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Enhance cell monitoring accuracy in energy storage BMS

Posted: 30 Jul 2015     Print Version  Bookmark and Share

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

To mitigate the system noise before it can affect the BMS performance, the LTC6804 converter uses a delta-sigma topology, aided by six user-selectable filter options to address noisy environments. The delta-sigma approach reduces the effect of electromagnetic interference (EMI) and other transient noise, by its very nature of using many samples per conversion, with an averaging, filtering function.

Cell balancing: The need for cell balancing is an unavoidable consequence in any system that uses large battery packs arranged as groups of cells or modules. Although most lithium cells are well matched when first acquired, they lose capacity as they age. The ageing process can differ from cell to cell due to a number of factors, such as gradients in pack temperature. Exacerbating the whole process, a cell that is allowed to operate beyond its SOC limits will prematurely age and lose additional capacity. These differences in capacity, combined with small differences in self-discharge and load currents, lead to cell imbalance.

To remedy the cell imbalance issue, the LTC6804 directly supports passive balancing (with a user-settable timer). Passive balancing is a low cost, simple method to normalise the SOC for all cells during the battery charge cycle. By removing charge from the lower capacity cells, passive balancing ensures these lower capacity cells are not overcharged. The LTC6804 can also be used to control active balancing, a more-complicated balancing technique which transfers charge between cells through the charge or discharge cycle.

Whether done using active or passive approaches, cell balancing relies on high measurement accuracy. As measurement error increases, the operating guardband which the system establishes must also be increased, and therefore the effectiveness of the balancing performance will be limited. Further, as the SOC range is further restricted, the sensitivity to these errors also increases. The LTC6804's total measurement error of less than 1.2mV is well within system-level requirements.

Connectivity/data integrity considerations: Modularity in the battery-pack design adds to scalability, serviceability, and form-factor flexibility. However, this modularity requires that the data bus between packs has galvanic isolation (no ohmic path), so failures in any one pack do not affect the rest of the system or put high voltages on the bus. Further, the wiring between packs must tolerate high levels of EMI.

A two-wire isolated data bus is a viable solution to achieve these goals in a compact and cost-effective way. Therefore, the battery monitor offers an isolated SPI interconnect called isoSPI, which encodes the signals for clock, data in, data out, and chip select into differential pulses, which are then coupled through a transformer an isolation component (figure 3).

Figure 3: The LTC6804 supports an isolated SPI interface which can be "daisy-chained" for larger arrays, which results in a robust, EMI-resistant interconnect which also minimises cabling requirements and the number of isolators.

Devices on the bus can be connected in a "daisy chain" configuration, which greatly reduces harness size and enables modular designs for large, high-voltage battery packs, while maintaining high data rates and low EMI susceptibility (figure 4).

Figure 4: Test results on the LTC6804 and isoSPI interface showed no data errors despite 200mA of injected RF with the isoSPI operating at 20mA signal strength.

Linear Technology also performed BCI testing on the LTC6804, which involved coupling 100mA of RF energy into the battery-wiring harness, with the RF carrier swept from 1MHz to 400MHz and with 1kHz AM modulation on the carrier. The LTC6804 digital filter was programmed for a 1.7kHz cut-off frequency, and an external RC filter and ferrite choke were added as well. The result: the error in voltage reading was below 2mV over the entire RF sweep range.

An array of self-assessment and self-test features adds to the suitability of the LTC6804 for BMS applications. These checks include open-wire detection; a second internal reference for ADC clock; multiplexer self-test, and even measurement of its internal power-supply voltages. The device is engineered for systems that are intended to be compliant with ISO 26262 and IEC 61508 standards.

Conclusion
There's a lot of "glamour" associated with backup and carry-through supplies for grid-level systems. It seems so straightforward: just keep an array of batteries charged (whether from the grid-AC line, or solar, wind, or other renewable sources), then use the batteries with a DC/AC inverter when you need to provide line-equivalent AC power.

The reality is that batteries are not "simple" in any of their behaviour or performance characteristics, and they need carefully controlled charging, monitoring of their voltage, current, and temperature, and discharging. As power levels increase, a practical, efficient, and safe system is not a trivial design, and so a grid-connected multi-cell BMS is a complex system. Many unique problems need to be understood and addressed, with safety a major concern as well.

A successful and viable system design needs a modular, structured, top-down architecture that is supported from bottom up by optimised components such as the LTC6804. When combined with sophisticated, secure data-acquisition and control software, the result is a high-performance, reliable BMS that requires minimal operator involvement, and will function autonomously for years of reliable service.

About the authors
Mike Kultgen is Design Manager for Signal Conditioning Products at Linear Technology.

Greg Zimmer is Senior Product Marketing Engineer for Signal Conditioning Products at Linear Technology.

Stefan Janhunen is Design Engineer for BMS Products at Nuvation Engineering.


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