Lithium iron phosphate battery collection system based on LTC6802

Abstract: Design a lithium iron phosphate power battery pack status acquisition system based on the battery monitoring chip LTC6802. The embedded microprocessor MC9S08DZ32 is used to collect the status data of 12 battery packs and upload it to the host through the CAN (Controller Area Network) bus. The overall structure and working principle of the system are introduced, and specific hardware and software design schemes are given.
Keywords: LTC6802; MC9S08DZ32; lithium iron phosphate power battery pack; monitoring system

Introduction
As new energy and environmental pollution issues are highly concerned in the process of global economic development, power batteries as clean energy are increasingly valued. Lithium iron phosphate power batteries have been rapidly applied in the fields of industry and transportation due to their long life, safe use, high current 2C fast charging and discharging, high temperature resistance, large capacity, no memory effect, and green environmental protection. The working status of the battery is a key link in the application, and the intelligent acquisition system of the power battery status based on embedded technology has also become a hot spot in technical research. Traditional battery working status acquisition chips support a small number of lithium batteries in series. For example, the bq2060 and bq2085, which are widely used in laptop battery cell working status acquisition, support up to 4 lithium batteries in series. Power batteries are mostly used as power sources, and generally require about 100 batteries in series. For example, pure electric bus batteries require 108 lithium iron phosphate batteries in series. If traditional battery working status acquisition chips are used, the number of chips required will be large, which will greatly increase the cost. Therefore, famous chip manufacturers in the world have begun to develop acquisition chips that support more than a dozen lithium batteries in series. The LTC6802 developed by Linear is a dedicated battery status monitoring chip that can measure the voltage of up to 12 batteries in series, complete all battery voltage detection in 13 ms, and the overall measurement error is less than 0.25%. It has the advantages of high electromagnetic compatibility and low power consumption.
This article uses LTC6802 to collect the voltage of 12 batteries in series. The microprocessor MC9S08DZ32 uses the SPI bus to read the voltage of the series-connected cells collected by LTC6802, and controls LTC6802 to balance any cell. At the same time, the built-in ADC is used to collect the surface temperature of the 12 series-connected cells, and the collected voltage, temperature and balance status of the 12 cells are uploaded to the upper control system through the CAN bus to monitor the working status of the 12 cells in real time. With this as the basic system, multiple basic systems are used to realize the status monitoring of the battery pack composed of a large number of batteries in series.

1 System composition and working principle
1.1 System composition
The series lithium iron phosphate power battery pack acquisition system studied in this paper can realize the online monitoring of 12 power batteries. The system includes the cell voltage acquisition and balancing part with LTC6802 as the core, and the temperature acquisition and CAN bus communication part with MC9S08DZ32 as the core, as shown in Figure 1.

1.2 Working Principle
The monitoring system uses LTC6802 to collect the voltage of the 12 cells connected to it, uses the SPI bus to complete the communication between MC9S08DZ32 and LTC6802, and reads the collected voltage value. The surface temperature of the 12 cells is obtained through the thermistor NTC attached to each battery shell, and the switch of the balancing circuit of the 12 cells is determined according to the voltage and temperature of the 12 cells read, and the corresponding register of LTC6802 is set through SPI to start and close the balancing. The entire system is centralized controlled by an embedded chip and online monitoring of the power battery pack.
2 System Hardware Design
2.1 Voltage Collection and Balancing Part
The voltage collection part is based on LTC6802. LTC6802 is a complete battery monitoring IC launched by Linear, which has a built-in 12-bit ADC, a precise voltage reference, a high voltage input multiplexer and a serial interface. Each LTC6802 can measure the voltage of up to 12 series-connected batteries with an input common-mode voltage of up to 60 V, and multiple LTC6802 devices can be connected in series to monitor the voltage of each battery in a long string of series-connected batteries. By using a unique level-shifted serial interface, multiple devices can be connected in a daisy-chain manner without the use of optocouplers or opto-isolators. Each battery input has an associated MOSFET switch for discharging overcharged batteries. The microcontroller can read data from the LTC6802 via the SPI bus and control the on and off of the MOSFET of the corresponding battery input to achieve cell balancing. In order to protect the LTC6802 voltage acquisition pin and prevent the voltage from exceeding the maximum input voltage, a 6.2 V voltage regulator is connected in parallel to each battery cell acquisition input port, and a resistor-capacitor filter circuit is added in front of each voltage acquisition pin, thereby effectively filtering out high-frequency interference and ensuring the correctness of voltage acquisition. The acquisition circuit of the lithium iron phosphate power battery pack acquisition system is shown in Figure 2.

