Bench Talk

Rarely Thought About and Yet Essential: Electrical Connection Technology for Energy Storage Systems

(Source: Phoenix Contact)

Energy storage systems play a crucial role in the future development of renewable energy. A stable energy supply is only guaranteed if the volatility of the source has an efficient and reliable storage system. This heavily depends on the electrical connection technology and the typical design of an energy storage system through the flow of energy and data information (Figure 1).

Figure 1: Energy storage systems must be fast, reliable, and economical. Plug-in battery connections play an important role in this. (Source: Phoenix Contact)

Generally, a galvanic battery energy storage system is modular in design (Figure 2). With a few limitations, users can apply this modularity in large storage systems to smaller, home storage systems. The first element is the connection of battery cells to either increase current or voltage (Figure 3). This is where electrical connections are found at this very basic level.

Figure 2: Typical energy storage system design—from the storage module in a rack to the entire system with its external interfaces. (Source: Phoenix Contact)

Figure 3: Overview of a storage module with battery packs, BMS PCB, and internal and external interfaces. (Source: Phoenix Contact)

A large number of cells integrated into one housing is known as a storage module. The Battery Management System or BMS monitors and controls the storage module. The BMS oversees the charging and discharging processes of the battery cells so that all cells are charged and discharged uniformly. This is also called balancing. To enable balancing, each of the parallel-connected cell groups is controlled separately. Along with the two connections—positive and negative—the BMS also has a large number of connections to the individual cell groups (see Figure 3).

A logic system processes all information on the status of the battery module. Each BMS needs a data interface. Data and power connections are routed from the BMS PCBs to the module housing. Screw connections are used for the power, while the plug-in is mainly used for the data interface.

The voltage of an individual battery module is typically within a range of 24V to 96V. However, the system voltage is often at a much higher level. Therefore, several battery modules are connected in series and, in most cases, integrated physically into corresponding racks (see Figure 2). The individual modules are also connected together for data communication, with at least one data line routed from each module.

Each rack also includes a controller—known as the rack BMS. The rack BMS is designed to be connected to assigned battery modules via power connections. An additional power interface connects the rack to the higher-level system. The rack BMS has several data interfaces for the data traffic and several sensor technology inputs. From the external interfaces on the housing of the rack BMS, the connections are routed back internally to various PCBs.

Depending on the scale, several storage racks are integrated together to form a higher-level system. This also has its own controller for remote communication of charging and discharging. Air-conditioning equipment as well as fire sensors and fire suppression equipment should be included.

A multitude of different electrical connections plays a crucial role with the system components and the external connection of the storage system. A misperception is that only the power flows need to be considered, but this is not true. Using a digital connection of the storage system to the grid from the solar or wind turbine generator, creates the most efficient use of an energy storage system.

The number of potential errors in connection technology is absolutely manageable. The probability of errors depends on the quality of components used, the design of the technology for the application, and the user’s qualifications. A fundamental prerequisite for avoiding malfunctions in complex electronic systems is device design. This begins with understanding the connection technology strengths, environmental requirements (corrosion if exposed to moisture), and the degrees of protection. Also, consider vibrations on connectors where bolts/screws are not tightened or maintained (Figure 4).

Figure 4: With screw connections, there is an increased risk of high-temperature rises due to increased contact resistance. (Source: Phoenix Contact)

The failure or even the complete destruction of an energy storage system due to an error is the worst-case scenario. Suboptimal designs or system states can also cause hidden costs that are easy to avoid in most cases.

The external power connectors of a battery module are used here as an example. Even a correctly designed and directive-conforming connection can lead to losses of more than 1,000kWh per module over the entire service life. The initial investment in a higher-quality connector quickly pays for itself.

The design, quality, and correct use of the connection technology are closely related to cost, reliability, and efficiency of these systems. A compromise that initially appears to be inexpensive leads all too often to high running costs and system failures. It is important to use skilled manufacturers when designing the connection technology at all levels of an energy storage system. Phoenix Contact can provide all the connectors required to build a complete energy storage systems.

Dr. Rüdiger Meyer, Phoenix Contact GmbH & Co. KG
Dr. Rüdiger Meyer is an electrical engineer with over 25 years of experience in renewable energies (solar, storage). Dr. Meyer currently works as an Application Expert Energy Storage Business Area DC at Phoenix Contact GmbH & Co. KG in Blomberg, Germany. During his tenure at Phoenix Contact, he has served as a representative in BVES (German Energy Storage Association), BSW (German Solar Association, working group “energy storage”), and ZVEI (German Electrical and Electronic Manufacturers' Association, “Batteries Division”). Additionally, Dr. Meyer does active standardization work on energy storage systems in DKE (German standardization body) UK 261.1 as well as in IEC TC 120 MT8.