Hidden Defects in BESS: What EPCs Must Watch

Battery Energy Storage Systems (BESS) are becoming a critical part of solar projects in India. With hybrid tenders, RTC (round-the-clock) obligations, and C&I storage deployments on the rise, EPCs must now ensure that battery systems perform reliably under real-world conditions.

Unlike solar modules or inverters, BESS involves complex electrochemical and electronic subsystems. Many defects don’t show up during basic inspections but later lead to capacity loss, safety risks, or warranty disputes.

Recently, Sinovoltaics hosted a technical webinar on “Hidden Defects in BESS,” bringing together diagnostic experts from Valitica Diagnostics and quality specialists from Sinovoltaics. The discussion offered a deep look at what typically goes wrong in BESS and why EPCs must rethink their QA strategies.

1. Why BESS Quality Needs Special Attention

BESS manufacturing still involves a high degree of manual processes. A single error at cell, pack, or container level can compromise the entire asset. As Arthur Cleair from Sinovoltaics explained, “It’s not because we have inspected 99 containers that the 100th will be fine.”

Traditional FATs (Factory Acceptance Tests) focus mainly on basic capacity and round-trip efficiency. But many hidden defects live outside these parameters:

• Thermal anomalies that standard FATs ignore
  
• Fire suppression systems not functioning in 25% of inspected projects
  
• Voltage imbalances between highest and lowest cells that don’t trigger alarms but affect usable capacity
  
• Sampling FATs that miss container-specific issues
  

These gaps can cause serious trouble later, especially when warranty claims require proof.

2. The Four-Stage QA Framework for BESS

The webinar outlined a clear quality assurance sequence EPCs should follow:

1. Factory Audits
   Qualify new suppliers with factory audits (quality, ESG, supply chain mapping). This helps compare manufacturing maturity across vendors.
   
2. Factory Acceptance Testing (FAT)
   This is the last step where defects can be easily fixed. FAT includes visual inspection, electrical testing, performance testing (full charge/discharge), and safety system verification. Unfortunately, many suppliers cut corners here by doing partial or sampled tests.
   
3. Site Acceptance Testing (SAT)
   Once the BESS arrives onsite, EPCs must verify there’s no transportation damage and that the system integrates properly with the grid and co-located assets. Transport shock and humidity can cause invisible defects.
   
4. Continuous Monitoring
   Using monitoring platforms like Valitica’s VDX Engine, operators can detect early deviations in KPIs (capacity, voltage, temperature). Having this data is critical for future warranty claims and performance management.
   

3. Real Defects Found During FAT

The experts shared several examples from recent projects:

a. Fire Suppression Failures

The most frequent FAT defect found was related to fire suppression systems, which were found non-functional in 25% of inspected containers. This is especially concerning because BESS thermal events can escalate rapidly.

b. Voltage Imbalance Between Cells

In one 50 MWh project with 23 containers, performance tests showed a noticeable deviation between the highest and lowest cell voltages within a container. Although no voltage limits were breached, the imbalance trapped part of the capacity and raised concerns about long-term degradation.

c. Thermal Inhomogeneity

In another example, one container showed 25% higher temperature spread compared to its peers. This indicated a thermal management problem or a damaged cell. The manufacturer had to take the container back for module replacement.

d. Sampling FAT Pitfalls

Some EPCs only test 1 out of 60 containers to save time. This creates massive blind spots. Sinovoltaics strongly recommends 100% FAT, especially when working with a new technology or supplier.

4. Quantifying the Financial Risk

Sinovoltaics and Valitica analyzed multiple projects and found that:

• Thermal issues accounted for two-thirds of the hidden defects detected during advanced performance tests.
  
• In some cases, financial losses exceeded $600,000 due to undetected thermal issues leading to accelerated degradation, downtime, and claim costs.
  
• Across three thermal defect cases in 2025, they estimated $1.3 million of potential losses prevented thanks to advanced FAT analysis.
  

For EPCs, this highlights a simple truth: investing in QA upfront is far cheaper than fixing BESS failures later.

5. Practical EPC Actions to Catch Hidden Defects

Based on the webinar insights, here are practical steps EPCs can take:

• Include full performance testing clauses in BESS procurement contracts (full charge/discharge, maximum power, 100% container coverage).
  
• Insist on receiving raw FAT data (voltage, current, temperature traces) for independent analysis.
  
• Use independent inspection agencies for factory audits and FAT witnessing.
  
• Deploy on-site IQC and SAT checks to compare FAT vs on-site data.
  
• Adopt battery monitoring platforms to track early KPI deviations.
  

6. Key Takeaways

• Hidden BESS defects often can’t be detected by traditional FAT focused only on RTE and capacity.
  
• Thermal behavior, fire suppression, cell balancing, and data transparency are critical.
  
• Sampling FAT is not enough. 100% testing is recommended, especially for first projects.
  
• Thermal issues are the most common hidden defect, and the most financially damaging.
  
• EPCs who establish robust QA frameworks can avoid major losses and deliver more bankable projects.
  

Conclusion

As India ramps up solar + storage deployment, BESS quality will define project reliability and profitability. Hidden defects may be invisible at first, but their impact grows over time, often surfacing years later when warranty claims are complex and expensive.

By applying structured QA steps and learning from Sinovoltaics’ insights, EPCs can catch issues early, strengthen their contracts, and safeguard project returns.At Heaven Designs, we help EPCs adopt advanced QA strategies for both solar and storage systems. Our team follows global best practices, including detailed FAT protocols, SAT supervision, and monitoring setups to ensure long-term performance.

Key insights adapted from Sinovoltaics Webinar Series 2025: “Hidden Defects in BESS.”