BESS Factory Acceptance Test to Commercial Operation: FAT, SAT, and COD Explained

For every utility-scale battery energy storage project, the journey from manufacturing to commercial operation follows a standardized lifecycle of testing and verification milestones: FAT (Factory Acceptance Test), SAT (Site Acceptance Test), and COD (Commercial Operation Date). Each milestone serves as a contractual gate, and passing each one determines whether the project moves forward, gets delayed, or triggers warranty claims.

This guide walks through each phase of the BESS project testing lifecycle — what is tested, how it is measured, who is involved, and what documentation is required for a clean handover to operations.

The BESS Project Testing Lifecycle

A typical BESS project passes through these major stages:

1. Manufacturing — Cells are produced and assembled into modules, racks, and containers at the factory
2. FAT — Tests conducted at the factory before shipping to verify the system meets specifications
3. Shipping & Storage — Logistics, customs, and potentially months of on-site storage before installation
4. Installation — Civil works, rack mounting, busbar connections, HVAC, fire suppression, and auxiliary systems
5. SAT & Pre-Commissioning — On-site testing after installation to verify system integrity
6. Commissioning — Full-system testing including SOH verification, RTE measurement, and grid interconnection
7. COD — The date the system enters commercial operation and begins generating revenue

Each stage has its own testing protocols, pass/fail criteria, and documentation requirements.

Factory Acceptance Test (FAT)

The FAT is conducted at the battery manufacturer's or system integrator's factory before equipment ships. Its purpose is to verify that each manufactured unit meets the agreed specifications and is free from manufacturing defects.

What is tested during FAT:

• Visual inspection — Physical condition, labeling, weld quality, torque checks on busbars
• Insulation resistance — Hi-Pot testing between DC terminals and ground (typically >1 MΩ at 1000V DC)
• Open-circuit voltage (OCV) — Cell-level and module-level voltage matching within specified tolerance
• Internal resistance (DCIR) — Measured at module level, compared to spec
• BMS communication — Verify CAN/MODBUS communication between battery management system and SCADA
• Capacity test — A partial or full charge/discharge cycle to verify nameplate capacity (typically to within ±2% of spec)
• Thermal management — Verify cooling/heating system operation under load
• Functional safety — Test emergency stop, smoke detection, and fire suppression system triggers

FAT typically samples a percentage of units rather than testing every single container. The sampling protocol is defined in the EPC contract and should follow recognized standards such as IEC 62619 or UL 1973.

Shipping, Logistics, and Storage Verification

Between FAT and installation, BESS equipment may spend weeks or months in shipping and on-site storage. This period can cause real — and often overlooked — degradation:

• Storage SOC matters — Most manufacturers recommend storing at 30-50% SOC. Storing at 100% SOC accelerates calendar aging
• Temperature exposure — Containers in transit may experience temperatures well outside the recommended range, accelerating degradation
• Transport vibration — Road and sea transport can cause mechanical loosening of connections if not properly secured
• Storage duration — Six months of storage at 40°C and 100% SOC can cause 2-3% additional SOH loss before the system even starts operating

Many EPC contracts include a pre-installation inspection that repeats some FAT tests to verify no damage occurred during transit. This is especially important for projects where batteries are stored on site for extended periods.

Site Acceptance Test (SAT) and Pre-Commissioning

The SAT is conducted after the equipment is installed and all mechanical and electrical connections are complete, but before the full-system commissioning begins. Its purpose is to verify that the equipment survived installation and that all auxiliary systems are functioning.

What is tested during SAT:

• Continuity and wiring verification — All DC and AC connections checked against the single-line diagram (SLD)
• Grounding and bonding — Verify grounding resistance meets local code (typically <5 Ω)
• Auxiliary power systems — HVAC, fire suppression, lighting, BMS, and SCADA power-up
• Communication check — End-to-end communication from BMS to plant controller to SCADA
• Insulation resistance repeat — Repeat Hi-Pot testing after installation
• Polarity verification — Confirm all connections are wired with correct polarity
• Safety system function — Test E-stop, gas detection, smoke detectors, and alarm annunciation

A failed SAT holds up COD and can trigger delay liquidated damages (LDLs) under the EPC contract. SAT is typically a pass/fail gate with a defined punch list process for minor items.

SOH Verification and Capacity Test

The SOH verification test is arguably the most important commissioning test for a BESS project. It establishes the actual usable capacity at COD, which forms the baseline for the project's performance guarantee and warranty.

