Abstract: This paper introduces a fast charging method for lead-acid batteries: variable current pulse charging. Based on DSP technology, a switching power supply-type charging device for coal mine locomotive batteries is designed. The principle of its main power conversion section is analyzed, and the hardware structure and control method of the charger are given. Experimental results show that variable current pulse charging can effectively eliminate battery polarization under high current charging, thus accelerating the charging speed, increasing charging efficiency, reducing battery gas evolution, and lowering temperature rise. Keywords: lead-acid battery; switching power supply; charger; pulse charging; variable current charging [b]0 Introduction[/b] The intelligent charger introduced in this paper is a high-frequency switching power supply type, which greatly reduces the overall system size and improves charging efficiency. The variable current pulse charging method allows the charging current to better approximate the acceptable charging current curve of the battery, thereby accelerating the charging speed, effectively protecting the battery, and extending the battery's cycle life. 1. Lead-acid Battery Charging Theory and Intelligent Charging Technology In 1967, American scientist J.A. MAS, based on the premise of the lowest gas evolution rate during battery charging, proposed the maximum acceptable charging current and the acceptable charging current curve (called the MAS curve) for a battery. As shown in Figure 1, the acceptable charging current of a battery at any charging moment is large at the beginning of charging, but it decays rapidly. This is because polarization occurs inside the battery during charging, hindering further charging. Battery polarization consists of three parts: ohmic polarization, electrochemical polarization, and concentration polarization. Ohmic and electrochemical polarization disappear when charging stops, while concentration polarization is eliminated more slowly, gradually decreasing and disappearing within seconds. Furthermore, according to the electrochemical theory of lead-acid batteries, when the charging current exceeds the battery's acceptable current, the excess electrical energy is used for the electrolysis of water, leading to the formation of bubbles on the battery plates, an increase in internal battery temperature, and ultimately, damage to the battery. Therefore, the current during charging must approximate the Mass curve as closely as possible. Common charging methods include constant current decreasing charging and pulse charging. In the late 1990s, Professor Chen Tixian of Xiamen University proposed the variable current intermittent charging method (see Figure 2) based on VRLA battery charging experiments. Its characteristic is that a stop-charge voltage is set in the variable current intermittent constant current charging segment. When the battery terminal voltage reaches the stop-charge voltage, charging is stopped for a period of time, and then the charging current value is gradually reduced. To restore the battery to a fully charged state, constant voltage equalization charging is used in the later stages of charging, with the charging current gradually decreasing until it reaches the trickle charge current and remains constant. If there is no change within the set time, the charging process ends. The intelligent charging method introduced in this paper is based on the variable current intermittent charging method, adding very short stop-charge intervals in each constant current charging segment. Thus, the constant current charging in each segment can be considered as a series of pulse currents with the same amplitude and pulse width (see Figure 3). This method is more conducive to eliminating the three polarization phenomena mentioned earlier. 2 System Composition [b]2.1 Main Circuit Design[/b] The hardware circuit structure of the intelligent charger is shown in Figure 4. The system adopts an AC-DC-AC-DC circuit structure. The input is three-phase 380V AC power, which is rectified by a three-phase bridge to obtain a DC voltage of 486~530V, with a filter capacitor and a voltage equalization resistor added in between. The DC-AC conversion part adopts an H-bridge conversion circuit. The selection of power switching device IGBT is as follows: (1) withstand voltage value, Voc=537V, leave a margin of 2 times, take vcEs=1200V; (2) on-state current value, =52A, take, =100A; (3) switching frequency is 30~40kHz. Therefore, the DB-FF100R12KS4 series IGBT module of EUPEC is selected. The module integrates two IGBT power transistors, and a protection diode is connected in parallel on each power transistor. The IGBT power transistor's on and off states are controlled by the drive signal generated by the PWM generator SG3525, thereby controlling the output voltage and current. The secondary output of the high-frequency transformer uses a full-wave rectifier circuit, which, after filtering by inductors and capacitors, charges the battery. The charger has a maximum output current of 80A, a maximum output voltage of 280V, and a maximum power of 22.4kW, making it a high-power charger. 