Circuit design of battery pack monitoring platform based on LTC6802

As environmental and energy issues become increasingly severe, electric vehicles and hybrid electric vehicles (EV/HEV) have become the focus of attention in today's world. The battery is the power link of EV, but its single-cell terminal voltage and capacity are relatively small. For example, the terminal voltage of the widely used lithium iron phosphate (LiFePO4) battery generally does not exceed 3.65 V, so multiple single-cell series and parallel combinations are often required to meet the needs of the vehicle [1-3]. For vehicle-mounted battery packs, a fully functional monitoring system is very necessary. At present, there are two major problems with domestic battery pack monitoring equipment: one is that the battery voltage detection accuracy is not high, and the other is that the implementation of battery pack balancing control is relatively complex. In response to these problems, this paper uses the newly launched battery pack monitoring chip LTC6802 from Linear Technology to design a hardware monitoring platform for lithium-ion battery packs. The functions designed and implemented by this platform include single-cell voltage/temperature detection, battery pack balancing, and distributed CAN communication.

1 Introduction to the battery pack monitoring chip LTC6802

LTC6802-1 is a chip specially used for battery pack monitoring. Each chip can detect the voltage of up to 12 single cells connected in series, with a total input voltage of up to 60V. It can detect the voltage of more single cells in series through a distributed bus structure or by directly connecting the chips in series. In addition, LTC6802-1 also has the following features:

(1) It has a 12-bit ADC with high voltage acquisition accuracy, up to 5~8 mV; (2) It has a passive balancing function, which can discharge overvoltage monomers through on-chip (or externally extended) MOSFET switches; (3) It has a 1 MHz serial communication interface that is compatible with SPI; (4) It has strong anti-electromagnetic interference capabilities.

In general, the LTC6802 has comprehensive battery pack monitoring functions, high chip integration, and high voltage acquisition accuracy. Its main application scenarios, in addition to EV/HEV, also include high-power portable device power management and backup battery pack system monitoring.

2 Design of Battery Pack Monitoring Platform

2.1 Overall structure of battery monitoring system

The overall structure of the battery pack monitoring platform is shown in Figure 1. This platform design adopts a distributed CAN bus structure. First, LTC6802 is used to realize the acquisition of single cell voltage and passive balancing control of the series battery pack; the main control chip is responsible for receiving the voltage acquisition information from LTC6802 and setting the relevant parameters of LTC6802. In addition, MCU is also used to realize the acquisition of battery pack node temperature and current; finally, MCU sends the configuration information of the battery pack to the CAN communication network.

Figure 1 Overall structure of the battery pack monitoring platform

2.2 LTC6802 and MCU connection circuit design

The peripheral circuit of LTC6802 and the connection circuit between it and the microcontroller are shown in Figure 2. The MCU selected in this circuit is the Freescale series microcontroller MC9S08DZ60, whose main functions are to collect current and temperature, receive information from LTC6802 and send battery pack configuration information to the distributed CAN communication network.

Figure 2 Connection circuit between LTC6802 and MCU

LTC6802 can communicate with MCU through its own SPI-compatible serial interface. For LTC6802, CSBI is the chip select signal; SDO is the serial data output; SDI is the serial data input; SCKI is the serial clock input.

In addition, in order to ensure the stability and reliability of the communication process, an electrostatic interference suppression circuit is also introduced in this design, see D7-D15 in Figure 2. The circuit consists of 8 diodes and a Zener diode, and can actually be implemented using a dedicated ESD electrostatic protection device PRTR5V0U4D.

Another task of the MCU is to send the battery pack configuration information to the CAN communication network. In this design, the CAN isolation driver chip ISO1050 is selected, see U1 in Figure 2. In order to further improve the anti-interference performance of CAN communication, the transient voltage suppression chip PSM712 is also used at the CAN output end of the platform.

2.3 Voltage acquisition and equalization circuit design

The main function of LTC6802 is to detect the voltage of the cells in the battery pack and to control the balance under the overvoltage condition of the cells. LTC6802 has a 12-bit ADC, which can detect the voltage of up to 12 series cells. The voltage acquisition circuit outside the chip is also relatively simple. It only needs to connect the positive and negative poles of the cells to the corresponding cell voltage input terminals of the chip. In order to suppress the high-frequency noise in the voltage signal, an RC low-pass filter is added to the circuit. In addition, LTC6802 also has MOSFET drive output capability. The drive output terminal has a built-in 10k pull-up resistor, which can be used to drive external MOSFET.

For the cell n in the series battery pack, its corresponding voltage acquisition circuit and balancing control circuit are shown in Figure 3, where the upper figure is the voltage acquisition circuit and the lower figure is the balancing control circuit. In the figure, CELLn and CELLn-1 are connected to the positive and negative electrodes of cell n respectively; Cn and Cn-1 are the voltage acquisition input terminals of LTC6802; DCn is the MOSFET drive output terminal of LTC6802. When cell n is over-voltage, Q1 will turn on to discharge it, and the discharged energy will be consumed on resistor R1.

2.4 Temperature acquisition circuit design

The node temperature of the battery pack is also an important parameter in the configuration information. In this platform, the detection of node temperature is implemented by MCU. The design takes one node for each monomer, and a total of 12 nodes can be detected. The temperature acquisition circuit is shown in Figure 4, which shows the connection circuit of node 1. First, the thermistor RT103 is selected as the temperature sensor element in the design to convert the temperature signal into a voltage signal; then, the voltage signal is input into the analog switch device CD4067D, and the input signal can be selected by configuring its four control terminals ABCD through the MCU, and output from its common terminal, that is, pin 1; finally, the signal output by the analog switch is input to the AD input terminal of the MCU after RC filtering and limiting processing, and the node temperature acquisition is realized.

Figure 3 Voltage acquisition and balancing circuit

Figure 4 Temperature acquisition circuit

3 Conclusion

Based on the battery monitoring chip LTC6802 and the Freescale series microcontroller MC9S08DZ60, this paper designs a monitoring platform for series lithium-ion battery packs. Combining the characteristics of the chip and the application scenarios of the platform, the paper specifically designs the voltage detection circuit, the balance control circuit, the temperature acquisition circuit, the SPI communication and the CAN communication circuit. The platform makes full use of the characteristics of the LTC6802, such as high integration, high voltage acquisition accuracy and strong anti-interference ability, and greatly improves the problems of poor voltage acquisition accuracy and complex circuit structure in traditional battery monitoring circuits. It can be asserted that in the EV/HEV industry, this battery pack monitoring platform based on LTC6802 has strong application value and good application prospects.