The capacity test protocol:

1. Charge the battery to 100% SOC at the rated C-rate (e.g., 0.5C for a 2-hour system)
2. Constant voltage (CV) hold until current drops to the termination threshold
3. Discharge at rated C-rate to the minimum SOC specified in the warranty (e.g., 5% or 10% SOC)
4. Measure the total energy discharged (MWh) at the DC bus and at the AC point of connection
5. Repeat for a minimum of three cycles and average the results
6. Compare to the nameplate capacity to calculate SOH

SOH at COD is typically expected to be between 97% and 100% of nameplate, depending on the time elapsed since manufacture and the storage conditions. If SOH measures below the contract threshold, this triggers warranty provisions.

Important nuance: SOH at COD is not 100% in most real projects. The battery has already experienced calendar aging during manufacturing, transport, and storage. A well-documented FAT and shipping log helps distinguish between factory defects and expected pre-commissioning aging.

Round-Trip Efficiency Measurement

Round-trip efficiency (RTE) is measured during commissioning to establish the baseline for the project's efficiency guarantee. RTE is defined as:

RTE = Discharge Energy (AC) / Charge Energy (AC) × 100%

The RTE measurement protocol involves:

• A full charge and discharge cycle at the rated C-rate
• Measurement at the point of common coupling (PCC) — includes transformer and auxiliary losses
• A minimum of three cycles to average out measurement noise
• Temperature normalization — RTE varies with ambient temperature, so results are corrected to 25°C reference

Typical RTE at COD for LFP BESS systems ranges from 84% to 89% AC-to-AC, depending on transformer configuration, auxiliary load, and operating C-rate. The EPC contract usually specifies both an initial RTE guarantee and an RTE degradation curve over the project life.

Commissioning Milestones and COD

The path from SAT completion to COD is managed through a structured commissioning plan with defined milestones:

• Milestone 1: Auxiliary systems commissioning — HVAC, fire suppression, BMS, lighting, and SCADA verified operational
• Milestone 2: DC system commissioning — All battery strings connected, BMS communication verified, insulation resistance passed
• Milestone 3: Inverter/PCS commissioning — Power conversion system startup, grid synchronization, reactive power capability verified
• Milestone 4: Full-system capacity test — SOH verification and RTE measurement completed
• Milestone 5: Grid code compliance test — Voltage regulation, frequency response, ramp rate, harmonics, and islanding detection verified (see Grid Code Compliance)
• Milestone 6: COD declaration — All punch list items resolved, performance tests passed, commercial operations begin

The Commercial Operation Date (COD) is the contractual date on which the system begins earning revenue under its PPA or merchant agreement. COD triggers several important events:

• Start of the revenue stream (capacity payments, energy payments, ancillary service payments)
• Start of the warranty period
• Start of the performance guarantee measurement period
• Transition from EPC to O&M responsibility

Warranty Compliance and Documentation

Battery manufacturer warranties (typically 10 years or 6000 cycles, whichever comes first) depend on proper testing and documentation at each stage:

• FAT documentation — Required for warranty validation. Missing or incomplete FAT reports can void coverage
• Shipping records — Temperature and SOC logs during transit needed to prove storage conditions were within warranty limits
• SAT documentation — Installation verification records and punch list sign-offs
• COD capacity test report — Establishes the baseline SOH for the warranty degradation curve
• Ongoing O&M records — Monthly capacity tests, cycling logs, and temperature data required for warranty compliance during operations

Most warranty disputes in BESS projects arise from inadequate documentation of the pre-COD phase. Investing in a structured commissioning management system that tracks all test results against contractual requirements is essential.

Common Failure Modes at Each Stage

FAT failures:

• Cell voltage mismatch beyond spec — requires cell sorting or module replacement before shipping
• BMS communication errors — firmware updates needed
• Insulation resistance below spec — typically caused by moisture ingress or manufacturing defect

SAT failures:

• Damaged connectors or busbars from transport vibration
• Ground fault from compromised insulation during installation
• HVAC or fire suppression not functioning after installation — delays COD

Commissioning failures:

• SOH below contract threshold — may require warranty claim or augmentation earlier than planned
• RTE below guarantee — often caused by excessive auxiliary load or transformer losses not accounted for in the design
• Grid code test failure — inverter settings or firmware not compliant with local utility requirements

Modeling Pre-COD Degradation in Energy Optima

Energy Optima's degradation modeling platform tracks battery SOH from the factory floor through COD and across the full project life. The platform enables project developers and EPC contractors to:

• Model expected SOH loss during transport and storage based on shipping conditions and duration
• Compare FAT test results against manufacturer degradation curves to validate baseline SOH
• Forecast SOH at COD based on expected installation timeline and storage conditions
• Model the impact of pre-COD degradation on 25-year financial projections, including augmentation timing and costs
• Generate commissioning documentation packages that align with warranty requirements

For more on degradation modeling methodology, see our complete guide to BESS degradation modeling. For planning augmentation schedules based on SOH thresholds, see battery augmentation planning.