What Actually Happens During a BESS DC Block FAT

If you’ve never attended a factory acceptance test for a BESS DC block at a major Chinese manufacturer, you probably have a cleaner picture in your head than what actually happens. And if you’ve attended one without understanding the distinction between the manufacturer’s internal FAT and the customer-facing FAT, you may have walked away thinking you saw the whole process when you only saw the performance.

That distinction matters. Getting it wrong doesn’t just waste your time on the factory floor — it creates misaligned expectations between the buyer and the manufacturer, and it leads to the wrong people making the wrong calls about what’s acceptable and what isn’t.

The Manufacturer Knows What They’re Doing

Let’s start with something that needs to be said plainly: the major DC block manufacturers producing standardized, containerized products at scale are not winging it. These are companies running high-volume production lines where several complete DC blocks — sometimes ten or fifteen — roll out of a single factory in a day. They employ dedicated engineers, manufacturing equipment specialists, and quality managers. They hold third-party certifications for safety, performance, and quality. They have invested heavily in repeatable production processes because that’s the only way to operate at this volume.

Every one of these manufacturers has an internal FAT procedure — a detailed, standardized testing protocol that every unit passes through before it leaves the factory. This internal process covers electrical testing, functional verification, safety checks, and quality inspection. It’s built into the production flow, optimized not to create bottlenecks, and executed by engineers who run the same tests dozens of times a week. The internal FAT is the manufacturer’s own assurance that the product meets design specifications and complies with the documentation package.

This is important context for what comes next.

The Customer-Facing FAT Is Not the Real FAT

When the equipment manufacturer invites you to “attend the FAT,” what they’re actually inviting you to is a customer-facing factory acceptance test. This is an additional process, layered on top of the manufacturer’s internal FAT, specifically designed to give the buyer visibility and confidence in the product quality.

The customer-facing FAT is performed after the internal FAT is already complete. The unit you’re inspecting has already passed the manufacturer’s own testing protocol. What you’re doing is reviewing the results, walking through the product physically, and verifying that the quality meets your expectations — not discovering whether the product works.

This is where expectations go sideways. EPCs and system integrators who show up thinking they’re witnessing the definitive quality gate for their equipment are misunderstanding the process. You’re not the gatekeeper. The manufacturer is. They carry the ultimate responsibility for the product having passed their own acceptance criteria and being built in conformance with the documentation package. That responsibility doesn’t shift to you because you attended a walkthrough.

The customer-facing FAT is one layer of a three-stage quality assurance chain — not the whole picture.

What the Customer-Facing FAT Actually Looks Like

For a large order — say twenty or more DC blocks — the customer-facing FAT is typically performed on a sample, not on every unit. One in ten, or one per production batch, is common. This is a practical reality: having a customer team physically inspect every single unit would grind the production line to a halt, and no major manufacturer operating at scale will allow that.

The procedure itself usually follows a standard document that the manufacturer shares with the customer ahead of time. There’s typically a small window for customer-requested additions or modifications, but in most cases, the manufacturer’s standard procedure is what gets used — especially with top-tier producers who protect their production flow.

The process is thorough but time-consuming. Expect a full day or several days of work. You’ll move back and forth between the factory floor and a meeting room, walking through checkpoints, examining the physical product, discussing findings with the manufacturer’s testing engineers, and working through any issues that come up.

Common findings during the customer-facing FAT tend to be practical rather than catastrophic:

Cosmetic and appearance issues. Scratches on enclosure panels. Paint color inconsistencies between units. Welding points that aren’t fully aligned. These look minor on paper, but they matter — they signal manufacturing discipline, and they’re visible to everyone who walks past the equipment once it’s installed on site.

Cable routing and workmanship. Internal cabling that isn’t straight, bend radii that are too tight, cable ties that are inconsistent. None of these are typically functional failures, but they indicate whether the production team is maintaining attention to detail across a high-volume run.

Labeling and documentation. Logos placed incorrectly. Labels still in Chinese on equipment destined for an overseas market where English or another local language is required. Nameplate data that doesn’t match the project specification. These are easy to fix but should not pass a FAT.

IP testing failures. Ingress protection tests that fail because a gasket on a container door isn’t properly seated or aligned. Straightforward to correct, but it’s exactly the kind of thing that the customer-facing FAT is designed to catch — issues that passed the internal process but are visible under closer inspection.

Firmware and DoD Configuration: The Detail Most Buyers Miss

Here’s one that doesn’t appear in most FAT checklists but should be on every buyer’s radar: firmware configuration and depth of discharge settings.

A standardized DC block coming off the production line will typically be loaded with the manufacturer’s standard testing firmware. This firmware is optimized for the internal FAT process — it allows the manufacturer to run their standard tests efficiently across every unit. But your project may have specific requirements that differ from the standard configuration.

