Design of multifunctional battery charging system based on P89V51RD2

　　1 Introduction

　　Maintenance-free batteries (referred to as batteries) are widely used in electric bicycles, electric tour buses and uninterruptible power supply systems due to their ease of use, low price and high energy storage ratio, becoming the most popular electric energy storage device. Most of the current battery chargers charge in a constant current and constant voltage mode, without considering the impact of ambient temperature changes on the battery charging process, which affects the full performance and service life of the battery. In combination with the charging characteristics of the battery, the author uses the P89V51RD2 microprocessor as the control core to develop a multifunctional digital battery charger, which realizes the initial charging, activation, fast charging and normal charging of batteries below 36 V and within 100 Ah. At the same time, it automatically adjusts the charging termination voltage according to the changes in ambient temperature, realizing the intelligent charging process.

　　2 Battery charging characteristics

　　Charging a battery is a complex electrochemical process. There are many factors that affect the charging effect, and temperature is one of them. Figure 1 shows the charging characteristic curve of a new 12 V/100 Ah battery at different ambient temperatures with a standard constant current of 0.1 CA (CA is the rated capacity of the battery, in units of A·h). As can be seen from Figure 1, during the charging process, changes in temperature will have an important impact on the charging voltage. When the temperature is between 0℃ and 5℃, the charging terminal voltage will rise by about 2%, when it is between 10℃ and 25℃, the charging terminal voltage will rise by about 1.5%, and when it is between 35℃ and 40℃, the charging terminal voltage will drop.

About 1%; when the temperature is higher than 55℃, the charging terminal voltage drops by 5%. It can be seen that the constant voltage charging mode may not be enough in winter, and the battery may be overcharged in summer. Practice has also proved that the battery voltage decreases exponentially with time during the charging process. Even for batteries of the same model and capacity, their charging performance is very different due to different discharge states, usage and storage periods. Therefore, it is impossible to charge at constant current or constant voltage.

　　3 Main components

　　TLC2543 is an 11-channel high-speed A/D converter with a sampling rate of 200 kHz. Its input command format is shown in Table 1 and its operating timing is shown in Figure 2.

　　OCM2X8C is a 128x32 dot matrix LCD display module that can display Chinese characters and graphics. It has 8192 Chinese characters (16x16 dot matrix) and 128 characters (8x16 dot matrix). It can be directly interfaced with the CPU, provides 8-bit parallel and serial connection modes, and has multiple functions such as cursor display, screen shift, sleep mode, etc. Due to the limited number of microprocessor pins, the serial communication mode is used in the system. The functions of each pin are listed in Table 2.

　　P89V51RD2 is a high-performance microcontroller fully compatible with 80C51 MCU, with 64 KB Flash and 1024 bytes RAM integrated, providing 6 machine cycles and 12 machine cycles, with a maximum clock of 40 MHz. It supports ISP programming, PWM output, PCA programmable counter array and programmable watchdog timer, etc.

　　4 System Working Principle and Interface Circuit Design

　　The system is mainly composed of a microprocessor control system, Chinese LCD display, PWM charging output, A/D converter and keyboard scanning, and its structural block diagram is shown in Figure 3.

　　The microprocessor is the control core of the system. It simulates the working sequence of TLC2543, controls TLC2543 to sample the terminal voltage, charging current and temperature of the battery, completes A/D conversion, calculates and analyzes the sampling results, and then controls the PWM output circuit to change the charging current and adjust the charging terminal voltage. The interface circuit is shown in Figure 4.

　　The keyboard system has 4 buttons: the ON/OFF button is the start and stop control button for the charging process; the S button is the cycle function selection button, which mainly includes working mode, battery voltage, capacity, charging mode, time limit, etc.; the "+" key and "-" key are the working parameter adjustment keys. The working modes are divided into automatic and manual. The battery voltages are 6 V, 12 V, 18 V, 24 V, 30 V, and 36 V. The capacity items range from 2Ah to 120Ah, a total of 26 items. The charging modes are normal, initial charge, activation, and fast charge. The time limit function can set the start time and end time of charging, etc.

