Batteries and similar devices accept, store, and release electricity on demand. Batteries use chemistry, in the form of chemical potential, to store energy, just like many other everyday energy sources. For example, logs and oxygen both store energy in their chemical bonds until burning converts some of that chemical energy to heat. Gasoline and oxygen mixtures have stored chemical potential energy until it is converted to mechanical energy in a car engine. Similarly, for batteries to work, electricity must be converted into a chemical potential form before it can be readily stored. Batteries consist of two electrical terminals called the cathode and the anode, separated by a chemical material called an electrolyte. To accept and release energy, a battery is coupled to an external circuit. Electrons move through the circuit, while simultaneously ions (atoms or molecules with an electric charge) move through the electrolyte. In a rechargeable battery, electrons and ions can move either direction through the circuit and electrolyte. When the electrons move from the cathode to the anode, they increase the chemical potential energy, thus charging the battery; when they move the other direction, they convert this chemical potential energy to electricity in the circuit and discharge the battery. During charging or discharging, the oppositely charged ions move inside the battery through the electrolyte to balance the charge of the electrons moving through the external circuit and produce a sustainable, rechargeable system. Once charged, the battery can be disconnected from the circuit to store the chemical potential energy for later use as electricity.
Batteries were invented in 1800, but their complex chemical processes are still being studied. Scientists are using new tools to better understand the electrical and chemical processes in batteries to produce a new generation of highly efficient, electrical energy storage. For example, they are developing improved materials for the anodes, cathodes, and electrolytes in batteries. Scientists study processes in rechargeable batteries because they do not completely reverse as the battery is charged and discharged. Over time, the lack of a complete reversal can change the chemistry and structure of battery materials, which can reduce battery performance and safety.
DOE Office of Science Contributions to Electrical Energy Storage Research
Research supported by the DOE Office of Science, Office of Basic Energy Sciences (BES) has yielded significant improvements in electrical energy storage. But we are still far from comprehensive solutions for next-generation energy storage using brand-new materials that can dramatically improve how much energy a battery can store. This storage is critical to integrating renewable energy sources into our electricity supply. Because improving battery technology is essential to the widespread use of plug-in electric vehicles, storage is also key to reducing our dependency on petroleum for transportation.
BES supports research by individual scientists and at multi-disciplinary centers. The largest center is the Joint Center for Energy Storage Research (JCESR), a DOE Energy Innovation Hub. This center studies electrochemical materials and phenomena at the atomic and molecular scale and uses computers to help design new materials. This new knowledge will enable scientists to design energy storage that is safer, lasts longer, charges faster, and has greater capacity. As scientists supported by the BES program achieve new advances in battery science, these advances are used by applied researchers and industry to advance applications in transportation, the electricity grid, communication, and security.
Electrical Energy Storage Facts
- The 2019 Nobel Prize in Chemistry was awarded jointly to John B. Goodenough, M. Stanley Whittingham, and Akira Yoshino "for the development of lithium-ion batteries."
- The Electrolyte Genome at JCESR has produced a computational database with more than 26,000 molecules that can be used to calculate key electrolyte properties for new, advanced batteries.
Resources and Related Terms
Scientific terms can be confusing. DOE Explains offers straightforward explanations of key words and concepts in fundamental science. It also describes how these concepts apply to the work that the Department of Energy’s Office of Science conducts as it helps the United States excel in research across the scientific spectrum.
Battery packs contain a multitude of cells that provide the power to the electrical load of a device. Battery chemistries such as lithium-ion can become unstable due to a number of factors. This instability can cause thermal runaway which could lead to an explosion or fire.
To monitor an individual battery cell or modules in a pack, a Battery Management System (BMS) is designed into the battery pack. The battery management system ensures that the battery continues working in a safe operating level.
BMS for Battery Chemistries
Not every battery chemistry requires a battery management system. Some batteries, such as lead acid, typically have very stable chemistries. Batteries designed with lower energy capacity and simple functions for devices also usually do not have a battery management system. However, lithium-based batteries are usually equipped with a battery management system due to their high energy density. This setup includes both lithium-ion and lithium-phosphate batteries. Another time when a battery pack will come equipped with a BMS is if the pack is designed for fast charging characteristics.
Battery Management System's Role
The battery management system's main role is monitoring. It can monitor a host of factors such as temperature, capacity, current, and voltage. It may include additional functions as well depending on the complexity of the device's power consumption, battery charging, and battery discharging needs. There is no one-size-fits-all for the design of the BMS. The monitoring system will be designed and programmed based on battery chemistry and device.
Custom battery pack being assembled with a BMS.
Using sensors, the BMS analyzes the gathered data from the battery cells and other electronics to further understand the state of health (SoH) and state of charge (SoC). It compares the information to the programmed benchmarks to evaluate whether the data is within acceptable limits.
The secondary role of the battery management system is its protection circuits. If the battery cells are entering dangerous levels, the BMS will automatically shut down the battery's charging and discharging state. This safe mode protects the rest of the batteries in the pack, the device, and the user. This BMS information can be evaluated by the user to determine the reasons why the battery pack entered into an unstable, dangerous state. This information can later be used to redesign the battery pack or perform recalls.
Battery management systems may also provide additional functions depending on the design. For some battery packs, the BMS will provide cell balancing. Cell balancing redistributes the charging and discharging states between cells to prevent overcharging and over-discharging while prolonging the longevity of the cells.
Designs of a BMS
The design infrastructure of the BMS is based on how it will interact between the battery cells, the charging, and the device. There are several different designs for the BMS based on where it is located and its function.
Centralized BMS
Centralized BMS has a single battery management system that handles all the battery cell modules and multiple packs. With many ports, the BMS is connected to all the cells to perform monitoring and measurements. This compact setup can be economical for smaller battery pack designs yet can become more complicated and costly for several packs that will require numerous connectors, cabling, wires, and other components.
Distributed BMS
Distributed BMS are the opposite of the centralized BMS. Instead of a single battery management system, a distributed BMS has a control board with the monitoring electronic components mounted onto each battery module or cell. Each component system is connected to the adjacent BMS which leads to the single controller. Every BMS is self-contained to monitor the connected cell as it relays the communication through fewer wires to the controller. By encapsulating the BMS components onto the cells or modules, it reduces the amount of cabling and wiring required to make a simple design. However, the costs for distributed BMS are higher due to the number of BMS units that are needed for each cell.
Primary/Secondary BMS
In a primary/secondary BMS infrastructure, specific functions are reserved for each BMS unit. The primary BMS handles the communication, computation, and control functions. The secondary BMS is restricted to monitoring the cells, performing measurements, and relaying this information to the primary BMS. This infrastructure allows for a simpler design for the secondary BMS when multiple systems are required and allows fewer features to be wasted or become redundant.
Mobile BMS
Mobile BMS has similar features to centralized, distributed, and primary/secondary BMS infrastructures. There are multiple BMS systems like a distributed system, however, it takes a more centralized approach where each BMS will monitor multiple cells or modules. Each BMS will have the same functions and features, although in some design cases, there will be a primary BMS that will monitor the other BMS and communicate information to external devices much like in the primary/secondary BMS. A main advantage is that each BMS will have duplicate functions to ensure full monitoring. A disadvantage is that there may be duplicated features that will go unused due to this setup.
Summary
Battery pack functionality, safety, and performance are the key goals of a BMS unit. Monitoring high-performance and high-demand battery packs can prevent damage to the cells caused by extremely high or extremely low temperatures, electrical shorts, overcharging, and overly discharging. By managing the cell voltage, charge, temperature, current, and balancing, you can prolong the life of the battery pack and further understand the energy demands of your devices.