Application design of isolation transmitter in battery charge and discharge monitoring

1. Introduction

As a common device for providing DC power, batteries are increasingly used, especially in power automation equipment, rail transit, communications, offline solar photovoltaic power generation systems, and military applications that require uninterrupted power supply. Battery voltage and charge and discharge current are important indicators of battery performance, and are also important parameters for judging charge and discharge. Therefore, monitoring its voltage and current is an indispensable part of reasonable battery charge and discharge. The traditional detection method uses tools such as voltmeters to measure the voltage value and then adjust the charging parameters. There are problems such as large human error in recording, poor real-time performance, poor accuracy, unscientific recording, and high detection costs. The introduction of microcomputer technology has led to the gradual intelligence of battery charge and discharge monitoring systems, realizing real-time monitoring of battery working conditions. However, due to the presence of ground potential interference signals and AC interference signals introduced by charging power supplies, MCU resets, erroneous signal acquisition, and even direct burning of the main control chip, it is completely necessary to choose a powerful signal isolation and anti-interference solution. The following solution gives you a detailed introduction on how to use the isolation transmitter module manufactured by Guangzhou Jinshengyang to isolate interference signals, accurately transmit signals, and improve the stability and reliability of the monitoring system.

2. Introduction to Isolation Transmitter

The isolation transmitter module is a signal processing module that uses electromagnetic isolation to isolate and transmit the input non-standard industrial signals to standard industrial signals. Jinshengyang's isolation transmitter module adopts a modular and small-volume design. Users can use it directly without adding any peripheral circuits, which simplifies the user's circuit design, improves the space utilization of the PCB, and facilitates customers to analyze the cause of the fault and maintenance. Because the module uses efficient electromagnetic isolation technology inside, it has better temperature drift characteristics, linearity, accuracy, conversion speed and life than optocoupler isolation. The signal input and signal output, power input and isolated power output (optional) of the module are isolated from each other, and the isolation withstand voltage value is as high as 2500VDC, which can effectively isolate interference signals. Compared with the isolators on the market, it has a smaller size, lower cost, higher integration, and can be directly welded on the PCB board, which greatly simplifies the design and installation of signal acquisition and control systems, and improves the overall stability and reliability of the system.

Features:

No need for adjustment, ready to use

Modular & Small Size

Extremely low temperature drift (35PPM/℃)

High accuracy grade & high linearity (0.1% FS)

High isolation (2500VDC/60s between input, output and power supply)

High reliability (MTBF>500,000 hours)

Industrial grade (operating temperature range: -25℃～71℃)

3. Introduction to the principle block diagram of the isolation transmitter

An isolation transmitter is an electrical device that can convert the measured electrical signal into a specified electrical signal that is electrically isolated from each other in a linear proportional output. As shown in Figure 2, the isolation transmitter module is divided into four parts: power input part, isolated power output part (optional by the customer), signal input part, and signal output part. Its working principle is to first modulate and transform the measured signal through a semiconductor device, then perform electromagnetic isolation conversion through a signal transformer, and then convert it into the required signal at the output end. At the same time, the power supply of the isolated signal is isolated to ensure electrical isolation between the power supply end, the isolated power output end, the signal input end, and the signal output end, thereby effectively isolating signal interference and enabling the main control chip to work stably and reliably.

4. Application of Isolation Transmitter in Battery Charging and Discharging Monitoring System

When the battery is fully charged, it will directly supply power to the DC load at the downstream stage. The battery supply voltage and current will gradually decrease as the power is consumed. If the power is still not replenished after it drops to the minimum, the battery will easily be damaged due to over-discharge. On the other hand, when the battery is insufficient, the charger will charge the battery, and the battery voltage will gradually increase as the power is replenished. If the battery voltage reaches the limit and the charging is still not stopped, the battery will easily be damaged due to over-charging. Therefore, it is completely necessary to monitor the battery voltage and current in real time with high precision, otherwise the battery will easily be damaged due to over-charging or over-discharging.

