Battery Rack: what it is and how it works in a BESS

A battery rack is a mechanical and electrical structure that groups multiple battery modules or packs to form a storage unit with higher voltage and capacity within a BESS (Battery Energy Storage System).

In a large-scale storage system, individual cells are not connected directly to the inverter or the electrical system. They are arranged hierarchically at different levels (cell, module, rack and system) to achieve the voltage, energy and power required to operate the asset.

The battery rack is therefore one of the fundamental levels of this architecture. In addition to physically supporting the modules, it incorporates electrical connections, monitoring, protection and communications that enable the batteries to operate together safely and in a coordinated manner.

In large Utility Scalesystems, multiple battery racks can connect to a common DC bus and work together with the PCS (Power Conversion System), which converts the batteries’ direct current into alternating current for exchange with the electricity grid.

What is a Battery Rack made up of?

The exact architecture of a battery rack depends on the manufacturer, the chemistry used, the system voltage and the overall BESS design. There is no universal configuration.

However, a battery rack usually includes several fundamental components:

  • Battery modules: contain groups of electrically connected electrochemical cells and are the basic building blocks of the rack.
  • Mechanical structure: supports and secures the modules, maintaining the spacing, accessibility and strength required during operation.
  • Busbars and DC connections: electrically connect the modules and provide the required total voltage.
  • Rack BMS: monitors parameters such as voltage, current, temperature, State of Charge (SoC) and various alarm states.
  • Contactors: electrically connect the rack to, or isolate it from, the system’s DC bus.
  • Fuses and protection devices: provide protection against certain overcurrent or electrical fault conditions.
  • Precharge circuit: limits certain transient currents before the rack is fully connected to the DC bus.
  • Sensors: can monitor temperature, voltage, current, insulation or other relevant parameters.
  • Service disconnect: some designs include components that isolate the rack during maintenance work.

In modern systems, all this information is integrated into a hierarchical control architecture. Each module’s BMS communicates with the rack controller, which in turn can communicate with a higher-level BMS and the EMS (Energy Management System).

Infographic showing the main components of a Battery Rack in a BESS, including modules, busbars, BMS, contactors, sensors and structure
A Battery Rack integrates battery modules, electrical connections, control systems, protection and communications within the architecture of a BESS.

From cell to BESS: Cell, Module, Rack and Container

To understand what a battery rack is, it helps to examine how a storage system is built up from its smallest element.

The typical architecture can be represented as follows:

Cell → Module / Pack → Battery Rack → Battery System → BESS

Battery Cell

The cell is the basic electrochemical unit. In a lithium-ion battery, it is where energy is physically stored and released through reversible electrochemical reactions.

Battery Module

Several cells are connected together to form a battery module. Depending on the design, they may be connected in series, in parallel or using a combination of both configurations.

The module simplifies the assembly, monitoring, maintenance and manufacture of the complete system.

Battery Rack

Several modules are integrated into a battery rack. In many high-voltage stationary systems, the modules are connected mainly in series within the rack to achieve the required DC operating voltage.

Battery Cabinet or Container

One or more racks can be installed inside a battery cabinet, equipment room or container.

These concepts should not be used interchangeably. A battery rack is the structure that electrically groups the modules, whereas a cabinet or container is the physical enclosure that can house racks together with climate control, fire protection, communications and other auxiliary systems.

The precise terminology may vary between manufacturers, particularly the use of words such as module, pack, cluster, rack or string, so the architecture defined by each OEM should always be checked.

How are modules connected inside a Battery Rack?

One of the battery rack’s main functions is to achieve the DC voltage needed for the system to operate efficiently with the PCS. When identical modules are connected in series, their voltages add together while their capacity in Ah remains approximately the same.

Vrack = Nseries × Vmodule

Nominal energy can be calculated in simplified form as:

Erack = Vrack × Ah / 1,000

where:

  • Erack = nominal rack energy in kWh.
  • Vrack = nominal rack voltage.
  • Ah = nominal capacity in ampere-hours.

Battery Rack capacity example

Assume, purely as an illustrative example, that a rack contains 16 modules connected in series and each module has:

  • Nominal voltage: 51.2 V.
  • Capacity: 280 Ah.

The nominal rack voltage would be:

16 × 51.2 V = 819.2 V

And its approximate nominal energy:

819.2 V × 280 Ah / 1,000 = 229.4 kWh

That battery rack would therefore have approximately 229 kWh of nominal energy. This does not mean that all this energy can be used under any conditions. Usable energy depends on parameters such as the operating SoC range, depth of discharge, temperature, degradation, current limits and the strategy defined by the BMS.

Battery Racks in series and in parallel

The difference between series and parallel connections is fundamental to understanding the electrical architecture of a BESS. Within a rack, series connections are often used to increase voltage. Several racks can then be connected in parallel to a DC bus to increase the energy capacity and current available to the system. In simplified terms:

  • Series connection → increases voltage.
  • Parallel connection → increases capacity and available current.

