The LTC6803 is specifically designed for hybrid and electric vehicles (HEVs and EVs), as well as other high-voltage, high-performance battery systems. This advanced battery monitoring IC features 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. Its unique design allows multiple LTC6803 devices to be connected in series without the need for optocouplers or isolators, making it ideal for long series battery configurations.
The LTC6803 series chip is capable of measuring up to 12 series-connected battery voltages with an overall measurement error of less than 0.25% and a response time of just 13ms. It also offers excellent electromagnetic compatibility and low power consumption. In this application, two LTC6803-3 chips are used to monitor 24 series-connected batteries.
**First, the system operation**
The ATMEGA128 microcontroller communicates with the LTC6803-3 via the SPI bus to read the 24 channel voltages. It also uses an ADC to collect battery temperature data, which is then sent to the control board through the SJA1000 CAN controller. Once the control board receives the data, it processes and controls the equalization board accordingly.
**Second, the system hardware design**
**2.1 Voltage acquisition**
The LTC6803 is a battery monitoring IC from Linear Technology, an improved version of the LTC6802 series. It includes a 12-bit ADC and a precision voltage reference. The device can measure up to 12 series-connected battery cells with a common-mode input voltage of up to 60V. It can be connected in series or parallel to support high-voltage applications. Each cell has an associated MOSFET switch that allows for discharging overcharged cells.
To protect the LTC6803’s voltage acquisition pins and prevent overvoltage, a voltage regulator is connected in parallel at each cell input. Additionally, an RC filter is added before each voltage acquisition pin to reduce high-frequency interference and ensure accurate voltage readings.
**2.1.1 Selection of Zener Diodes**
Since the LTC6803 has an internal 12V regulator, the external zener diode should be set below 12V. To account for potential open-circuit scenarios in adjacent batteries, it is recommended to select a zener diode with a voltage higher than twice the single-cell voltage. This paper suggests using a zener diode between 7.5V and 9V.
**2.1.2 Selection of RC filter circuit**
Larger RC values provide better filtering performance and improve surge resistance but may slightly reduce measurement accuracy. In environments with significant interference, increasing the resistor value in the RC filter is advisable.
**2.2 SPI Communication Loop**
The ADUM3401 digital isolator is used to enable communication between the MCU and the LTC6803. It offers enhanced ESD protection and consumes only 1.4mA at 2Mbps. With four isolated channels, it uses iCoupler technology for isolation. The ADUM3401 is powered by the VREG of the LTC1603. It's important to note that the SDO pin of the LTC6803 requires a pull-up resistor; otherwise, it will not function correctly. When multiple SPI devices are connected, the SCK and MOSI signals can be shared, but SS and MISO signals must remain separate.
**Third, system software design**
The LTC6803 starts in standby mode and must be initialized before data acquisition. A command is sent to start the ADC, and data transmission includes a CRC check, which enhances reliability compared to the LTC6802.
**Fourth, test results**
**4.1 LTC6803 accuracy test**
According to Table 1, the LTC6803 demonstrates high accuracy, with a maximum error of 0.449% at 3.8V. While slightly higher than the manual-specified 0.25%, it still meets the required standards. At lower voltages, such as 0.8V, the error increases to 0.732%. There are also minor differences between different chips, particularly between the high and low channels of the same LTC6803.
**4.3 Type test**
The LTC6803-3 was tested under various environmental conditions, including high and low temperatures, humidity, smoke, and vibration. After testing, the system continued to operate normally, proving its strong environmental adaptability.
**V. Conclusion**
This article presents a lithium battery monitoring system using ATMEL’s ATMEGA128 and Linear’s LTC6803-3. The system can measure the voltage of 24 individual cells and upload the data to the control module via CAN. After real-world testing, the system achieves ±20mV voltage accuracy and operates stably and reliably, making it a valuable solution for battery management in EVs and HEVs.
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