| | MAY 20259E ERGYTech ReviewThe auxiliary power supply circuit must be designed to meet the BESS product's technical requirements, which vary by product. For example, the rated voltage of the auxiliary power supply might be 400V, 480V, or 208V.The circuit must also be sized based on the peak auxiliary load of the selected BESS product and the specific project configuration. Each BESS product has a unique auxiliary load design and peak auxiliary load. Even for a specific product, the peak auxiliary load may vary depending on the use case (e.g., C-rate, charging/discharging profile) and ambient temperature. Additionally, the peak auxiliary load for a project may increase over time due to augmentation--adding new BESS equipment to offset battery capacity losses caused by degradation. Accurately determining the peak auxiliary load is essential to properly size the auxiliary power supply circuit.Backup Auxiliary Power SupplyFor certain projects, backup power must be provided for the BESS auxiliary load as required by the BESS supplier or fire codes. Some BESS suppliers mandate uninterrupted power to maintain the operation of thermal management systems, ensuring battery temperatures remain within desired limits to minimize degradation.BESS fire safety standards, such as NFPA 855, outline minimum requirements for backup power for fire safety systems. For instance, NFPA 855 specifies a minimum of two hours of standby power for gas detection systems and combustible gas concentration reduction systems. If a BESS product cannot meet these backup power requirements as mandated by the code or the Authority Having Jurisdiction (AHJ), an external backup power source needs to be provided. Options for backup power include local distribution network feeders (if available with sufficient kVA rating) or backup generators.BESS Auxiliary Power CostThe cost of the auxiliary power supply circuit and any required backup power sources must be accounted for in the project's capital expenditures. Project owners are also responsible for the electricity costs associated with the BESS auxiliary load during operation.The electricity cost for auxiliary loads depends on the energy consumption (kWh) and the pricing structure set by independent system operators or utilities. For example:· In ERCOT, the BESS auxiliary load must be metered separately from energy used for battery charging and is charged at the retail rate.· In CAISO, only the BESS auxiliary energy consumption in idle mode is charged at the retail rate. Accurately estimating auxiliary power energy costs is a critical aspect of evaluating the financial viability of a BESS project.ConclusionAs discussed above, auxiliary power is a vital consideration in BESS project design and development. While it is an important aspect, a comprehensive approach, such as the total cost of ownership method, should be used for BESS product evaluation and selection. While it is an important aspect, a comprehensive approach, such as the total cost of ownership method, should be used for BESS product evaluation and selection
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