Understanding battery energy storage system (BESS) | Part 5

Detailed Characteristics of BESS

In February 2024, Rahul Bollini had written about the latest trend of 314Ah Cell and 5MWh BESS in 20 feet container. In this article, he discusses the 5MWh BESS in more detail.

The cell used in this solution is a 314Ah LFP prismatic cell. Below are its cycle life characteristics:

• 10,000 cycles at 0.3C/0.3C (80% SoH) at cell level at 100% DoD at 25°C.

• 15,000 cycles at 0.3C/0.3C (70% SoH) at cell level at 100% DoD at 25°C.

• 8,000 cycles at 0.3C/0.3C (70% SoH) at BESS level at 90% DoD with calendar ageing up to 20 years at up to 35°C temperature.

• 8,000 cycles at 0.5C/0.5C (80% SoH) at cell level at 100% DoD at 25°C.

• 12,000 cycles at 0.5C/0.5C (70% SoH) at cell level at 100% DoD at 25°C.

• 6,000 cycles at 0.5C/0.5C (70% SoH) at BESS level at 90% DoD with calendar ageing up to 15 years at up to 35°C temperature.

C Rate of Operation: 0.3C/0.3C indicates 0.3C rate of charge and 0.3C rate of discharging. Theoretically, it is 3.3 hours of energy storage backup.

State-of-Health: 80% SoH indicates the retention capacity that will remain in the battery after a particular number of cycles.

Depth-of-Discharge: DoD indicates the depth of cell discharge in each cycle. 100% DoD would mean the cell would operate between 0% and 100% SoC (state-of-charge). To achieve 100% DoD in LFP, the cell must work between 2.5V and 3.65V, i.e. charge to 3.65V and discharge to 2.5V.

Calendar ageing: Calendar ageing refers to the degradation of the cell capacity with respect to inactivity during the non-usage time. It contributes to the system level cycle life because a system is not constantly charging or discharging at a given time like in the case of cycle life testing done for cells. For projects with 2 cycles per day, there is lower calendar aging as compared to one cycle per day projects because there is less time of inactivity per day.

Temperature: The 25°C temperature condition allows for a longer cycle life for cells. BESS can operate up to 35°C on a regular basis because most cooling systems (air cooling or liquid cooling) activate at 35°C and come with various cooling levels based on the temperature inside the system. The cooling system activates at 35°C to save the auxiliary energy required for cooling the system. Higher auxiliary energy consumption increases the total cost of ownership of BESS.

Price: Pricing is the most misunderstood part of BESS. There are various levels of pricing depending on multiple factors, such as mentioned below.

Design

• Sizing ratio of PCS with BESS. More hours of backup would have smaller PCS sizing and hence lower cost of the overall solution per kWh.

• Container and other auxiliary utilisation. Larger containers with larger batteries tend to have lower costs.

Type of Components

• BESS pricing without PCS.

• BESS pricing with PCS (with or without isolation transformer) for Commercial and Industrial applications.

• BESS pricing with PCS and Transformer for Grid-scale applications.

Type of Cooling

• Air cooling

• Liquid cooling

Type of DC Voltage Architecture

• 1500V architecture

• 1000V architecture

Warranty Period

• 5-year standard warranty

• Extended warranty from 6th year

Any change in the above parameters can cause a shift in the price, which varies between $110/kWh and $140/kWh.

Image: 5MWh 20ft container liquid cooling design from Xingdong / Lu Xiao Ye (For representation only)

The BESS market is growing, and with battery prices coming down in 2023 and 2024, BESS is more affordable than ever. Combine that with increasing cycle life capabilities in the cell, and many record-low pricing projects are being executed in the BESS space.

The next article, Part 6 of Understanding BESS, will focus on deeper aspects of the architecture of the 5MWh liquid cooling container, which is gaining popularity across large-scale grid-connected projects.

About the author

Rahul Bollini is an R&D expert in Lithium-ion cells with 9 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.

Also read: Understanding battery energy storage system (BESS) | Part 3 – Project planning