The most common example is depth of discharge. If your project specifies a 90% DoD, the operational voltage limits, alarm thresholds, and minimum state-of-charge setpoints will be different from those used during a 100% DoD test. The energy capacity test performed during FAT — which is typically one of the final steps — is almost always run at 100% DoD to verify nameplate capacity against the product specification. That’s correct for validating the hardware. But the firmware that ships with the unit for actual project operation should reflect your project’s DoD requirements, not the test configuration.

This means there are two firmware states to be aware of: the testing firmware used during FAT, and the project-specific firmware that gets loaded either as a final production step or during commissioning. If you don’t ask which firmware is currently loaded, when the project-specific settings will be applied, and whether the correct voltage limits, alarm levels, and DoD restrictions have been hard-coded for your project, you risk receiving equipment that’s technically functional but not configured for your operational requirements.

Verifying this during the customer-facing FAT is the right time to raise it. Many buyers don’t.

Send the Right People with the Right Mandate

This brings up something that doesn’t get discussed enough: who you send to the FAT matters as much as the inspection itself.

The customer-facing FAT will surface findings. Some will be clearly unacceptable. Some will be clearly cosmetic. And some will fall in between — issues where the decision to accept, reject, or require a repair is a judgment call that weighs commercial risk against project timeline and technical materiality.

Those decisions need to be made on the factory floor, in real time. If the people you’ve sent to the FAT don’t have the mandate to make commercial and technical decisions, every borderline finding turns into a remote escalation — calls and emails to colleagues back home, often in a different time zone, adding days to a process that was supposed to take one or two.

Both parties need decision-makers present. From the customer side, that means someone who understands the technical requirements and someone who can assess the commercial implications of a delay. From the manufacturer side, it means having their quality manager and a production lead available, not just a junior testing engineer.

And one more thing that matters more than most people acknowledge: be respectful of the manufacturer’s team. You’re a guest in their factory. The people walking you through the FAT are professionals who build these products every day. Approaching the process as a collaborative review rather than an adversarial audit gets better results — more transparency, more willingness to address findings, and a working relationship that extends well beyond the factory visit.

Ask for the Full Picture: Internal FAT, Manufacturing Procedure, and End-of-Line

The customer-facing FAT is valuable, but it’s a subset of a much larger quality assurance chain. To understand what’s actually been done to the equipment before it reaches your site, you want visibility into three layers — not just one.

The internal FAT procedure is the manufacturer’s own testing protocol — the electrical, functional, and safety checks that every unit passes through. This is typically more rigorous than the customer-facing version and is tightly integrated into the production flow.

The manufacturing procedure and protocol is broader. It covers the full production sequence — assembly steps, component integration, quality checkpoints at each stage, and the inspection criteria applied throughout the build. This is where you get a real understanding of how the product is actually made, not just how it’s tested at the end. The manufacturing procedure often aligns closely with the FAT procedure, but it captures process steps that a standalone FAT document won’t show you.

The end-of-line process is the final stage before the unit ships from the factory, and it’s distinct from both the FAT and the manufacturing procedure. This is where the last practical preparations for delivery happen: personnel doing final cosmetic touch-ups — scrubbing, cleaning, paint corrections. Wrapping the container in protective sheeting for transportation. Installing the printed manuals and documentation packages inside the enclosure doors. Making the final physical adjustments that ensure the product arrives in the condition the customer expects. None of this is part of the FAT, but it’s part of the overall manufacturing process, and it directly affects what you receive on site.

Asking the manufacturer for documentation across all three layers — internal FAT records, the general manufacturing procedure, and the end-of-line protocol — gives you a complete picture of what was done to your equipment from first assembly to final shipment. Some manufacturers will share this openly. Others may hesitate for confidentiality reasons — they don’t want competitors getting access to their production processes and internal quality standards. That’s a reasonable concern, and it can usually be addressed with an NDA or by providing redacted versions that show the scope and rigor without exposing proprietary details.

Together, these documents form the quality assurance trail from the equipment manufacturer to the EPC or system integrator. They’re valuable as project handover documentation, and they provide a clear reference point if any performance or quality issues arise after the equipment reaches site.

The Bottom Line

The factory acceptance test is not a single event. It’s two parallel processes: the manufacturer’s internal quality assurance, which is their responsibility and runs on their terms, and the customer-facing FAT, which is your opportunity to verify, ask questions, and catch the things that matter to your specific project.

Understanding this distinction changes how you prepare, who you send, and what you focus on when you’re standing on the factory floor. Go in with the right expectations, the right people, and the right questions — starting with what firmware is loaded on the unit you’re inspecting — and the FAT becomes what it’s supposed to be: a productive quality gate, not a procedural formality.

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learnBESS covers the full BESS project lifecycle — from procurement and factory acceptance through commissioning and operations — in our Project Lifecycle course.