State-of-the-art high-voltage battery pack monitor performance testing solution
If you want a lithium-ion battery to work reliably for a long time, you need to treat it very carefully. Such batteries cannot operate at the limit of their state of charge (SOC). As time passes and continues to be used, the capacity of lithium-ion batteries drops and deviates, so each cell in the system must be managed to keep all cells within a defined SOC range. In order to provide sufficient power to the vehicle, tens or hundreds of batteries are required. These batteries are configured in a long string to provide voltages up to 1000V or higher. Battery management electronics must operate in such a very high voltage environment and reject common mode voltage effects while differentially measuring and controlling each cell in the battery string. The battery management electronics must be able to transfer information from the battery pack to a central point for processing. In addition, the use of high-voltage battery packs in vehicles or other high-power applications can be accompanied by some thorny conditions, such as very high electrical noise during operation and a very wide temperature range. Battery management electronics are expected to maximize operating range, extend life, improve safety and reliability while minimizing cost, size and weight. In 2008, Linear Technology announced the launch of the first high-performance multi-cell battery pack monitor LTC6802. Key features include: measuring up to 12 lithium-ion batteries in 13ms with a maximum overall measurement error of 0.25%; many LTC6802 ICs can be connected in series to simultaneously monitor all cells in a long high-voltage battery string. Over the years, Linear Technology has continued to improve the LTC6802 and offers many new versions. All of these versions are designed to provide accurate battery management in hybrid/electric vehicles (HEVs), electric vehicles (EVs), and other high voltage, high power battery packs. State of the art high voltage battery pack monitor The LTC6811 is Linear Technology's new multi-cell battery pack monitor with an ultra-stable voltage reference, high voltage multiplexer, 16-bit delta-sigma ADC and 1Mbps isolated serial interface. The LTC6811 measures the voltage of up to 12 series connected batteries with better than 0.04% accuracy. With eight programmable 3rd-order low-pass filters, the LTC6811 is extremely good at reducing noise. In the fastest ADC mode, all battery measurements can be made in 290μs. Multiple LTC6811s can be interconnected and operated simultaneously through Linear Technology's proprietary two-wire isoSPI interface. Each LTC6811 integrates an isoSPI interface to provide high RF noise immunity up to 1Mbps and allows up to 100 meters of cable in the case of twisted pair only. The LTC6811 offers two communication options: when using the LTC6811-1, daisy-chain multiple devices, all connected to one host processor; when using the LTC6811-2, multiple devices are connected in parallel to the host processor, each The devices are individually addressed. The LTC6811 is fully specified for operation over the -40°C to 125°C temperature range. Designed for ISO 26262 (ASIL) compliant systems, the device provides exceptional voltage reference, logic test circuitry, cross-channel testing, open-circuit detection, watchdog timer, and serial interface packet error checking. Large tolerance range. For existing designs using the Linear Technology LTC6804, the LTC6811 is a pin-compatible replacement with additional filter cutoff frequency, additional passive and active balance control, new ADC commands, and additional Fault tolerance range to ensure functional safety. Figure 1: Battery pack monitor for multi-cell battery ISO 26262 and functional safety The LTC6811 can be used to implement systems with high reliability, high stability and high measurement accuracy that will operate for many years in high voltage, extreme temperature, hot swap and electrical noise environments. The LTC6811 also supports the functional safety of cars as defined by the ISO 26262 standard. ISO 26262 systematically addresses the potential hazards posed by electrical and electrical system failures in automobiles. This requires the system to constantly confirm that critical electronic circuits are in normal operation, such as battery voltage measurement electronics. To do this, the LTC6811 offers a wide range of internal diagnostics to verify that such electronic circuits are functioning properly: Open circuit detection between battery and monitor Auxiliary voltage reference to confirm main reference accuracy within ±5mV Voltage measurement of the battery pack consisting of 12 batteries, confirming that the battery measurement accuracy is within ±0.25% Dual channel measurement to verify multiplexer and ADC accuracy within 0.01% Simultaneously perform dual filter measurements to confirm that the filter is working properly Internal supply voltage measurement Memory self test Redundant monitoring with sensors and external devices with general purpose I/O Accuracy For superior accuracy, the LTC6811 uses a dedicated buried