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.