2.2 Temperature acquisition and CAN bus communication part
The temperature acquisition part uses CD4067 to convert the temperature into voltage by resistor voltage division for acquisition by the built-in ADC of MC9S08DZ32. CD4067 is a digitally controlled multi-channel analog switch with the advantages of low on resistance, small off leakage current and internal address decoding, and the on resistance is relatively stable in the full input range. CD4067 selects one of the 16 pins to be connected to the common pin through 4 binary control pins A, B, C, D and 1 Inhibit pin. MC9S08DZ32 sequentially collects the 12-channel NTC resistor voltage division value by controlling the level state of A, B, C, D and Inhibit pins, and then calculates the corresponding resistance value through software, and then calculates the temperature value through the corresponding relationship between the NTC resistor resistance value and temperature. Since the resistance value and temperature value of the NTC resistor are exponentially related, the segmented linearization method is used in the software design to improve the accuracy of temperature acquisition.
This part is based on MC9S08DZ32. MC9S08DZ32 is a 32-pin 8-bit microprocessor from Freescale. It is small in size and powerful in function. It has 32 KB Flash memory and 2 KB EEPROM online programmable memory, supporting 8-byte single page or 4-byte double page erase partitions; it can perform programming and erase operations while executing Flash programs; it supports erase cancel operations and a maximum of 4 KB of random access memory (RAM). In addition, it has the following features: 24 channels, 12-bit resolution, 2.5μs conversion time, and internal ADC with automatic comparison function; the internal integrated CAN module supports CAN protocol V2.OA/B; supports standard and extended data frames; supports remote frames; has 5 receive buffers with FIFO storage mechanism and flexible receive identifier filters; the internal SPI supports full-duplex or single-line bidirectional; double buffer transmission and reception; has master-slave mode selection; supports high-order or low-order shifting.

MC9S08DZ32 communicates with LTC6802 through SPI bus, reads the collected voltage value, and determines the battery that needs to be balanced according to the voltage and temperature of each battery. Then, the corresponding register of LTC6802 is set through SPI to start and close the balance, and the voltage, temperature data and balance status are uploaded to the upper control system through CAN bus. The CAN chip used in this article is ISO105 0, which has high integration and is easy to use, simplifying the hardware design of the CAN bus communication part.

3 System software design
In software design, SPI communication between MC9S08DZ32 and LTC6802 is the most critical step. Because only by ensuring reliable communication can MC9S08DZ32 control LTC6802 to collect battery cell voltage and control the opening and closing of the balancing circuit. In order to realize SPI communication, the read and write timing of LTC6802 must be clear first. The read timing of LTC6802 is shown in Figure 3, and the write timing is shown in Figure 4.

It can be seen from the read and write timing that the high bit is sent first when each byte is sent. In the write timing, the logic state of the SDI pin is latched at the rising edge of SCKI. In the read timing, the logic state of the SDO pin is valid at the rising edge of SCKI. In addition, it should be noted that the clock phase and polarity requirements of LTC6802: the SPI interface of LTC6802 is configured to operate in CPHA=1 and CPOL=1 mode. According to the read and write timing of LTC6802 and the working mode of the internal SPI of MC9S08DZ32, the C language code of the MC9S08DZ32 SPI initialization function, the function of writing 1 byte of data to LTC6802, and the function of reading a group of data from LTC9802 can be written.
(1) MC9S08DZ32 SPI initialization function

(2) Write 1 byte of data to LTC6802

(3) Read a set of data from LTC6802

MC9S08DZ32 mainly implements a series of advanced read and write controls on LTC6802 by calling these three most basic functions. The overall process of the monitoring software is shown in Figure 5. First, the system clock is initialized, and then the CAN, SPI, ADC and various parameters of LTC6802 integrated in the microcontroller are initialized. The 12 battery voltages and balancing states are read from LTC6802 in a loop, and the ADC is used to read the temperatures of the 12 batteries. The working state of the balancing circuit of the battery cell is set according to the voltage and temperature, and the data is uploaded to the upper-level controller through the CAN bus to realize the monitoring function.

4 Conclusion
This paper uses Freescale's MC9S08DZ32 and Linear's LTC6802 to design a lithium iron phosphate power battery pack status monitoring system. The system can collect the voltage and temperature of 12 series-connected power battery cells, balance the cells, and upload the voltage, temperature and balance status of the cells to be tested through the high-speed CAN bus. After actual testing, the system's voltage acquisition accuracy is ±4 mV, and the temperature acquisition error is ±1℃. The system works stably and reliably and has important application value.