　　The microprocessor simulates the working sequence of TLC2543 and is connected to TLC2543 through P20, P21, P22, P23 and P24 to control the A/D conversion process. AIN0 of TLC2543 monitors the charging current of the first PWM output channel, and AIN1 monitors the charging terminal voltage of the first PWM output channel. AIN2 and AIN3, AIN4 and AIN5, AIN6 and AIN7 are used for the second, third and fourth PWM output channels respectively. AIN8, AIN9 and AIN10 are connected to AD590 temperature sensors through T_IN terminals respectively. AIN8 and AIN9 measure the battery charging temperature, and AIN10 measures the ambient temperature.

　　PIN1 of the PWM charging circuit is the AC16 V/30 A, AC33V/25 A and AC50 V/20 A power input terminal. The microprocessor automatically selects the appropriate AC power supply through relays J2 and J3 according to the terminal voltage of the different input batteries. The PWM pulse is output by CEX0 and drives the N-channel power MOSFET output through the TLP250 optocoupler. R6 is a charging current sampling resistor with a resistance of 0.1 Ω. IC2A constitutes an amplifier with a gain of 3.3, which amplifies the voltage of the charging current flowing through the sampling resistor and outputs it to the AINO terminal of TLC2543 for A/D conversion. When the charging current is detected to be too large, the PWM duty cycle is increased, otherwise the duty cycle is reduced. When the charging current is greater than 15 A, if the PWM control circuit has not been adjusted to the normal range in time, when the output level of IC2A is higher than 5.4 V, the 4.7V voltage regulator tube V7 will be broken down, and the transistor N4 will be turned on through V2, and the output power will be turned off through TLP250 and P1 to protect the power supply system.

　　The charging terminal voltage is divided by R2 and R9 and then transmitted to the AIN1 terminal. The charging terminal voltage is the main basis for judging the charging process. If it is lower than 13% of the nominal battery voltage, it is generally due to over-discharge or long storage time. An average pulse current of 0.1 CA is used for charging; when the charging terminal voltage is within ±13% of the nominal value, a charging current of 0.35 CA is used for fast charging; when the charging terminal voltage is close to or higher than the nominal +13%, the charging current gradually decreases. When the charging terminal voltage reaches the upper limit after temperature correction, the PWM duty cycle is changed to use a very small current for charging. The use of segmented pulse charging can improve battery performance and increase the battery charging acceptance rate.

　　The second PWM, the third PWM and the fourth PWM are the same.

　　The CEX4 pin, P3, N1 and Bl of the microprocessor P89V51RD2 form an independent power supply to power the TLP250 and drive the N-MOSFET output. The voltage is adjusted by software.

　　5 Software Design

　　The 5-channel PWM outputs share one PCA counter, with the same output frequency and independent duty cycles. The special counters related to PWM output include the PCA counting mode register CMOD, the counting control register CCON, the PCA counters CH and CL, the 5 module working mode registers CCAPMO_4 and the 5 capture counters CCAP0_4H and CCAP0_4L.

In this mode, when the counter CLWhen CCAP0L, CEXO=1; when CL=CCAPOL, CEX0 flips. When the counter CL changes from 255 to 0, the value of CCAPOL is reloaded by CCAPOH. Changing the value of CCAPOH can change the duty cycle of the PWM output. Therefore, the charging current, charging terminal voltage and ambient temperature fed back by the A/D converter continuously adjust the value of CCAPOH according to the optimization scheme to change the charging current.

　　The program is compiled and debugged with KEIL C51 Ver 6.12. The main program logic block diagram is shown in Figure 5.

　　6 Conclusion

Experiments have shown that the intelligent multifunctional battery charging system　　using segmented constant current charging method and PWM pulse charging technology, combined with the temperature characteristics of the battery, with the P89V51RD2 microprocessor as the control core has strong adaptability, can effectively improve the battery's charging acceptance rate, improve battery performance, and shorten charging time.