Abstract: A smart maintenance system, or battery activation device, for large-capacity nickel-cadmium (NiCd) battery packs was developed using a Siemens miniature PLC as the control center, a 5.7-inch touchscreen as the operation display platform, thyristor rectification, and active inverter technology. This system overcomes the "memory" effect caused by long-term use of NiCd batteries, significantly improving battery lifespan. Keywords: Battery activation, PLC, analog PI, NiCd battery, "memory" effect. Large-capacity NiCd batteries, with their advantages of high discharge current and long lifespan, have long held a crucial position in the military, railway, power, and petroleum industries. However, a significant drawback of NiCd batteries is the "memory" effect, which greatly reduces battery efficiency and, in severe cases, prevents the battery pack from discharging. For example, battery packs composed of GN300Ah NiCd batteries, widely used in railway passenger cars, require maintenance and repair during maintenance, primarily focusing on capacity restoration. To meet the technological requirements of this work, with the support of the Railway Bureau's Science and Technology Commission, and in cooperation with professional manufacturers, a battery intelligent maintenance system was developed, utilizing a Siemens S7200 programmable controller as the control center and incorporating thyristor charging and discharging technology. This system features charging, discharging, and automatic maintenance (e.g., automatic three-charge-two-discharge) functions for nickel-cadmium batteries, with a maximum charging/discharging current of 300A and a maximum voltage of 500V. Coupled with a large-screen touchscreen interface, it offers advantages such as convenient parameter setting, automatic data storage, automatic data printing, and automatic charge/discharge conversion, significantly improving work efficiency, reducing the labor intensity of charging workers, and enhancing equipment quality and performance. The system is briefly described below: 1. System Hardware Composition: The entire system consists of two main parts: an automatic control and management center centered on the Siemens S7200 programmable controller, and a main circuit centered on a thyristor rectifier circuit. The system schematic is as follows: [align=center] Figure 1: System Block Diagram[/align] 1.1 Control and Management Center: Centered on the Siemens S7200 small PLC CPU-224, it has 14 inputs, 10 outputs, 12K program memory, and 8K data memory; the peripheral expansion EM235 is used as an analog input/output module, which has 4 analog inputs and 1 analog output, with A/D and D/A conversion accuracy of bipolar 12-bit precision; the system voltage and current are converted into 0-5V standard signals by voltage and current transmitters for PLC acquisition, calculation, and display. The human-machine interface is provided by Stepper Technology's eview 5.7-inch touch screen human-machine interface. The MT4300L and CPU224 exchange data via RS485 communication. The operator can set the charging and discharging current value, charging and discharging end voltage value, charging and discharging time, battery pack rest time, recording interval time, and other working parameters of this equipment; select the charging, discharging, charging-discharging, and charging-discharging-charging working modes according to actual needs; and automatically record 100 historical records for storage and review, with reports automatically printed out. The CPU224 receives switch signals from the external environment and analog and digital signals from the EM235, automatically judging the working process and status of the equipment. If an external AC fault is detected, the CPU receives a command to immediately stop all work and issue an audible and visual alarm signal to prompt the staff to carry out maintenance; the CPU uses an analog PI regulation program to output a 0-10V analog signal through the EM235 to control the trigger pulse of the BSC6F-1 digital thyristor trigger board, thereby controlling the conduction degree of the thyristor conduction angle α and controlling the output current and output voltage of the equipment, realizing the digitalization and intelligence of the equipment. 1.2 Charging Inverter Main Circuit: Mainly composed of an isolation transformer, a three-phase full-bridge combined thyristor, and a fully digital thyristor trigger board. The isolation transformer uses a Y/D-11 connection, and the thyristor trigger board uses a BSC6F-1 type digital trigger. The six-channel pulse symmetry does not require adjustment and has a phase sequence self-detection circuit. When a phase reversal or phase loss occurs, it automatically blocks the trigger pulses and issues an alarm signal. The trigger board itself has overvoltage and overcurrent protection and soft-start functions. During rectification operation, the thyristor conduction angle α operates between 0-150°; during inverter operation, the β angle operates between 30-90°, effectively preventing inverter failure. 