Cummins whitepaper shares key concepts behind BESS

As the global energy landscape shifts toward renewables, battery energy storage systems (BESS) are making inroads in a variety of areas. On jobsites and in commercial and industrial applications, they offer clean, emissions-free power while also avoiding another type of emissions: noise.

“Battery energy storage systems (BESS) have emerged as a pivotal technology in modern energy management, offering a solution to the intermittent nature of renewable energy sources and enhancing grid stability,” said Hassan Obeid, global technical sales leader for New Energy Solutions at Cummins. He penned a new whitepaper called, “Battery Energy Storage Systems: Understanding Key Concepts and Applications.” In it, Obeid discussed what BESS are, how they work and how they can be effectively applied.

Hierarchy of Components

While a BESS may resemble a simple box, Obeid said numerous components are integrated inside that box in complex ways to ensure safe and efficient operation.

Image: malp via Adobe Stock

“Each component plays a critical role in the overall functionality and performance of the system,” he said. “Understanding these key components is essential for grasping how BESS operates and the various benefits they provide.”

As you would expect, every BESS contains battery cells. These serve as the fundamental units of energy for the system.

“Cells are grouped into modules for easier handling and management,” Obeid said. “Multiple modules are then assembled into racks for a more structured and efficient configuration.”

Like many EVs, BESS also have battery management systems (BMS). Obeid said it’s the BMS that manages each level of the component hierarchy.

At the battery cell level, for instance, he said the BMS performs several tasks. These include voltage and temperature monitoring as well as charge and discharge balancing. The BMS also estimates the state of charge (SoC), which Obeid called “the equivalent to a fuel gauge for a battery.” SoC indicates how much energy is available in the battery at a given moment as a percentage of its total capacity, he said. The BMS also monitors the batteries’ state of health (SoH), which is a measure of their lifetime capacity.

Going up a hierarchical level to battery modules, the BMS monitors the status of each module, Obeid said. This includes thermal management to ensure heating and cooling systems keep temperatures within an optimal range. It also manages any faults that may occur. Finally, it facilitates communication between the cells and the rack level.

At that rack level, Obeid identified several key responsibilities for the BMS. It performs system integration and load management, for example, and manages energy for optimized storage and uses. At this level, the BMS is also responsible for implementing safety protocols, including emergency shutdowns. Finally, he noted that the BMS manages communication with systems outside the BESS, such as those used by a facility or the grid.

Other BESS components Obeid identified include the power conversion systems (PCS), which turn DC battery power into AC power via inverters and rectifiers. They run the conversion in reverse during charging.

There’s also a BESS control system, which “serves as the central hub that integrates the BESS with other parts of the system,” Obeid said. These include the grid, microgrid or other distributed energy resources.

Some BESS may also contain an uninterruptible power supply (UPS). “This addition provides backup power to maintain critical functions and allows the system to restart independently,” Obeid said.

Capabilities and Limitations

As with any type of equipment, it’s important to be able to understand the limits of a BESS as well as having the means to determine how it is faring over time.

A battery energy storage system (BESS) consisting of several lithium battery modules placed side by side. (Image: malp via Adobe Stock)

To those ends, Obeid said every BESS has a rated power capacity and energy capacity.

He called rated power capacity the total possible instantaneous discharge capability of the BESS or the maximum rate of discharge from a fully charged state. It is described in terms of kW or MW.

By comparison, energy capacity “is the maximum amount of energy stored or consumed in kWh or MWh,” Obeid said.

He added that it’s crucial to understand the difference between kW/MW and kWh/MWh. “Knowing the difference is essential for accurately assessing energy needs, system capacities, sizing, applications and operational costs,” Obeid said. “It also aids in making informed decisions about energy usage, efficiency and sustainability.”

Using kW and kWh as examples, Obeid said that kW is a unit of power, while kWh is a unit of energy.

Of kW, he said “it indicates how quickly energy is being used at any given moment,” adding that a 10kW system can deliver 10kW of power instantly. It’s about the speed of power flow into or out of a BESS.

“Kilowatt-hour (kWh) is a unit of energy,” Obeid said, “representing the total amount of energy consumed or generated over time. It indicates the cumulative energy usage or production.” As an example, he said a 10kW system running for an hour will consume or generate 10kWh of energy.

Understanding C-Rate

One aspect of the BESS is known as the C-rate — also described as storage duration and charge/discharge rate.

“A C-rate measures the rate at which a battery is discharged relative to its maximum capacity,” Obeid said. “It is defined as the reciprocal of the time (in hours) need to fully discharge the battery.”

As examples, Obeid said a BESS with a C-rate of 1C will either be fully charged or discharged in an hour, because 1/1 — the reciprocal of 1 — equals 1. However, a C-rate of 0.5C means a charge/discharge rate of 2 hours, because 1/0.5 equals 2.

He said that managing the C-rate ensures safe BESS operation and a longer useful life.

“Batteries with a low C-rate take longer to charge,” Obeid said, “but can provide power for an extended period. Conversely, batteries with a high C-rate can deliver a large current quickly, making them suitable for high-power, short-duration applications such as grid frequency regulation. However, they cannot sustain this power output for as long as batteries with a lower C-rate.”

As an example, Obeid said that a battery with a C-rate of 5C rate can provide five times its rated power but only for 12 minutes (1/5 of 60 minutes is 12 minutes).

BESS with lower C-rates are often used in energy applications, as they provide a steady, prolonged energy supply. Power applications, by contrast, often need higher C-rates — 1C or above — because they can provide quick bursts of power.

Obeid added that while the C-rate can be reduced, it cannot exceed the rated capacity of the BESS.

“For instance, if a system is rated at 1C, it can be charged or discharged at lower C values, such as 0.5C or 0.25C,” he said, “but it cannot be operated at higher C values, such as 2C or 3C.

BESS assists decarbonization of tower cranes, job sites Known as the Enertainer — a combination of “energy” and “container” — the BESS stores enough power to address the energy peaks needed by up to three tower cranes.

Another UK contractor adopts BESS for tower cranes, construction sites Bowmer & Kirkland has adopted BESS as its preferred power solution for cranes and has discovered that only a single BESS is needed to effectively power an entire construction site.