells contains 8 single cells. The normal working voltage is 2.5~4.2V. The voltage of each group of cells is collected in each collection cycle. The processor sends instructions to the protection execution circuit according to the voltage to perform the corresponding protection action. The balancing circuit is implemented with a single-chip microcomputer and a triode, replacing the dedicated balancing chip. The system will record the maximum value of voltage, current and temperature, the battery usage time, the remaining power and other abnormal information in the storage device. The processor provides a TTL communication interface, and the computer on site can read the log in the storage device through a TTLRS232 conversion module. In order to prevent the MCU from freezing and other abnormalities during the charging process, the protection fails. A secondary protection circuit is added. If the voltage exceeds the preset value, the secondary protection circuit will be activated to blow the three-terminal fuse to prevent accidents.

　　2. Hardware Design

　　2.1 Protection execution circuit

　　The protection execution circuit is the execution mechanism of the protection action. CH is the charging control switch and DISCH is the discharging control switch. The corresponding protection action is performed by controlling CH and DISCH. The circuit diagram is shown in FIG2 .

　　Figure 2 Protection execution circuit

　　CH and DISCH are set to low level in normal operation, and M1 and M2 are both turned on. When there is a discharge overcurrent or over-discharge state, DISCH is set to high level, Q2 is disconnected, Q3 is turned on, and the charge of the M2 gate capacitor is quickly discharged, so that M2 can be turned off instantly to complete the protection. When there is a charge overcurrent or overcharge state, CH is set to high level to turn off M1. The MOSFET in the circuit uses IRF4310, which has an on-resistance of only 7kΩ and a current capacity of up to 140A.

　　2.2 Balancing circuit and secondary protection

　　Figure 3 (a) shows a schematic diagram of a charging equalization circuit for a group of cells. The charging equalization circuit is composed of four such units connected in series. The voltage at the ADV terminal is collected by the single-chip microcomputer to obtain the voltage of the group of cells. If the voltage exceeds 4.2V during the charging process, the single-chip microcomputer control pin BLA is set to a high level. At this time, the group of cells is short-circuited, and the charging current flows through R4 to charge other groups of cells, thereby ensuring that the power of each group of cells has good consistency after charging is completed.

　　The secondary protection is irreversible and will only be activated in extremely critical situations. The circuit is shown in Figure 3 (b). BQ29411 is a secondary protection chip with a quiescent current of only 2μA. If the voltage of any group of cells exceeds 4.4V, OUT will output a high level, and the three-terminal fuse F3 will start to heat up. When the temperature exceeds 139℃, the fuse will blow.

　　Figure 3 Charge balancing and secondary protection circuit

　　3. Bidirectional high-end micro-current detection circuit

　　In the application of small signal detection with single power supply, the sampling voltage is very small and it is often restricted by the power supply rail of the op amp, making it difficult to complete the detection of small signals. This design uses a current high-end detection circuit to get rid of the limitation of single power supply on small signal detection. The high-end detection circuit uses the LT1495 ultra-low power op amp of Linear Technology. The circuit diagram is shown in Figure 4.

　　Figure 4 Current detection circuit

　　This circuit can realize the sampling and amplification of bidirectional small current and determine the direction of current. R9 is a sampling resistor. Considering that the current is large during short circuit, its resistance is generally very small. In this scheme, the resistance of R9 is set to 25mΩ. When the battery is in the discharge state, assuming that the direction of the loop current composed of the current source, R9 and LOAD is clockwise, DIR1 is low, DIR2 is high, M1 is cut off, and M2 is turned on. The current flowing through R4 is IR4=R9×IR9/R4, and the voltage signal at the output end of R5 is VCUR=R9×IR9×R5/R4. When the battery is in the charging state, the loop current is counterclockwise. At this time, the op amp U1 completes the amplification of the current signal, DIR1 is high, and DIR2 is low. When the battery is in the idle state and there is no current in the loop, DIR1 and DIR2 are both low. The logical state of DIR1 and DIR2 can be used to determine whether the lithium battery is in the discharge, charging or idle state.

　　4. Power supply design

　　The power supply design adopts a button battery to power the system, eliminating the DC/DC and LDO chips and reducing the power consumption of the buck chip. The circuit diagram is shown in Figure 5.

　　Figure 5 Schematic diagram of digital power supply

　　In the figure, R is a digital potentiometer, and the AD5165 of ADI is selected. Its adjustment range is from 0 to 100kΩ, and the static current is only 50nA. V1 and V2 are button batteries, and the MS920SE of Seiko is selected. This model supports a maximum current discharge of up to 800μA. When the acquisition time comes, adjust the resistance of the potentiometer according to the battery pack voltage value CELL4+, R= (R1+ R2) [(CELL4+)-3.6V)], close switches W1 and W2 and collect the voltage of POW_DET to determine the power of the button battery. If the anode voltage value of D1 is less than the charging threshold voltage, it means that the button battery voltage is too low, then disconnect W2 and adjust the digital potentiometer to charge the button battery with an appropriate current. When the next acquisition cycle comes, readjust the digital potentiometer R, close W1 and W2 and collect the voltage of POW_DET to determine whether the button battery is fully charged. If the anode voltage of D1 is greater than the charging completion threshold voltage, it means that the button battery is fully charged, and then disconnect W1 and W2. In this way, the charging regulation control of the button battery is completed. The 3.3V digital power supply is converted into an analog power supply through LC filtering.

　　5. Software Design

　　The software adopts modular design, which mainly includes 4 parts: initialization module, button battery power detection and control module, battery pack status detection and exception handling module, and power estimation module. The software flow chart of the battery pack status detection and exception handling module is given in this paper, as shown in Figure 6.

　　After the system collects various information of the battery pack, it will compare the current measurement value with the historical record value. If the current measurement value is determined to be the maximum or minimum value, the value will overwrite the historical value and be saved in the storage device. Each abnormal situation will also be recorded and saved. The PC on site can read the log in the storage device through the serial port to view the abnormal information.

　　Figure 6 Battery pack status detection and control software flow chart

　　SOC estimation uses an estimation method that combines open circuit voltage and ampere-hour integration. There are many factors that affect the accuracy of SOC estimation. Temperature, discharge current, number of cycles, etc. will cause errors. There is an SOC estimation formula:

　　Among them: SOC is the current charge, SOC0 is the charge in the initial state, C is the capacity of the battery, K is the correction factor, which is an empirical value. I is the measured instantaneous current, which is a negative value for charging and a positive value for discharging. In order to obtain an accurate SOC estimate, it is necessary to correct SOC0 regularly or irregularly when using the ampere-hour integration method.

　　The working current of a gas meter is relatively stable, and the power P=U×I is a fixed value. It can be seen from the formula that as the battery voltage decreases, the working current of the meter increases. In view of the slow change of battery voltage, the current sampling circuit in this scheme is set to sample once every 5 minutes to achieve the purpose of reducing power consumption. The nth sampling current in is regarded as the average current in this sampling period, thus we can get

　　The lithium battery management system can estimate the remaining battery life based on the current operating current and SOC.

　　Conclusion

　　Some low-power instruments have special requirements for battery life. This design aims at the application requirements of longer battery life. Through hardware and software low-power technology, a lithium battery management system for low-power instruments is designed, which can complete the management function of 4 series and 8 parallel 32 low-temperature lithium battery packs. After the operation test of a gas remote monitoring instrument, the lithium battery management system has good performance in all functions, and the working current is only 145μA, which is much lower than the existing lithium battery intelligent management system.