Zener voltage reference. The buried Zener voltage reference varies over time and operating conditions, providing excellent long-term stability and accuracy. As a result, the LTC6811 is capable of measuring all batteries with an error of less than 1.2mV. Figure 2: Excellent temperature drift performance of a buried Zener voltage reference In addition, the LTC6811 ensures excellent measurement accuracy by filtering out noise on the battery voltage even in the presence of noise. This is done by using an incremental accumulation ADC converter. When using an incremental accumulation converter, the input is sampled multiple times during the conversion and then digitally filtered. As a result, the built-in low-pass filtering eliminates the noise as a source of measurement error, at which point the cutoff frequency is established according to the sampling rate. The LTC6811 uses a fast 3-step incremental accumulation ADC with programmable sample rate and eight selectable cutoff frequencies. As a result, the noise is excellently reduced and eight programmable measurement rates are available, allowing measurements of 12 cells to be completed in as little as 290μs. Figure 3: LTC6811 incremental accumulation converter More features The LTC6811 is designed to operate in the most critical position in the battery system and is directly connected to the battery. This position is between the BMS microprocessor and the external device, so the LTC6811 can monitor the battery sensor for values ​​such as battery current or temperature and show a close correlation between these values ​​and battery measurements. The LTC6811 has several ways to play in this position. The LTC6811 offers very flexible general purpose I/O that can operate as a digital input, digital output or analog input. When operating as an analog input, the LTC6811 can measure any voltage from V- to 5V with the same measurement accuracy as when measuring the battery. In addition, the LTC6811 allows battery measurements to be synchronized with these external signals or with battery pack voltages containing 12 batteries. The LTC6811 also has a built-in feature that controls I2C or SPI slaves via digital I/O. This allows the LTC6811 to control more complex functions such as controlling the multiplexer to add analog inputs or controlling the EEPROM to store calibration information. The LTC6811 has an internal passive balanced FET that can discharge a single battery or directly control a larger, high power external FET. The LTC6811 can be configured to discharge the battery in a low power state, such as when the battery pack is stationary. In addition, the discharge output of each battery can be turned on within a separate period of time. This allows the battery to remain balanced for long periods of time when the battery monitor is not activated. These balanced pins can also be used as a serial interface to control Linear Technology's LT8584 active balancing circuit. The LT8584 is a monolithic flyback DC/DC converter that can be used to recover more than 99% of the capacity in a battery pack with mismatched batteries. With the LTC6811 SPI master function, the LTC6811 can be connected to Linear Technology's SPI-based active balancing IC LTC3330. The LTC3300 is a fault protection controller IC for bidirectional active balancing that efficiently transfers charge to 12 or more connected cells or transfers the charge from these cells. Figure 4: Battery pack monitor with active balancing LINDUINO ONE To facilitate integration of the LTC6811's features and reduced development time, Linear Technology's new LinduinoTM One provides full support for the LTC6811. The Linduino One is an Arduino Uno-compatible microcontroller board that is completely isolated from USB and directly connected to the LTC6811 demo board. This platform has a built-in bootloader that can quickly update firmware online, making it a simple and stable hardware development platform. Because Arduino is an open source platform, BMS designers have easy access to the easy and powerful Arduino Integrated Development Environment (IDE). A codebase called bmsSketchbook provides the LTC6811 sample code for compiling in any standard C compiler. For example, bmsSketchbook includes many routines, including reading and writing configuration values, reading and writing battery voltages, running self-tests, running redundancy tests, and controlling passive balancing. Figure 5: The Linduino One is an Arduino Uno-compatible microcontroller board that is completely isolated from the USB and directly connected to the LTC6811 to sum up Linear Technology's LTC6811 is the fourth generation of high performance multi-cell battery monitors. The LTC6811 is pin compatible and software compatible with its predecessor, the LTC6804. Compared to the LTC6804, the LTC6811 offers lower cost, additional filter cutoff frequency, additional passive and active balance control, new ADC commands, and additional fault tolerance for functional safety. The LTC6811 is also fully supported by Linear Technology's Linduino One for ease of evaluation and development.
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