The “Sibling Effect” in Battery Energy Storage: A Hidden Performance Bottleneck

As battery energy storage systems (BESS) grow in size and capacity, their operational behavior becomes even more complex. One such nuance – often overlooked until it causes performance bottlenecks – is what we at TWAICE refer to as the “Sibling Effect.” In this article, we’ll discuss what the Sibling Effect is, how it can degrade your system’s output, and what strategies you can adopt to mitigate it.

What Is The “Sibling Effect”?

In a typical BESS, multiple battery racks or strings are grouped together and are connected—via a DC combiner—to a shared PCS (Power conversion system), which converts power between the batteries (DC) and the grid (AC). Because of the direct coupling at the DC combiner, all racks are electrically tied to the exact same voltage rail.

The “Sibling Effect” describes a scenario where a single underperforming rack limits the output of all other racks connected to the same DC combiner. In certain parallel configurations, the weakest rack—whether because of aging, imbalance, or fault conditions—can limit the output of its sibling racks, even if they are still healthy and capable of higher performance.

How Does It Impact System Performance?

The effects of the Sibling Effect can be substantial, particularly in aging systems or systems with inconsistent maintenance:

• Reduced usable energy: A group of racks may appear fully functional, but their output is defined by the least capable rack.

• Lower efficiency: The system may operate below its designed power levels, increasing cost per kWh delivered.

• Hidden downtime: Because racks are technically “online,” standard monitoring may not flag the issue—masking a significant performance drop.

• Maintenance challenges: Without precise monitoring, the issue may persist for months before being noticed.

• Reduced ROI: The underutilized group of racks means lost revenue opportunities

How You Can Prevent the Sibling Effect From Impacting System Performance

During planning, procurement and commissioning:

• Smart Grouping: When commissioning new sites or replacing components, group racks with similar health profiles to avoid imbalance.

• Bypass Design: Design systems to isolate underperforming racks where feasible by avoiding centralized PCS architectures, this helps minimizing the impact on healthy units.

During operation

TWAICE Energy Storage Analytics can help you identify and fix underperforming racks that are impacting the total usable energy of your system. With TWAICE’s feature Usable and Recoverable Energy, you can see how much energy is truly usable, and how much you can recover by fixing issues. If your system is per design affected by the sibling effect, you receive recommendations on how to deal with it, depending on the magnitude of the effect.

The calculation of the Usable Energy takes the sibling effect into consideration: The usable energy of the whole DC combiner is calculated by the usable energy of the worst rack multiplied by the number of sibling racks within this DC combiner (and not by the sum of usable energy of individual racks). TWAICE recommends the "quick fix" to take the worst rack offline if the remaining usable energy of the worst rack is less than the energy that would be recovered in the whole DC bank by taking the worst rack offline plus an additional 20% buffer.

If the difference within a DC combiner is above that 20% buffer, you will see the following indication (orange info icon):

The highlighted string also has such an icon and has as recommendation "Balance or Take offline". By clicking on the affected rack you find more detailed information in the side panel, for example, on the short term gains in usable energy by switching off the worst rack:

Behind the Scenes: Why the Sibling Effect Occurs

There are a few technical drivers behind the Sibling Effect:

1. Shared currents

In BESS that use central power conversion systems without rack-level control, battery racks connected in parallel must share the current evenly. If one rack degrades due to reduced capacity or higher impedance, the PCS reduces total output to protect that specific rack while limiting the others.

2. Control system safeguards

Both the Battery Management System (BMS) and the PCS controller include safety protocols designed to protect equipment and ensure stable operation. If one rack approaches a thermal or electrical threshold, the BMS reduces its current, which in turn slows down the other racks (see point 1).

3. Uneven rack aging

As BESS assets age, racks do not degrade uniformly. Differences in thermal exposure, cycling history, and manufacturing variability can create uneven wear. If one rack’s usable capacity declines significantly, it restricts the group’s available energy.

Conclusion: Capture Hidden Performance Potential

The Sibling Effect can cause performance bottlenecks and affect the revenue generation potential of your system. However, with BESS Analytics, you can identify when this situation occurs and receive recommendations to mitigate its effect. This means BESS operators can unlock more value from existing infrastructure and improve long-term system performance.