Introduction
Capacity testing is a critical step in verifying the usable energy and performance of your Battery Energy Storage System (BESS). It ensures that your system meets contractual obligations, supports warranty claims, and validates operational readiness.
However, capacity tests are time- and resource-intensive, they take your system offline and require careful setup, environmental control, and data validation. That’s why it’s essential to perform them correctly and efficiently and keep monitoring the system continuously in between.
In this article, we explain how to perform capacity tests, share best practices, and show how continuous analytics can help you keep the frequency of the tests to minimum.
When and How Often Should You Perform Capacity Tests?
Capacity tests are essential, however the goal is to minimize unnecessary tests by combining well-planned validation cycles with continuous monitoring.
Good Practice: Commission, Verify Annually, and Monitor Continuously
1. Initial Commissioning
Performed after installation and before commercial operation to establish a baseline capacity reference.
2. Annual Tests
Follow contracted guidelines, in most utility-scale projects, annual testing is considered the standard practice.
Done after significant hardware replacements or system upgrades to verify no negative impact on capacity.
3. Continuous Monitoring
With tools like TWAICE BESS Analytics, continuously track usable capacity, SoH, and round-trip efficiency, turn what used to be a snapshot test into a continuous performance view. This reduces the need for repeated manual tests while keeping warranty confidence and operational assurance high.
Common Capacity Test Framework
Based on TWAICE’s experience with utility-scale BESS, the following validation cycle ensures consistent, bankable, and comparable results.
Pre-Requirements
System State
BESS and all strings are fully charged (100% SoC) and balanced across all strings and DC blocks (voltage spread < 20 mV tolerance, above 70-80% SoC).
All PCS and strings are online and operating properly, all contractors closed, all doors and vents closed.
All alerts are cleared.
Make sure that the system cannot be used for other purposes during the entire test.
Temperature Control
Strings must be in thermal equilibrium before the test (recommended to keep the system idle 30 mins to 1 hour before the test).
Keep ambient/string temperature between 23 ± 3 °C (some warranties allow 10 –27 °C).
Measurement Setup
Capacity test should be based on full usable capacity. Test the total capability, not the contracted capacity. The objective is to achieve the largest feasible Depth of Discharge (DoD) (ideally ~100%).
End-of-Charge (EoC) & End-of-Discharge (EoD) must be defined clearly and aligned with system operation (to be able to define gross vs net usable energy).
Usually there are two cut-offs: BMS SoC limits or cut-off voltage. As best practice, voltage cut-off is preferred, due to possible drifts in BMS SoC.
Use the AC meter at the Point of Connection (PoC) as the reference for measuring energy during the test.
Balance-of-System (BOS) efficiency factor: Some integrators apply a fixed efficiency factor (often 95%) to account for inverter, transformer, and collection losses when calculating DC usable capacity from AC meter data. Always check your warranty documents for the specific factor.
Verify that power ramps align with expected system performance, and clarify whether energy consumed or delivered during ramp phases is included in usable capacity calculations
Step-by-Step Procedure
1. Initial Charge:
Charge at constant power setpoint (e.g., project-rated MW or agreed test power) until SoC = 100% (until upper cut-off voltage).
Record “Meter Start.”
Note that BMS SoC limits or cut-off voltage may stop you short of 100%. The objective is to achieve the largest feasible Depth of Discharge (DoD) (ideally ~100%). As best practice voltage cut-off is preferred over SoC limits.
2. Rest: Hold system idle for 15-30 minutes (longer if warranty requires).
3. First Discharge:
Discharge at constant power setpoint until SoC = 0% (lower cut-off voltage).
Note that BMS SoC limits or cut-off voltage may stop you short of 0%. The objective is to achieve the largest feasible Depth of Discharge (DoD) (ideally ~100%).
Record discharge energy and AC meter value (“Meter Finish”).
Make sure charge and discharge is always with the same CP-rate and the cut-offs are not changing.
4. Rest: Hold idle for 30–60 minutes (longer if warranty requires).
5. Recharge:
Recharge again at constant power to 100% SoC.
Record charge energy.
6. Rest: Hold system idle for 15-30 minutes (longer if warranty requires).
7. Second Discharge:
Repeat the discharge step under identical conditions.
This second discharge capacity defines the validated system capacity.
⚠️ Important Note:
Most integrator and warranties require tests at constant power (MW setpoints). Some warranties may specify constant current (e.g., C/3) or CC–CV charging, CP discharge. Always check your specific warranty conditions to ensure compliance.
Results & Documentation
Measured Capacity
Calculate usable energy from the second discharge cycle.
Adjust for BOS losses if required by warranty (typically 95%).
Round Trip Efficiency (RTE)
Use the same test data to calculate RTE with charged and discharged energy (second discharge cycle).
Typical acceptance threshold for utility-scale systems is 85–92% at commissioning, depending on the CP rate.
Reporting Package
Test date, location, and environmental conditions (temperature, humidity).
Full charge/discharge profiles (SoC vs. Power, Voltage/Current curves).
AC and DC measurements (raw data + processed values), down to the string-level to ensure meaningful State of Health (SoH) tracking.
Test procedure followed, including rest times and setpoints.
Calculated usable capacity and efficiency.
Comparison to warranty
Best Practices and Key Takeaways
1. Test Annually, Monitor Continuously
Perform a full capacity test at commissioning and repeat it annually in line with your warranty agreement.
Between tests, use TWAICE BESS Analytics to continuously track usable energy, SoH, and round-trip efficiency, reducing the need for additional manual testing.
2. Know Before You Test
Avoid failed or inconclusive tests by verifying system readiness beforehand.
Capacity test results are highly sensitive to system conditions such as:
Cell or string imbalances
Temperature control or HVAC issues
PCS availability or limited power capability
These issues can cause a test to fail or require a costly repeat without providing new insights.
With TWAICE Analytics, you can detect such problems early (explained how in detail below), so you only perform a test when the system is ready, ensuring reliable results the first time.
3. Standardize Test Protocols
Use consistent procedures and clearly defined end-of-charge (EoC) and end-of-discharge (EoD) limits.
Standardized test parameters make long-term capacity and degradation comparisons more meaningful and defendable.
4. Consider Environmental and Operational Effects
Document ambient temperature, depth of discharge (DoD), and component availability during testing.
Even small variations in these parameters can affect measured capacity and make tests harder to compare.
5. Leverage TWAICE Expertise
BESS Analytics: Use TWAICE platform to track system performance, identify early deviations, and confirm readiness before each capacity test:
Recoverable & Usable Energy: Identify whether any component is limiting usable capacity. Drill down into causes (such as SoC imbalances), view TWAICE recommendations, and take corrective action before running the test.
Temperature: Detect temperature imbalances across strings and flag potential HVAC issues that could affect test results.
Smart Alerts: Review active alerts to identify components that may compromise test validity, such as high string cell voltage spread detected.
Safety: see early if there is any safety anomaly detected (e.g. temperature or self-discharge) that could be a blocker in your test.
Data Explorer: Examine detailed time-series data, zoom into specific areas, and trace root causes. Export relevant data to include in your capacity test documentation or third-party validation reports.
Digital Commissioning: Integrate TWAICE’s Digital Commissioning offering for enhanced validation at commissioning stage of your system.
Pre-Test Validation: Get expert review of your test method and a pre-capacity test punch list to optimize outcomes.