1.3 Touchscreen Operation Interface: The MT4300L touchscreen is a 5.7-inch 256-color human-machine interface launched by Shenzhen Buke Electric. It can connect and communicate with multiple PLCs. This system exchanges data and controls data with a Siemens S7 PLC via RS485 signals and the PPI protocol. The use of the touchscreen completely eliminates the need for numerous buttons and indicator lights in previous control systems, and the addition of Chinese character display makes information display clearer and more intuitive. Figure 2: [align=center] Figure 2: Display Interface[/align] 2. Working Process and Characteristics: Battery activation, i.e., capacity recovery, consists of three stages. The first stage is deep discharge of the battery. The DC power from the battery pack is inverted and fed back to the grid using thyristor active inverter technology, which is the constant current discharge process of the battery pack. The battery pack voltage, discharge current, and discharge time are displayed through the MT4300L human-machine interface. The discharge current is generally 0.2C5 (C5 is the battery pack capacity), and the battery pack discharge termination voltage is 1×Nv (N is the number of batteries in the pack). The battery discharge process is actually a chemical reaction process. When the voltage of a single battery discharges to 1V, it is equivalent to deep discharge. Therefore, the battery pack cannot be recharged immediately after discharge, otherwise it will affect the battery pack's lifespan. The second stage is the battery pack resting process. The CPU224 automatically records the battery pack resting time. When the resting time reaches the set time (generally, the resting time should be set to 2 hours), it automatically enters the third stage, i.e., equalization charging of the battery pack. The equalization current of the battery pack is 0.2C5A. The entire equalization process consists of two stages: constant current charging and constant voltage current limiting. The entire process is coordinated and completed by the control center. Generally, the battery pack ends charging after 8 hours or when the voltage of a single battery reaches 1.65V. The entire activation process, from the start of discharge to the end of charging, is completed by the PLC control center without human intervention. To ensure good applicability, taking advantage of the flexible control features of the Siemens S7200 PLC, the equipment is equipped with four operating modes: charging, discharging, discharging-charging, and charging-discharging-charging. The number of cycles in the charging-discharging-charging mode can be set. For example, if the number of cycles is set to two, the equipment can automatically achieve "three charges and two discharges" for the battery pack. [align=center] Figure 3: Main Program Flow[/align] 3 Software Design Philosophy The Siemens S7200 PLC can be programmed using three languages: ladder diagram, statement list, and function chart. This system uses ladder diagram, the programming language that is closest to engineering design and easy to understand; the programming software is STEP7-MICRO/WIN V32. The entire program adopts a modular structure design, divided into seven subroutines: main program, initialization subroutine, charging subroutine, discharge program, historical data storage and retrieval subroutine, AD sampling subroutine, and DA output control. The touchscreen HMI MT4300L and CPU224 communicate via RS485 (PPI communication protocol) through the variable data memory VW for data exchange and control. The above four functional programs respectively incorporate analog voltage and current acquisition, calculation, display, and control programs. The ladder diagram mainly utilizes timer and mathematical logic operations. Because the activation device requires a constant current output, the CPU224 needs to perform analog PI calculations to output the current setpoint to the BSC6F-1 thyristor trigger board via the EM235 analog signal; that is, the actual current digital value (feedback value) acquired is compared with the setpoint current value set by the operator through the HMI. After corresponding mathematical calculations, the CPU outputs a 0-10V DC voltage signal via the EM235 analog signal. This signal is transmitted to the thyristor trigger circuit to achieve digital voltage regulation, thereby ensuring constant current charging and constant current discharging of the system. The ladder diagram program is as follows: [align=center] Figure 4: Ladder Diagram Program for Charging Current Adjustment[/align] In the ladder diagram program, VW302 is the voltage display value register, VW68 is the voltage setting value register; VW300 is the current display value register, VW66 is the charging current setting value register; VW632 is the analog-to-digital converter digital register. Q0.3 being closed indicates charging. When the battery pack voltage value VW302 is less than the setting value VW68 and the current value VW300 is less than the set charging current value VW66, the analog output value VW632 continuously increases until the charging current equals the set current value (the maximum value of VW632 is 31999); conversely, the value of VW632 continuously decreases. When the charging voltage is higher than the set voltage value, the analog output value continuously decreases to ensure that the charging output voltage does not exceed the set voltage value. 4. Conclusion This system was developed and put into trial use in 2007. After more than a year of field testing, it fully met the field requirements and began to be promoted and used both inside and outside the railway system, receiving unanimous praise from users. References: S7-200 Programmable Logic Controller Design Manual, SIEMENS, 2002; eView 4000 Series Touch Screen User Manual, Shenzhen Stepper Technology; Thyristor Control Trigger Board Principle, Xi'an Tiangong Power Electronics Research Institute; Power Electronic Converter Technology, Xi'an Jiaotong University, Huang Jun and Wang Zhaoan (eds.), 1993.