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 to a shared PCS (Power conversion system), which converts power between the batteries (DC) and the grid (AC).
The “Sibling Effect” describes a scenario where a single underperforming battery rack limits the output of all other racks connected to the same PCS or inverter. 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 capacity: 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 flagged.
Reduced ROI: The underutilized PCS 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 degraded 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. Usable Energy takes the sibling effect into consideration. If your system is experiencing this phenomenon, you receive recommendations on how to fix it, for example, to balance, replace or switch off the underperforming rack.
In Recoverable Energy, the higher levels, at which such an sibling effect will be experienced, will exhibit such a notification:
The highlighted string also has such an icon and has as recommendation "Balance or Take offline". In the side panel, the information on the short term gains are highlighted to support the justification of turning off a single string.
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.