For example, if each rack has 229 kWh and a system includes 20 equivalent racks, the combined nominal energy would be approximately:

20 × 229.4 kWh = 4,588 kWh ≈ 4.59 MWh

In practice, the electrical architecture must also account for current limits, protection, wiring, voltage drop, BMS coordination and PCS characteristics.

What does the BMS do inside a Battery Rack?

The Battery Management System is one of the fundamental components of the battery rack. Its role is not simply to display the percentage of battery capacity available. The BMS continuously monitors the electrical and thermal conditions of the cells and modules to keep the system within its operating limits. Typical parameters and functions include:

  • Individual cell voltages.
  • Total module and rack voltage.
  • Charge and discharge current.
  • Temperatures.
  • State of Charge (SoC).
  • State of Health (SoH).
  • Cell balancing.
  • Detection of abnormal conditions.
  • Contactor management.
  • Communication with higher-level systems.

Excessive differences between cells can limit the performance of the entire rack. If one cell reaches its upper or lower voltage limit before the others, the BMS may need to limit charging or discharging of the whole assembly. Therefore, a battery rack's performance depends not only on its nominal capacity, but also on the uniformity and condition of its cells.

Battery Rack, PCS and DC Bus: How Energy Flows

In a large-scale BESS, battery racks form the electrochemical part of the system, but they must be integrated with various pieces of equipment to exchange energy with the grid. In simplified terms, the electrical flow can be represented as follows:

Battery Cells → Modules → Racks → DC Bus → PCS → Transformer → Grid

During discharge, the energy stored in the racks flows through the DC bus to the PCS, which converts direct current into alternating current. Charging reverses this process: the PCS receives alternating current from the grid and converts it into direct current to charge the battery racks. The EMS coordinates when and how much the system should charge or discharge according to the asset's operating strategy.

In applications such as energy arbitrage, balancing services, Peak Shaving or renewable energy integration, this coordination may change continuously depending on prices, grid setpoints and battery condition.

Battery Rack Safety and Protection

A battery rack can operate at high DC voltages and contain large amounts of stored energy, so its design must address electrical, electrochemical and thermal risks. The IEC 62619 sets out safety requirements for secondary lithium cells and batteries used in industrial applications, including stationary energy storage systems.

The IEC 63056 sets out additional safety requirements for lithium cells and batteries used specifically in electrical energy storage systems and covers systems with nominal DC voltages up to 1,500 V within its scope. The elements that may contribute to rack safety include:

  • BMS and continuous monitoring.
  • Fuses and electrical protection devices.
  • Isolation contactors.
  • Insulation monitoring.
  • Temperature monitoring.
  • Climate control systems.
  • Gas or smoke detection, depending on the design.
  • Fire detection and protection systems at installation level.

One of the specific risks of lithium-ion batteries is thermal runaway, a condition in which certain internal reactions can cause a rapid, self-sustaining rise in temperature. System design must consider not only how to reduce the likelihood of a failure, but also how to prevent or limit its propagation between cells, modules, racks and other BESS components.

The standard IEC 62933-5-2 addresses the safety of grid-integrated electrochemical energy storage systems from a system-level perspective and considers interactions between their various subsystems.

Which Parameters Define Battery Rack Performance?

Comparing battery racks solely by their kWh can lead to incomplete conclusions. To assess a rack correctly, several technical parameters must be analysed together.

  • Nominal energy (kWh): theoretical amount of stored energy.
  • Usable energy (kWh): energy actually available within the specified operating window.
  • Nominal voltage (V): reference voltage of the assembly.
  • Voltage range: minimum and maximum values during charging and discharging.
  • Capacity (Ah): amount of stored electrical charge.
  • Maximum current: determines the maximum power the rack can deliver or absorb.
  • C-rate: ratio between power/current and the battery's energy capacity.
  • DoD: permitted depth of discharge.
  • Efficiency: losses during charging and discharging cycles.
  • SoH: battery state of health relative to its initial condition.
  • Cycles and degradation: changes in capacity and internal resistance with use and time.
  • Operating temperature: influences performance, ageing and safety.

Furthermore, the maximum power of a BESS cannot be determined by looking at the battery rack alone. It also depends on the number of connected racks, the limits of the BMS, the DC bus, the PCS, the transformer and the project's grid connection constraints.

Battery Rack and BESS Degradation

BESS degradation occurs at cell level, but its effects eventually become apparent at module, rack and overall system level. Two mechanisms are commonly distinguished:

  • Calendar ageing: degradation caused by the passage of time, even when the battery is not cycling.
  • Cycle ageing: degradation associated with charging and discharging cycles.

Factors such as temperature, average SoC, depth of discharge, current and cycling frequency affect the rate of degradation. Systems with many battery racks may also experience rack-to-rack imbalance, meaning differences in capacity, resistance or SoC between racks.

An effective strategy for O&M and monitoring makes it possible to identify deviations in temperature, voltage or performance before they cause significant losses of availability or capacity. For this reason, rack-level data analysis is an essential tool for assessing the technical and economic condition of a BESS throughout its service life.