The LTC6803 devices are targeted at hybrid/electric vehicles (HEVs), electric vehicles (EVs) and other high voltage, high performance battery systems. The LTC6803 is a complete battery measurement IC that includes a 12-bit ADC, a precision voltage reference, a high voltage input multiplexer, and a serial interface. Each LTC6803 can measure up to 12 individual battery cells connected in series. The device's proprietary design allows multiple LTC6803s to be stacked in series without the need for optocouplers or isolators, allowing accurate voltage monitoring of each cell in a long series of serially connected cells.

LTC6803 series chip can measure up to 12 series battery voltages, complete all cell voltage detection in 13ms, the overall measurement error is less than 0.25%, with high electromagnetic compatibility and low power consumption. In this paper, two LTC6803-3 batteries are used to collect 24 series battery voltages.

The single chip microcomputer ATMEGA128 uses the SPI bus to read the 24 channel voltages collected by the LTC6803-3 value, and uses the AD to collect the battery temperature value, and uploads the information to the control board through the SJA1000. After receiving the collected data, the control panel analyzes and controls the equalization board to work.

2.1 voltage acquisition

The LTC6803 is a battery monitoring IC from Linear, which is an upgraded version of the LTC6802 series. It has a 12-bit ADC and a precision voltage reference. Each LTC6803 is capable of measuring up to 12 serially connected battery voltages with an input common-mode voltage of up to 6 OV, and the LTC6803 can be connected in series or in parallel for high voltage range data acquisition. Each battery input has an associated MOSFET switch for discharging the overcharged battery. The MCU can read data from the LTC6802 through the SPI bus and control the turn-on and turn-off of the M0SFET of the corresponding battery input to achieve cell balancing.

In order to protect the LTC6803 voltage acquisition pin and prevent the voltage from exceeding the maximum input voltage, a voltage regulator is connected in parallel with each of the cell's acquisition input ports, and a RC filter circuit is added in front of each voltage acquisition pin to effectively filter out. High frequency interference ensures the correctness of voltage collection.

2.1.1 Selection of Zener Diodes

Because the LTC6803 has a built-in 12V regulator inside. The external voltage regulator tube needs to be less than 12V. Considering the special situation that the adjacent battery has an open circuit, it is recommended that the voltage regulator tube select more than 2 times the working voltage of the single battery. This paper suggests a choice between 7.5-9V.

2.1.2 Selection of RC filter circuit

The official choice of resistance selection parameters are as follows:

Larger resistance-capacitance parameters have better filtering effects, and can also improve the chip's surge-resistance capability, but will reduce the acquisition accuracy appropriately. If it is used in harsh conditions with large interference, it is recommended to increase the resistance value in the RC filter appropriately.

2.2SPI communication loop

The digital isolation optocoupler ADUM3401 is used to realize communication between the MCU and the LTC6803. The ADUM3401 features enhanced ESD capability and consumes 1.4mA/5V at 2Mbps communication speed. With 4 isolated signal channels (three outputs and one input), isolation is achieved using iCoupler technology. The operating power of the ADUM3401 comes from the VREG of the LTC1603. Note that the SDO of the LTC6803 requires a resistor pull-up, otherwise it will not work properly. When there are multiple SPI parallel communication on the MCU side, the SCK and MOSI signals can be multiplexed, and other SS and MISO signals can not be multiplexed.

The software flow chart of the LTC6803 works as follows:

The LTC6803 enters the standby mode by default. The chip needs to be initialized first, and then the AD start command is sent for data acquisition. It should be noted that the LTC6803 adds a data transmission CRC check compared to the LTC6802.

4.1 LTC6803 accuracy test

As can be seen from the data in Table 1, the LTC6803 has high accuracy. The maximum measurement error is 0.449% at 3.8V measurement. Although it is higher than the 0.25% accuracy of the manual, it can meet the requirements.

It can be seen from the data in Table 1 that when the voltage is low, the error is large, reaching 0.732% when measuring at 0.8V;

From the data in Table 1, it is seen that there are certain measurement deviations between different chips (high 12 channels and low 12 channels each realized by one LTC6803)

It can be seen from the data that a large input impedance causes a large error in the LTC6803-3. It is recommended to pay attention to the actual use process.

4.3 type test

The LTC6803-3 acquisition system was tested for high and low temperature, damp heat, smoke, vibration and other types. After the test, LTC6803-3 was working normally. It proves that LTC6803-3 has high environmental adaptability.

This article uses ATMEL's ATMEGA128 and Linear's LTC6803-3 chip to design a power lithium battery monitoring system. The monitoring system is capable of collecting the voltage of 24 single cells and can upload data to the control module via CAN. After actual testing, the system has a precision of ±20mV voltage accuracy, and the system works stably and reliably, which has important application value.

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