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8 ADI | Energy Storage Solutions To mitigate the system noise before it can affect the BMS performance, the stack monitor converter uses a Σ-Δ topology that is aided by six user selectable filter options to address noisy environments. The Σ-Δ 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. The need for cell balancing is an unavoidable consequence in any system that uses large battery packs arranged as groups of cells or modules, such as the big energy storage units used to supply hospital microgrids and subgrids. Although most lithium cells are well matched when first acquired, they lose capacity as they age. The aging process can differ from cell to cell based on several factors, such as gradients in pack temperature. Exacerbating the whole process, a cell that can 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 stack monitor IC directly supports passive balancing (with a user-settable timer). Passive balancing is a low-cost, simple method to normalize 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 IC can also be used to control active balancing, a more complicated balancing technique that 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 guard band that the system establishes must also be increased, and therefore the effectiveness of the balancing performance will be limited. Further, as the SOC range is restricted, the sensitivity to these errors also increases. A total measurement error of less than 1.2mV is well within system-level requirements for battery monitoring systems. In energy storage systems, a communication loop is mandatory to connect all battery cells. This loop transmits data from the system's battery to a cloud-based energy management algorithm that tracks charging and discharging events to determine the best way to maximize battery use or to keep the highest capacity battery fully charged in case of a power outage. ADI's LTC681x and LTC680x families represent the state of the art for battery stack monitors. The 18-channel version is called LTC6813. The battery stack monitor device needs to communicate with the master unit where a microcontroller or processor calculates the SOC and SOH values and regulates the charging and discharging profiles. Various forms of interconnection are possible, where the isolated communication channel is preferred for high voltage applications, such as energy storage systems (400V to 1500V) and portable devices with high capacity batteries (40V to 200V). The isoSPI feature built into the LTC6813 battery stack monitor, when combined with an LTC6820 isoSPI communications interface, enables safe and robust information transfer across a high voltage barrier. isoSPI is particularly useful in energy storage systems that produce hundreds of volts via series-connected cells, which require full dielectric isolation to minimize hazards to personnel. In these storage systems, where more than 18 cells are used, multiple LTC6813 BMS boards will need to be interfaced together. Here a robust interconnection of multiple identical PCBs, each containing one LTC6813, is configured for operation in a daisy chain. The microprocessor is located on a separate PCB. To achieve 2-wire isolation between the microprocessor PCB and the first LTC6813 PCB, the LTC6820 support IC is used. When only one LTC6813-1 is needed, it can be used as a single (non-daisy-chained) device if the second isoSPI port (Port B) is properly biased and terminated. The main design challenge for battery stack monitors with balancing and communication functions is to create a noise-free PCB layout design, with critical trace routes far from the noise sources—such as switching power supplies—giving clear signals to the stack monitor. With ADI solutions, the stack monitor's great accuracy and precision can help optimize already good designs. The batteries will then be efficiently used, they will have a 30% longer lifetime, and they will operate in a safer way. To support customers in designing their final products, ADI provides a full range of evaluation systems and platforms for the battery monitor devices, as well as a complete portfolio of variants to adapt to all needs. LTC6810 6-Channel Multi-Cell Battery Monitor Measures Up to 6 Battery Cells in Series Stackable Architecture Supports 100s of Cells 1.8mV Maximum Total Measurement Error LTC6811 12-Channel Multi-Cell Battery Monitor Measures Up to 12 Battery Cells in Series 1.2mV Maximum Total Measurement Error Built-in isoSPI™ Interface LEARN MORE LEARN MORE