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Boost cell monitoring accuracy in energy storage BMS

Posted: 27 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:battery management systems? energy storage? Battery Monitor IC? cyclic redundancy check?

The key advantage for this off-the-shelf BMS is its tiered, hierarchical topology (figure 1) with three sub-systems, each with unique functions, as shown in figure 2:

1. The cell interface provides tight management and monitoring of each battery cell in a stack; the system uses as many cell interfaces as needed, depending on the number of stacks. These interfaces can be daisy-chained as the number of cells and thus the stack voltage increases.

2. The cell interface is connected to a single stack controller which monitors and manages multiple cell-interface units. Multiple stack controllers can be connected together, if needed, to support large packs with many stacks in parallel.

3. The power interface connects the stack controllers to high voltage/current lines and is the interface to the inverter/charger. It isolates high-voltage and high-current components of the stack physically and electrically from the other modules. It also powers the BMS directly from the battery stack, thus eliminating the need for any external power supplies for the BMS operation.

The modular and hierarchical architecture of the Nuvation BMS supports battery-pack voltages ranging up to 1250Vdc, using cell-interface modules each containing up to 16 cells, stacks with up to 48 cell-interface modules, and battery packs that contain multiple stacks in parallel. The entire array assembly is managed as a single unit from the user's perspective.

Solid design also builds from the bottom up
Factors such as modular architecture, hierarchical topology, and error-aware design are essential to the integrity and expandability of the Nuvation BMS, but not enough. A successful implementation requires high-performance functional blocks to serve as the physical foundation.

That's why the LTC6804 Multicell Battery Monitor IC from Linear Technology Corp. plays a critical role in the Nuvation BMS implementation. It is expressly tailored for the needs of BMS systems and multi-cell designs, beginning with providing precise measurements on up to 12 battery cells stacked in series. Its measurement inputs are not ground-referenced, which is said to simplify measurement of those cells, and the battery monitor itself is stackable for use with higher-voltage arrays (and it also supports a variety of cell chemistries). It offers maximum 0.033% error with 16bit resolution, and needs just 290?sec to measure all 12 cells in the stack. Such synchronised voltage and current measurements are critical to yield meaningful analysis of power parameters.

Of course, performance in the benign environment of a prototype at the bench is not the same as actual achievable performance in an electrically and environmentally hostile real-world BMS setting. The battery monitor's analogue/digital converter (ADC) architecture is designed to resist and minimise those detrimental effects, using filters specifically designed for the noise of power inverters.

The data interface uses a single twisted-pair, isolated SPI interface which supports rates up to 1Mb and distances of up to 100m. To further enhance system integrity, the IC Includes an array of ongoing sub-system tests. As further indication of its reliability and ruggedness, the LTC6804 meets the stringent AEC-Q100 standard for automotive quality. This IC achieves its results due to an application-specific design which focuses closely on BMS issues and environments, including the unique system-level objectives of the application and its many challenges.

Three major issues resolved
The battery monitor addresses three major areas which affect system performance, conversion accuracy, cell balancing, and connectivity/data-integrity considerations:

Conversion accuracy: Due to the short- and long-term accuracy demands of the BMS application, it uses a buried-Zener conversion reference rather than a bandgap reference. This provides a stable, low-drift (20ppm/kHr), low-temperature coefficient (3ppm/C), low-hysteresis (20ppm) primary voltage reference along with excellent long-term stability. This accuracy and stability is critical since it is the basis for all subsequent battery-cell measurements and these errors have a cumulative impact on acquired-data credibility, algorithm consistency, and system performance.

Although a high-accuracy reference is a necessary feature to ensure superior performance, that alone is not enough. The A/D converter architecture and its operation must meet specifications in an electrically noisy environment, which is the result of the pulse-width modulated (PWM) transients of the system's high current/voltage inverter. Accurate assessment of the state of charge (SOC) and health of the batteries also requires correlated voltage, current, and temperature measurements.

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