Application of LTC6802 in lithium battery pack balancing circuit

1. Working principle of equalization circuit

The battery pack balancing circuit designed in this paper based on the LTC680221 lithium battery pack management chip consists of four parts: power supply system, embedded processor, LTC680221 data acquisition and balancing circuit. The circuit block diagram is shown in Figure 1.

The power supply system is the power source of the balancing circuit. The power is taken from the lithium battery pack and provided to the low-power embedded processor and the LTC680221 chip. The embedded processor is the core of the balancing circuit. On the one hand, it communicates with the LTC680221 through the SPI interface, and on the other hand, it performs simple processing on the acquired data. The data acquisition circuit is mainly composed of the LTC680221 chip, which has a built-in high-precision AD converter. Combined with the external filter circuit, it can achieve accurate sampling of the parameters of the lithium battery pack. The special design of the LTC680221 chip balancing interface can control the external balancing circuit to work, simplifying the balancing circuit.

1.1 Power supply system

LTC680221 can manage up to 12 lithium battery packs in series. Taking the WX11I3215 lithium battery produced by Wanxiang Electric Vehicle Co., Ltd. as an example, the voltage of a single cell changes between 2.8 and 3.8V during use. The total voltage of the battery pack with 12 WX11I3215 lithium battery packs in series changes between 33.6 and 45.6V during use. The embedded processor Atmega16LLTC680221 in the balancing circuit is powered by a 5V DC power supply, so the output voltage of the power supply system is selected to be 5V. Since low-power chips are used in the balancing circuit, the output power of the power supply system is selected to be 5W. Based on the particularity of the wide voltage input of the above-mentioned power supply system, the power supply system in this article uses a single-chip switching power supply chip TOPSwitch to design a switching power supply with a wide input voltage range.

1.2 Embedded processor and SPI interface

This article uses Atmel's AVR series processor Atmega16L, which has 16kb online programming Flash program memory, 512 bytes EEPROM, 2kbSRAM, 32 general working registers, 32 general I/O ports, and rich peripherals such as SPI and USART. At the same time, the At2mega16L chip has a low-power SMD package that can meet the design requirements of low-power balancing circuits. The chip's rich internal resources and interfaces can complete various functions of the lithium battery pack management system. Atmega16L communicates with LTC680221 through the SPI bus.

The SPI working mode of LTC680221 chip is fixed as CPHA=1, CPOL=1, and the byte is sent high first, which can support the host's maximum SPI shift frequency to 1MHz. Therefore, the SPI peripheral control register should have the same configuration in the SPI driver function writing of Atmega16L.

In the balancing circuit designed in this paper, Atmega16L reads the AD conversion value of each battery voltage through the SPI bus, then sorts the battery voltages, selects the battery corresponding to the highest voltage, and then sends instructions to control LTC680221 to operate the balancing circuit. The software flow is shown in Figure 2.