Lead-acid battery voltage automatic balancer-EEWORLD

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1 Introduction

Lead-acid batteries are widely used in post and telecommunications, railways, electricity and other communication departments. They are popular with users for their advantages of easy use, stable performance and low maintenance. However, in use, lead-acid battery packs are mainly used as backup power supplies and are in floating charge state most of the time. In this state, the energy loss caused by self-discharge of the battery pack is compensated during the floating charge process; when the DC power supply equipment fails, the battery provides power to the load to ensure the normal operation of the load. However, when the battery pack is in floating charge, the terminal voltage of the single cell will be unbalanced due to various reasons. If it is not handled in time, it will directly affect the life of the entire battery pack.

2 Analysis of unbalanced terminal voltage of single cell

When the battery pack is in floating charge, the charging equipment always gives a specific floating charge voltage and current value to the battery pack. In the long-term floating charge process, due to the influence of battery thermodynamic factors (electrolyte concentration, temperature, state of active substances, etc.) and kinetic factors (current density, battery structure, plate composition, etc.), the floating charge voltage of each single cell in the battery pack will inevitably be different, which is also a concentrated reflection of the working condition of each single cell. Batteries with high terminal voltage may have been fully charged. If they continue to be charged, most of the power will be converted into heat and lost, resulting in water loss of the battery, or even thermal runaway and damage; while for batteries with relatively low terminal voltage, normal floating charge current can no longer meet their needs, and large current must be added to catch up with the average floating charge voltage of the entire battery group. However, "equalized charging" cannot completely balance single cells with severely unbalanced terminal voltage. During "equalized charging", the charging equipment charges each battery in the battery pack at an average power level. If the charging current or charging time is not properly controlled, the difference between batteries may increase. Therefore, in order to fundamentally reduce the difference in terminal voltage of the battery pack, make it meet the requirements of the electrical equipment, and extend the life of the battery pack, it is usually necessary to manually handle each "laggard" battery separately. This is both time-consuming and labor-intensive. The current simple and feasible method is to install an automatic battery pack voltage balancer to ensure that the terminal voltage of each battery in the battery pack is always balanced during the entire working process, thereby reducing the maintenance of the battery pack and extending the working life of the battery pack.

3 System structure and working principle

(1) System structure

Figure 1 System structure diagram

The "lead-acid battery pack battery voltage automatic balancer" is composed of a charging circuit, a detection circuit, a comparison amplifier circuit, a control circuit (including temperature compensation circuit, drive circuit, protection circuit, A/D converter, etc.) and a real-time display circuit (clock, storage and printing, data and graphic transmission, and real-time alarm, etc.), as shown in Figure 1.

The charging circuit is an AC input charging device with a single-chip microcomputer as the core management. When the machine is turned on, it automatically detects the number of batteries in the battery pack, and can automatically complete the highest charging voltage setting according to the number of connected batteries, and charges the battery with a three-stage charging method of "constant current-constant voltage-floating charge". The three-stage charging design can avoid overcharging or undercharging of the battery due to failure to turn off the charger in time. The detection circuit monitors the technical parameters such as the capacity of the battery pack and the terminal voltage of each single cell in real time, and displays and transmits them to the control circuit through the display circuit. If the terminal voltage of the single cell is unbalanced, the voltage obtained by the detection circuit and the reference voltage are amplified by the error amplifier circuit, and the signal is transmitted to the control circuit to supplement the lagging single cell; and the single cell with a voltage higher than the floating charge voltage is discharged.

(2) Working Principle

The "Lead-acid Battery Cell Voltage Automatic Balancer" is a battery pack measurement and maintenance device with a high automation standard. When working, it charges the battery pack at the set floating charge voltage and floating charge current. When it detects a lagging battery in the battery pack, it will promptly provide supplementary balance. If the voltage of a battery lags too much, the built-in charging circuit (small charger) will charge the lagging battery separately in addition to floating charge. At this time, the current flowing through the battery is: I=Ifloat+Icharge. The total floating charge current in the battery pack remains unchanged and remains at the set value; in the battery with a voltage higher than the floating charge voltage, the discharge circuit is started. At this time, the current flowing through the battery is composed of two parts: Itotal=Idiv+Icell. The total current is composed of the current passing through the shunt and the current passing through the battery plate. We set the starting point of the battery shunt voltage at 2.25V and the shunt termination voltage at 2.30V. When the battery terminal voltage is between 2.25V and 2.30V, the balancer will bypass the power above 2.25V to keep the battery terminal voltage at 2.25V. Therefore, the device can play the role of shunting and supplementary charging, which can not only effectively prevent normal batteries from being damaged by overcharging, but also actively and automatically supplement the "lagging" batteries with appropriate amounts, so that the terminal voltage of the lagging batteries reaches the normal value, thus keeping the terminal voltage of the whole group of batteries consistent.

The signal collection and signal processing are completed by the single-chip microcomputer. The battery voltage signal received by the analog signal collector is converted by A/D and sent to the single-chip microcomputer for identification and quantization processing, and the processed signal is sent to the execution circuit to execute: balance the battery terminal voltage; control the connection and disconnection of the small charger; display the battery capacity; store and print various battery parameters in real time, etc.

(3) Temperature compensation

When the ambient temperature changes, the battery output capacity and terminal voltage will change accordingly. At this time, the battery float charge voltage and charging current need to be adjusted accordingly, otherwise it will cause battery damage and affect the battery life. This device is designed with a circuit that automatically adjusts the float charge voltage as the temperature changes. The control circuit can make the battery pack automatically adjust the float charge voltage as the external ambient temperature changes, in order to compensate for the change in battery terminal voltage caused by the change in ambient temperature.

4 Main control program

The software adopts modular design. The main program consists of initialization program, key scanning and analysis program. After booting, the working state is controlled by key input, and the terminal voltage of each battery is inspected and displayed. The corresponding subroutines are called respectively, as shown in Figure 2.

5 Main technical indicators

Balance voltage: 2.25V±0.01V
Discharge current: 0.3 A～5A
Charging current: 3A～6A
Total voltage measurement accuracy: 0.001V
Communication interface: RS232
Machine weight: about 3.5Kg
Volume: 88×265×480mm

6 Conclusion

"Lead-acid battery pack battery voltage automatic balancer" is a comprehensive measurement and control instrument. It not only has the automatic balance and control function of the battery terminal voltage, but also has the unique functions of active supplementary charging and discharge. It is mainly for users such as troops, mobile communications, power and telecommunications, and is particularly suitable for automatic maintenance and maintenance of batteries in base stations in high mountains and remote locations, islands and other unmanned base stations.

References:

[1] Wang Guohua. Power management technology for portable electronic devices [M]. Xi'an Electronic Science and Technology Press, 2004, 127-129.

[2] Gui Changqing. Online diagnosis technology of sealed lead-acid battery pack for communication [J]. International Power Supply Business, 2004.10.

[3] Hou Zuqing. Intelligent flow inspection instrument based on 8089 single chip microcomputer [J]. Electrical Age. 2000, 6.

Reference address：Lead-acid battery voltage automatic balancer

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