Power lithium-ion battery pack protection system based on "flying capacitor" technology

　　In recent years, more and more lithium-ion battery manufacturers have joined the research and development team of power lithium-ion batteries. Although power lithium-ion batteries have incomparable advantages over nickel-hydrogen, lead-acid and nickel-cadmium batteries in terms of specific energy, volume, life, environmental protection, etc., and its large-scale application is also a general trend, factors such as the cost and safety of battery packs still restrict the expansion of the power lithium-ion battery market. Lithium-ion batteries need to be equipped with electronic protection systems to prevent the battery from overcharging or over-discharging and exploding. However, since the materials and formulas used by various manufacturers to manufacture power lithium-ion batteries are not the same, the battery's overcharge and over-discharge protection voltages are varied. The existing single-cell or multi-cell lithium battery protection IC cannot meet so many voltage requirements. Therefore, this article will introduce a low-cost, reliable, and widely adaptable lithium-ion battery pack protection system to solve the current dilemma.

　　1 Lithium-ion battery pack protection system function

　　The lithium-ion battery pack protection system introduced in this article is designed for a 10-cell series-connected power lithium-ion battery pack, with the following functions:

　　a) Monitor the terminal voltage of each battery in the battery pack. Once it is found that a battery voltage is overcharged or over-discharged, the main circuit switch will be cut off.

　　b) The overcharge, over-discharge and other voltage protection values ​​of the protection system can be set arbitrarily by the user, and the setting range is 2.0V ~ 4.5V.

　　c) It has a disconnection protection function. The positive and negative poles of each battery have detection lines connected to the protection system. When the battery pack is broken due to vibration or other reasons, the protection system can immediately detect and cut off the main circuit. Currently, the design scheme based on single-cell or multi-cell lithium battery protection IC cannot perform disconnection detection.

　　d) With overcurrent and short-circuit protection function. When the protection system finds that the load is short-circuited or overcurrent occurs, it will immediately cut off the main discharge circuit until the load is removed, and then restore the main circuit switch.

　　2 “ Flying Capacitor ” Detection Principle

　　Figure 1 is a diagram showing the working principle of a "flying capacitor".

　　
       In the figure: K1 is a double-pole double-throw switch. When K1 is switched to the DC source signal Vi side, Vi charges C1. After a short period of charging, the terminal voltage across C1 should be the same as Vi. At this time, K1 is switched to the Vo side. Since the input impedance of the voltage follower is infinite and C1 has an energy storage function, Vo should be the same as the voltage across C1, thereby achieving the measurement of Vi. The "flying capacitor" measurement technology is suitable for measuring slowly changing DC signals. The power lithium-ion battery itself can be regarded as a capacitor with a large capacitance value, and its terminal voltage changes slowly. Therefore, this technology can be fully used for voltage measurement of power lithium-ion battery packs to effectively eliminate common-mode interference.

　　3 System Implementation Principle

　　FIG2 is a block diagram of a power lithium-ion battery pack protection system based on “flying capacitor” technology.

　　
       The system is divided into two sub-modules controlled by a single-chip microcomputer. Module 1 is responsible for managing the 7 low-end lithium-ion batteries, and module 2 is responsible for managing the 3 high-end lithium-ion batteries. The "grounds" of the two modules are independent of each other. The high-end and low-end modules control two pairs of switches in series (Ka1 and Ka2, Kb1 and Kb2) by outputting overcharge and over-discharge control signals to realize the control of the main circuit charging and discharging MOS tubes.

　　The single chip microcomputer used in the system is MICROCHIIP's PIcl6F676 single chip microcomputer, which has extremely low power consumption, 8 10-bit A/D conversion channels, 12 I/O pins, 1024 words Flash program storage area, and 60 bytes SRAM, which is very suitable for the detection and control of this system.

　　In module 1, since each switch has to withstand a DC high voltage of more than 30 V, the 4-channel switch switching array is implemented with a MAX309. MAX309 is a 4-to-1, dual-channel multi-way switch that realizes channel selection through site selection. Switch KA (1-4) is responsible for connecting the positive electrode of the battery to the positive electrode of the "flying capacitor" C1, and switch KB (1-4) is responsible for connecting the negative electrode of the battery to the negative electrode of the "flying capacitor" C1. The structure of the 3-channel switch switching array is similar to the 4-channel switch switching array, except that the number of channels is one less. When working, the microcontroller sends a channel site selection signal to connect the positive and negative electrodes of one of the batteries to C1, charge C1, and then disconnect the channel switch and connect to the switch of the follower amplifier. The microcontroller quickly detects the voltage of capacitor C1, thereby completing the voltage detection of a battery. If the detection voltage is found to be OV, it can be inferred that the battery may be short-circuited, over-discharged, or the detection line from the protection system to the battery is broken, and the microcontroller will immediately send a signal to cut off the main circuit MOS tube. Repeat the above process and the microcontroller will complete the detection of the battery managed by this module.

　　In module 2, since there are only three batteries to be managed, the voltage resistance of the multi-way switch only needs to be above 13 V, so the lower-priced MC14051 is selected for implementation. The control principle is the same as that of module 1.

　　The protection system is also equipped with an overcurrent and short-circuit detection comparator. By detecting the voltage drop caused by the current flowing through the discharge MOS tube and comparing it with the reference voltage, the microcontroller determines whether a short circuit or overcurrent occurs based on the output voltage of the comparator. After a delay period, if the overcurrent or short circuit still exists, the discharge circuit is immediately cut off until the overcurrent or short-circuit load is removed.

       4 Software Design

　　Since the battery voltage changes slowly, the battery voltage is monitored in an intermittent mode with a duty cycle of 1:9. Figure 3 is a processing flow chart of the main program. In the figure, except for the sleep process, the other processes are working processes. The working process only takes 1/10 of the time, while the sleep process takes 9/10 of the time.

　　
       The power consumption of the system when working is 200μA, and the power consumption when sleeping is 12μA, so the average power consumption of the system will be 30.8μA, and the power consumption of the system can be greatly reduced.

　　In addition, since the system is controlled by a single-chip microcomputer program, for power lithium-ion batteries with different voltage requirements, it is only necessary to modify the overcharge and over-discharge protection values ​​of the battery in the program before leaving the factory to adapt to different types of lithium battery protection requirements.

　　5 Conclusion

　　The monitoring of power lithium-ion battery packs is a brand-new topic. This paper proposes a design method for a lithium battery protection system with low cost, low power consumption, high reliability and wide adaptability. By adopting the "flying capacitor" technology, various protection functions such as overcharge, over-discharge protection and disconnection detection of the battery pack are realized.