In order for a lithium-ion battery to function reliably over a long period, it requires careful handling. These batteries cannot be operated at their maximum state of charge (SOC) continuously. Over time, the capacity of lithium-ion batteries degrades and becomes uneven, which means that each cell in the system must be monitored and kept within a defined SOC range.
To provide sufficient power for a vehicle, tens or even hundreds of batteries are typically connected in series, forming a high-voltage string capable of reaching 1000V or more. The battery management electronics must operate in this extreme voltage environment, effectively rejecting common-mode voltages while accurately measuring and controlling each individual cell. Additionally, these systems need to communicate data from the battery pack to a central processing unit.
High-voltage battery packs used in vehicles or other high-power applications often face challenging conditions, such as significant electrical noise and wide temperature variations. Battery management systems are designed to maximize performance, extend lifespan, improve safety, and reduce costs, size, and weight.
In 2008, Linear Technology introduced the LTC6802, the first high-performance multi-cell battery monitor. It could measure up to 12 lithium-ion cells with an accuracy of 0.25% in just 13ms. The device could be daisy-chained to monitor long high-voltage strings. Over the years, Linear Technology has continued to enhance the LTC6802, offering various improved versions suitable for hybrid and electric vehicles.
The LTC6811 is the latest in a line of high-voltage battery monitors from Linear Technology. It features an ultra-stable voltage reference, a high-voltage multiplexer, a 16-bit delta-sigma ADC, and a 1Mbps isolated serial interface. The LTC6811 can measure the voltage of up to 12 series-connected batteries with an accuracy better than 0.04%. With eight programmable third-order low-pass filters, it excels at noise reduction, making it ideal for demanding environments.
Multiple LTC6811 devices can be interconnected using Linear Technology's proprietary two-wire isoSPI interface. Each LTC6811 includes an isoSPI interface, providing high immunity to RF noise and allowing up to 100 meters of twisted pair cable. The LTC6811 offers two communication options: daisy-chaining multiple devices through one host processor (LTC6811-1), or connecting them in parallel with individual addressing (LTC6811-2).
Designed for operation between -40°C and 125°C, the LTC6811 is suitable for ISO 26262-compliant systems. It includes features like internal diagnostics, cross-channel testing, open-circuit detection, watchdog timers, and packet error checking. For existing designs using the LTC6804, the LTC6811 serves as a pin-compatible replacement with enhanced functionality, including additional filters, balance control, and fault tolerance.
The LTC6811 ensures high measurement accuracy by using a dedicated buried Zener voltage reference, which provides excellent long-term stability. This allows it to measure all batteries with an error of less than 1.2mV. It also employs an incremental accumulation ADC to filter out noise, ensuring accurate readings even in noisy environments.
In addition to its advanced measurement capabilities, the LTC6811 offers flexible general-purpose I/O that can act as digital inputs, outputs, or analog inputs. It can synchronize measurements with external signals and control I2C or SPI slaves. The device also includes internal passive balancing FETs for managing battery discharge and supports active balancing solutions like the LT8584 and LTC3330.
For developers, the Linduino One is an Arduino Uno-compatible board that simplifies integration with the LTC6811. It provides a fully isolated connection to the LTC6811 demo board and includes a bootloader for easy firmware updates. With access to the Arduino IDE and sample code, designers can quickly develop and test battery management systems.
In summary, the LTC6811 represents the fourth generation of high-performance multi-cell battery monitors from Linear Technology. It is pin and software compatible with the LTC6804 but offers improvements such as lower cost, enhanced filtering, additional balance control, and improved functional safety features. The LTC6811 is well-supported by the Linduino One platform, making it easier to evaluate and develop battery management systems.
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