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 MSP430 FG439 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. For intermittent working circuits with large working current, power management circuits are added 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 issues 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 and M1 is turned off. 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