Design and implementation of automatic charge and discharge controller for lead-acid batteries

    Abstract: This article introduces the composition of the acid battery automatic charge and discharge controller, gives the charge and discharge time flow and voltage control strategies, and briefly describes the software implementation method. Practical applications show that the use of hybrid fuzzy PID control can meet the requirements of fast charge and discharge control speed and high accuracy.

    Keywords: lead-acid battery charging and discharging machine fuzzy control PID control microcontroller

Lead-acid batteries are currently the main type of large-capacity batteries and are widely used in communications, transportation, electric power and other sectors. However, a considerable proportion of battery points are damaged due to unreasonable charge and discharge control. If the lead-acid battery is properly charged and discharged, it can work for 10 to 15 years. Nowadays, many batteries still use some simple charging and discharging equipment during the production and use process, which not only causes insufficient charging capacity, prevents the battery from exerting its maximum power effect, but also shortens its service life. To this end, we have developed an automatic charge and discharge controller for lead-acid batteries.

1 Charging and discharging method

The charging and discharging of the battery should be carried out strictly according to the requirements (especially float charging). According to the characteristics of the battery charging and discharging, it is most appropriate to use a computer to control the process.

A typical battery charge consists of four stages, as shown in Figure 1.

Each stage is charged with constant current or constant voltage, depending on the set values ​​of current and voltage Ib1, Ib2, and Eb2. In the first stage, charging is carried out at a constant current of Ib1, and when the voltage reaches Eb1, it switches to the second stage. In the second stage, constant voltage charging is performed at voltage Eb1. As the open circuit voltage of the battery increases, the current will gradually become smaller. When the charging current reaches Ib2, it will enter the third stage. The third stage is charging with Ib2 constant current. At this time, the battery voltage further increases. When the voltage reaches Eb2, it enters the fourth stage. The fourth stage is charging with Eb2 constant voltage. At this time, the current gradually becomes smaller and stops when the charging ampere-hour is reached.

The discharge of the battery is mainly to control the current to discharge at a given value and feed it back to the power grid through the three-phase fully controlled rectifier bridge. When the battery reaches the termination discharge voltage, it stops discharging.

2. Composition of automatic battery charging and discharging controller

The structure of the automatic battery charge and discharge controller is shown in Figure 2, which consists of a three-phase fully controlled rectifier bridge circuit, a trigger board, a computer control board, a current and voltage sensor and a filter circuit. The three-phase fully controlled rectifier bridge circuit is controlled by the trigger board and provides hundreds of amps of charging and discharging current to the battery pack. The trigger board is controlled by the computer control board and provides rectification or inverter trigger pulses for the three-phase fully controlled rectifier bridge circuit. The current and voltage signals of the charging and discharging process are measured using a current shunt and a voltage divider. After the signals are filtered, they are periodically sampled by the computer control board. The computer control panel, which is composed of the 16-bit single-chip computer 80C196KB as the core, includes a keyboard, a display circuit, a micro-printer, and a 12-bit adjustment output circuit. The 80C196KB samples the input signal through the embedded A/D channel to complete segmented curve control.

The output current of the three-phase fully controlled rectifier bridge is DC with ripples. The current and voltage sensor signals are filtered by a π- type filter. The output signal is used as the input of the hybrid fuzzy PID controller. The pre-filtered signal is sampled by the computer. for quick protection. The time constant of the filter is the period of the power frequency voltage. The control period of the fuzzy PID controller is smaller than the period of the power frequency voltage. If it is too large, it will cause the regulation to be out of control. The suppression and protection measures for overcurrent and overvoltage are completed by the computer quickly blocking the trigger pulse.

3 Control strategies

Due to fluctuations in the power grid and changes in surrounding loads, the charging current will change sharply within one or more cycles of the alternating current. The adjustment function of the automatic charge and discharge controller is to stabilize the charging flow or voltage. When the current is charged after deep discharge, the internal resistance of the battery is large at the beginning. As the charging voltage increases, the internal resistance becomes very small. Therefore, small voltage fluctuations in the power grid will also cause large changes in the charging current. This requires the charge controller to adjust quickly, otherwise it will cause tripping and even damage the rectifier circuit. In practice, we do not have a variety of control algorithms. To control the charge and discharge process of lead-acid batteries, it is more appropriate to use a hybrid fuzzy PID controller [1~3], as shown in Figure 3.

When the error e ≥ EP, the fuzzy controller is used for adjustment. E, EC, and U are the fuzzy language variables of the deviation, deviation change rate, and control amount respectively. The system uses appropriate control rules according to different states. Fuzzification is performed based on the current precise quantities e and ec, the fuzzy control variable U is calculated based on the fuzzy control rules, and the calculated fuzzy control variable U is made precise and added to the control object. Deviation and deviation change rate:

e(n)=y(n)-r(n) (1)

ec(n)=e(n)-e(n-1) (2)

Here, r(n) is the set value of the control system at time nT, y(n) is the output of the control system at time nT, e(n) is the deviation of the control system at time nT, and ec(n) is the deviation of the control system at time nT. Deviation change rate, T is the sampling period. The fuzzy subsets of E, EC, and U are defined as: E={NB, NM, NS, PS, PM, PB}, EC={NB, NM, NS, PS, PM, PB}.

The control rules of the fuzzy controller are in the form of conditional statements and are expressed as:

if Ei and Ecj then Uk.

Based on actual debugging and experience, fuzzy control rules were summarized, a total of 36 rules. The Mamdani inference synthesis algorithm is used, and the center of gravity method is used to make fuzzy judgments to calculate the precise output u. According to the above rules, all input combinations and status values ​​are calculated offline, and the control table is stored in E2PROM. In the fuzzy control process, the corresponding control table is checked according to e and ec to obtain the output control quantity.

When the error e

Δu(n)=Kp(e(n)-e(n-1))+Kie(n)+Kd(e(n)-2e(n-1)+e(n-2)) (3)

In the formula, Kp, Ki, and Kd are proportional, integral, and differential coefficients respectively. The output at time t(n) is:

u(n)=u(n-1)+ Δu(n) (4)

4 system software

The entire software is written in PLM96 language and is fully modularized. Use the 8 software timers provided by the system resources, set them to interrupt mode, and drive 8 events. Among them, the soft timer T0 is set to a 20 millisecond interrupt and is used to drive constant current or constant voltage control. Soft timer T1 is used to drive data processing and charge and discharge process curve control. Soft timer T2 is used to drive timing and accumulate charging and discharging ampere hours and time. The software also has fast protection function, soft start function and self-recovery function after suspension.

The application of the lead-acid battery automatic charge and discharge controller automates the charging and discharging process, which not only significantly improves the production quality and service life of the battery, but also saves about 5% of electric energy. After practical application and multiple corrections, the controller operates stably. Reliable and suitable for various industrial environments with heavy loads. It has been adopted by many manufacturers such as Luzhong Battery Factory and Zibo Battery Factory.