Figure 3 below is the circuit diagram of the battery charge and discharge monitoring system we designed, which is mainly composed of a charger, power supply circuit, battery, DC load, voltage and current sampling circuit, EMC circuit, isolation transmitter, MCU, display circuit, alarm circuit, etc. The following is a detailed introduction to the functions of each part:

4.1 Control circuit power supply scheme

For the control circuit power supply, we chose the AC-DC module power supply LH05-10B05 manufactured by Goldensun, which is used to power the isolation transmitter module, AD converter, MCU and LCD. The module outputs 5V, 1A, meets the supply voltage and power requirements and has the advantages of global input voltage range, AC/DC dual-use, low power consumption, high efficiency, high reliability, and safe isolation. The product is safe and reliable, with good EMC performance. The EMC and safety specifications meet the international IEC61000, UL60950 and IEC60950 standards, and has obtained UL, CE and other international certifications.

4.2 Voltage and current sampling circuit scheme

For the monitoring of battery voltage, we know that common battery voltages are 24V and 12V. In order to facilitate voltage signal acquisition, we use voltage division to reduce the battery voltage to the range of 0~5V for signal processing; for the monitoring of charge and discharge current, we connect the shunt in series into the charge and discharge circuit. The shunt will convert the discharge current into a millivolt voltage signal for signal processing.

4.3EMC circuit solution

The design of EMC circuit is often the most easily overlooked but extremely important part of signal isolation processing. We know that the EMC environment in the power grid in industrial sites is extremely harsh, and there are various interference signals. Since the battery pack is directly connected to the charger and DC load through the same line, the interference signal of the power supply circuit (such as surge interference) will be coupled to the subsequent signal processing circuit along with the signal circuit, causing the signal processing device to malfunction or be damaged. Therefore, it is completely necessary to add an EMC circuit before signal isolation to absorb the interference signal.

4.4 Signal Isolation Solution

In actual applications, it is found that when charging and discharging the battery, if the signal is not isolated, the interference signal of the power supply circuit will be directly transmitted to the subsequent MCU through the signal path, causing frequent signal loss or even crash of the MCU. There are two conventional signal isolation methods, electromagnetic isolation and optocoupler isolation. Optocoupler isolation cannot enable the monitoring system to monitor the battery voltage and discharge current in real time with high accuracy due to its slow response speed and low transmission accuracy. Therefore, we need to choose an electromagnetic isolation solution. Here we choose the isolation transmitter module manufactured by Goldensun to isolate interference signals and accurately transmit signals.

In the battery voltage monitoring circuit, we selected the isolation transmitter module T6S6D-3 to isolate and transmit the input 0~5V sampling signal to 0~3V for the convenience of subsequent ADC acquisition; because the battery charging and discharging circuits use the same circuit, the voltage signal collected by the shunt in the battery discharge circuit is positive and negative. We selected the isolation transmitter module TM6S6AP-3 of Jinshengyang Company, which can convert the signal linearly varying in the range of -100mV to +100mV into a 0~3V voltage signal for subsequent direct acquisition by the ADC.

The advantages of using these two modules are: first, they can achieve a high degree of electrical isolation between the signal input and the subsequent sampling circuit (isolation voltage is 2500VDC/60S), which can effectively isolate the interference signal of the previous circuit so that it will not affect the normal operation of the back-end equipment; second, the transmission accuracy of the module is 0.1%FS, which can meet the high-precision sampling requirements of the 12-bit ADC and accurately monitor the working status of the battery; third, the isolation transmitter adopts a modular design, and can realize the isolation and transmission of signals that are not easy to collect by AD (such as 0~5V, -100mV~+100mV) to signals that are easy to collect (0~3V) without any peripheral devices, which greatly simplifies the circuit design.

4.5 Data processing, fault alarm and status display

The data processing uses an MCU with integrated high-precision ADC, and the status display uses a 7-segment common cathode circuit digital tube, which mainly displays the current collected voltage value and displays alarm information. When the voltage is lower than the set value or exceeds the set value, an audible alarm is issued.

V. Summary

Due to the country's large investment in energy conservation and environmental protection, batteries have achieved rapid development. New types of batteries have emerged one after another, and higher requirements have been put forward for battery charging, discharging and scientific management. It is necessary to understand the working conditions of batteries in real time so as to scientifically and rationally manage and improve the utilization efficiency and service life of batteries, which has also promoted the application of battery monitoring systems. However, due to the presence of interference signals in the power supply circuit of various application sites, the entire system may not be able to monitor the working conditions of batteries in real time, accurately and reliably, and even damage the expensive instrument equipment connected later, causing immeasurable losses. The isolation transmitter module manufactured by Jinshengyang has the characteristics of high isolation, high precision & linearity, high reliability and low temperature drift. It can effectively isolate interference signals, accurately transmit signals, and improve the stability and reliability of system operation.