The High-Voltage storage unit

Reading time: approx. 5 minutes – Read this article to find out how a high-voltage storage system is constructed and what advantages it offers in practical use.

The recently published “Energy Storage Roadmap” from the Fraunhofer Institute for Systems and Innovation Research predicts a huge increase in demand for batteries in the coming years. The reasons for this are primarily due to a significant increase in battery performance in terms of the price-performance ratio. High-voltage batteries have particularly high growth potential, as technical progress is creating completely new and innovative application possibilities. The following section therefore deals specifically with the structure and functionality of high-voltage storage systems.

The advantages of high-voltage storage systems

A high-voltage storage system is a battery with terminal voltages greater than 60 VDC. The success of any battery system is defined by its cost, efficiency and flexibility. The advantage of using high-voltage storage systems lies in the lower currents as a function of the voltage compared to low-voltage systems. This reduces ohmic losses, simplifies thermal management and thus increases efficiency. Low currents also reduce the costs of peripheral electronic components and the use of thinner cables can save weight.

How is a high-voltage storage system constructed

Each high-voltage storage system consists of several individual battery cells. If these cells are connected in series, the total voltage of the storage system increases. The capacity and current carrying capacity can be increased by connecting them in parallel. The product of voltage and capacity provides the energy content of the storage system and is solely dependent on the number of individual cells used. It is therefore not possible to compare different batteries on the basis of ampere hours without taking the terminal voltages into account. This increasingly leads to a specification of the watt hours, which enables a comparison of different systems.

The individual cells connected in the overall battery have different internal resistances due to the manufacturing processes. These deviations drift further and further apart during the product life cycle, which has a negative impact on the usable capacity and service life. For this reason, so-called balancers and battery management systems are used in high-voltage storage systems, which monitor the individual cell voltages and can equalise them if necessary. In the event of a fault, many high-voltage storage systems use an interlock loop to protect the environment from the potentially dangerous voltage. This enables the opening of an integrated relay in the event of exposed contacts of devices connected to the high-voltage bus or internal battery defects, which de-energises the terminals accessible to the user or service personnel from the outside.

How battery cells work

As described above, a battery is made up of several individual galvanic cells. A distinction is made between primary cells, which cannot be recharged, and secondary cells, which can be recharged several times. Secondary cells are used almost exclusively in high-voltage batteries. Here, the electrical energy supplied during charging is converted into chemical energy.

Basically, every accumulator consists of two electrodes with different electrical potentials, the anode and the cathode. Through an electrochemical process, the electrodes undergo a material change in which charged atoms, so-called ions, are adsorbed or absorbed. When a galvanic cell is charged, oxidation takes place at the positive pole and reduction at the negative pole, creating a potential difference that can be measured as a clamping voltage on the cell. During a discharge process, these chemical reactions take place in reverse. By definition, the electrode at which oxidation takes place always acts as the anode. In contrast, a reduction always takes place at the cathode. Therefore, the negative pole is the anode when discharging and the positive pole is the anode when charging.

The ions released during this process can move freely in the electrolyte surrounding the electrodes. To protect the battery from cell closure, i.e. direct contact between the electrodes, they are physically and electrically separated from each other by a separator. However, the separator must be permeable to the ions.

If electrochemical equilibrium occurs during discharge, the cell is completely discharged and the terminal voltage is 0 VDC. This state causes irreparable damage to a secondary cell and should be prevented at all costs.

Chemical composition of battery cells in high-voltage storage systems

 The performance of high-voltage storage systems is determined by the cell chemistry. Depending on the chemical composition of the electrodes and the electrolyte, the internal resistance, the nominal voltage, the energy density and, of course, the price of the cell change. While the nominal voltage is generally not a problem in high-voltage batteries due to the serial connection of the individual cells, the internal resistance has an influence on the current carrying capacity and charging time as well as the energy density on the size and weight of the battery.

Commonly used electrode materials include lead, nickel-cadmium, lithium iron phosphate or lithium titanate, all of which differ in their electrical properties and are advantageous for certain applications. Due to the rapidly increasing demand for batteries, there are already bottlenecks in the extraction and supply of raw materials required for the production of rechargeable batteries. Examples include the production bottleneck for copper foil or the urgently needed element cobalt, which increases the energy density in lithium-ion batteries. These problems are likely to become even more acute in the future.

High-voltage batteries in practice

A fundamental distinction is made between mobile and stationary high-voltage storage systems. Examples of mobile storage systems include traction batteries in electric vehicles, while stationary storage systems are often used to bridge grid fluctuations or power failures in critical applications such as hospitals and data centres. The systems have to fulfil different requirements depending on the application. In contrast to the 12 VDC starter batteries used in most cars, the legislator has not yet prescribed a deposit system for other batteries. However, due to the scarcity of raw materials and the associated high costs, a sustainable recycling programme is essential.