2.2 Control System Design (1) DSP Chip 2407 The charger's control system uses a DSP chip, specifically the TI TMSLF240X series 2407 chip. This series of DSPs provides 32K words of FLASH program memory space, up to 1.5K words of data/program RAM, 544 words of dual-port RAM, and 2K words of single-port RAM. It contains two event manager modules, EVA and EVB, each including two 16-bit general-purpose timers and a 16-channel 10-bit A/D converter. Externally, it is equipped with sampling circuits (battery terminal voltage, charging current, and battery temperature, etc.), output control circuits, and EEP. ROM read/write circuit (reads and stores important charging parameters), keyboard scanning circuit and SCI serial communication circuit (used for host computer control and online communication), etc. The DSP is also connected to the display driver chip T6963C through a parallel line. Users can easily browse the menu, set charging parameters and control the entire charging process through the human-machine interface composed of keyboard and display. (2) Sampling circuit In this system, the DSP is responsible for sampling multiple analog quantities such as output current, battery terminal voltage, DC bus voltage, high frequency transformer temperature and battery temperature through the sampling circuit. Among them, the charging current, battery terminal voltage and battery temperature values ​​are displayed on the display screen in real time so that users can know the charging parameter values ​​and the stage of the charging process in a timely and convenient manner; at the same time, the DSP determines whether the system should be in working state (referring to the charging state of the battery) by detecting each temperature value. When any temperature value exceeds the allowable value, the system stops immediately. In addition, the two feedback quantities of charging current and battery terminal voltage and the output setpoint of the DSP constitute current and voltage closed-loop control. Their comparison value is used as the input signal of the PWM controller through the PI regulator. (3) PWM Control Chip SG3525 The PWM control chip is used to output the signal controlling the power transistor's on/off state. The SG3525 chip from Silicon General Corporation of the United States is selected in this control circuit. The SG3525 consists of a 5.1V output reference regulated power supply with a temperature coefficient of 1%, an error amplifier, a sawtooth wave oscillator with an oscillation frequency of 100-400Hz, a flip-flop, and a protection circuit. It can output two drive signals with equal duty cycles and a phase difference of 180°. After the output of the DSP chip passes through the voltage and current closed loop, it outputs two signals. After being amplified by the IGBT integrated driver chip M57959, the signals are transmitted to the gate of the IGBT and control the IGBT power switching transistors in the diagonal positions on the H-bridge inverter circuit. 2.3 Software Design The charger's software program is developed under the CCS2 (C2000) development system. The program is written in C language and adopts a modular programming method. The main program of the entire system is shown in Figure 5. The charger's software program provides users with a rich set of function menus. After entering the operating interface, users can select the charging method and set the charging parameters for each charging stage according to the different batteries, including: starting charging current, stopping charging voltage, charging time, variable current coefficient, and pulse duty cycle. These important parameters can be saved to EEPROM for future charging of the same battery. Depending on the specific charging operation, users can increase or decrease the charging current during charging via the keyboard. 3. Experimental Results In the charging experiment, the locomotive batteries used were provided by Huainan Guqiao Coal Mine. Each locomotive used a battery pack consisting of 96 cells connected in series, with a voltage of approximately 192V after full charging. Depending on the specific conditions, the charger's initial charging current was 80A, the stopping charging voltage was 2.55V/cell, and the current reduction coefficient was 0.6. In the experiment, after 3-4 stages of constant current pulse intermittent charging, it switched to constant voltage equalization charging. Multiple charging tests showed that this intelligent charger can safely and effectively charge the locomotive lead-acid battery pack. The entire charging time from fully discharged to fully charged can be controlled within 14 hours. During the charging process, only a small number of bubbles emerge from the battery, and the battery temperature remains within a low range. A fully charged lead-acid battery can power a mining car for 9 to 10 hours of continuous operation. Compared with traditional charging processes, not only is the charging time shorter, but the energy utilization rate is also greatly increased. [b]References:[/b][1] Guo Jun, Cao Yilong. New type of charging device for mining locomotive batteries[J]. Coal Mine Electromechanical, 2007(1): 63-65. [2] Yang Weizhen, Ren Xiaofeng. Research on intelligent charging device based on 80C196KB control[J]. Coal Mine Machinery, 2004, 25(3): 32-33. [3] Zhao Yutang, Wang Xiye. Lead-acid battery charging technology[J]. Power Supply Technology, 2001(5): 375-377. [4] Chen Tixian. VRLA Battery Intermittent Charging Method with Variable Current [J]. Battery, 1998(12): 274-277. Click to download: Design of Intelligent Charger for Coal Mine Locomotive Batteries. Editor: Chen Dong