18650 rechargeable battery lithium 3.7v 3500mah.Low temperature intelligent lithium battery management system design plan

　　Most of the current battery management systems are designed for applications with large-capacity battery packs and short battery life. The equipment served by this management system consumes a lot of power, the battery cycle time is short, and the power consumption of the management system itself is not low, so it is not suitable for Low power consumption instrument field use. A certain gas remote monitoring instrument has an average system current of only a few milliamperes and is required to operate continuously at low temperatures for more than 6 months. In order to meet the application of this project, this article introduces the design of a low-temperature intelligent lithium battery management system for 20Ah4 32-cell single cells are managed in series and in parallel. It has basic protection, power metering, charge balancing and fault recording functions. The experiment verified that the functional performance of the system was good and met the design requirements.

　　1. Overall structure of the system

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

　　Based on the consideration of low power consumption, many low-power devices are used in the design. For example, the processor uses MSP430FG439 low-power microcontroller; the voltage reference uses REF3325, which has an extremely low power consumption of only 3.9μA; the operational amplifier uses the operating current LT1495 with only 1.5μA; the digital potentiometer uses AD5165 with quiescent current as low as 50nA. For intermittent working circuits with large operating current, a power management circuit is 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.

　　Each collection cycle collects the voltage of each group of cells, and the processor issues instructions to the protection execution circuit based on the voltage to perform corresponding protection actions. The balancing circuit is implemented with a single-chip microcomputer and a transistor, replacing a dedicated balancing chip. The system will record the maximum value of voltage, current and temperature, battery usage time, remaining power and other abnormal information in the storage device. The processor provides a TTL communication interface, and the on-site computer can read the logs in the storage device through a TTLRS232 conversion module. In order to prevent the MCU from crashing and other abnormalities during the charging process, protection failure occurs. A secondary protection circuit is added. If the voltage exceeds the preset value, the secondary protection circuit will be activated and the three-terminal fuse will be blown to prevent the accident from happening.

　　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 discharge control switch. The corresponding protection action is made by controlling CH and DISCH. The circuit diagram is shown in Figure 2.

　　CH and DISCH are set to low level during normal operation, and both M1 and M2 are turned on at this time. When a discharge overcurrent or overdischarge state occurs, DISCH is set to high level. At this time, Q2 is turned off and Q3 is turned on, which quickly discharges the charge of the M2 gate capacitor so that M2 can be turned off instantly to complete the protection. When a charging overcurrent or overcharging state occurs, set CH to high level and turn off M1. The MOSFET in the circuit uses IRF4310, which has an on-resistance of only 7kΩ and a current flow capacity of up to 140A.

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