Design and implementation of a hybrid battery monitoring module

Distributed battery monitoring systems have the advantages of wide application and scalability; CAN bus has the advantages of high transmission rate and good reliability. Combining the two, the typical battery monitoring and management system structure is shown in Figure 1.

The remote data acquisition unit is the battery monitoring module.

Functional definition of monitoring module

The premise for realizing the function is that it is achieved without affecting or with negligible impact on battery performance. Without this premise, the design of the monitoring module will lose its meaning, because in actual applications, multiple batteries are often connected in series, and the failure of one battery will inevitably lead to problems with the entire battery pack.

The monitoring module will achieve the following functions under the above premise:

Accept control from upper controller;

Realize the collection of battery data and accurately reflect the physical parameters of the battery, such as voltage and temperature;

The collected data is transmitted to the upper-level controller to realize data sharing.

The physical properties that the monitoring module should achieve

When the sampling rate is >10khz,

Voltage sampling

The voltage acquisition accuracy is better than 0.5% at 25℃ and better than 1% at -40℃～85℃.

Temperature sampling

Temperature acquisition accuracy: ±2°C, -40°C~85°C.

Monitoring module system structure

As shown in Figure 2, the dotted box part is the basic structure of the monitoring module.

[page]The basic structure consists of three parts: low voltage circuit, isolation circuit , and high voltage circuit. Each of them consists of three parts:

The low-voltage circuit includes: signal processing and conversion circuit, which processes the control signal from the vehicle, such as ignition signal, which can be designed according to different application requirements; power supply circuit and main chip circuit, which supply power to the low-voltage circuit and realize the control logic and data processing of the monitoring module; communication circuit, which communicates with the upper controller and the lower data acquisition part. The upper communication circuit adopts CAN protocol communication, and the lower circuit adopts SCISPI, I2C, etc.

The isolation circuit includes: control isolation circuit, which is used to isolate the low voltage from the high voltage control circuit; power isolation circuit, which is used to achieve isolation control from the low voltage power supply to the high voltage power supply; communication isolation circuit, which is used to isolate the high and low voltage communication circuit levels and ensure normal communication.

The high-voltage circuit includes: signal processing, which converts the battery module signals, temperature sensor and other signals of the battery pack into electrical signals that can be directly sampled; power supply and sampling circuit, which provide power for the high-voltage system, and sample the processed signals to convert analog signals into voltage signals; communication circuit, which sends out the processed digital signals and receives control instructions from the low-voltage circuit.

Design and implementation

The design and implementation of this monitoring module is illustrated by taking the collection of only voltage and temperature as an example.

Low-voltage circuit: MC9S08DZ60 is used to implement the control logic, and the power is provided by an external 12V battery; the power circuit is used to convert the battery voltage into +5V voltage, which is implemented using the chip TLE4275G; the communication circuit uses CAN and SPI communication methods.

Isolation circuit: Photoelectric isolation relay (PhotoMos Relay) is used to achieve isolation of control signals; power isolation can be achieved by using isolated power supply or by directly taking power from high voltage to control isolation of low voltage. The design adopts the latter to achieve this; the communication circuit uses optical coupler or communication isolation core to achieve communication connection between high voltage circuit communication circuit and low voltage circuit.

High-voltage circuit: Adopt op amp plus resistor and capacitor to realize the conversion and conditioning of voltage signal, adopt NTC thermistor plus voltage divider circuit to realize the conversion of temperature signal; adopt high-voltage to low-voltage power conversion chip to realize the power supply of high-voltage control circuit; adopt 12-bit A/D converter AD7888 with SPI interface to realize the conversion of analog signal to digital signal and transmit the data. The voltage conditioning circuit, temperature conditioning circuit and AD7888 application circuit are shown in Figure 3, Figure 4 and Figure 5 respectively.