What is a Battery Management System (BMS)?

A Battery Management System (BMS) is a technology dedicated to supervising a battery pack, a configuration of battery cells organized in a matrix of rows and columns for electrical arrangement. This setup enables the provision of a target range of voltage and current over a period for anticipated load situations.

The supervision offered by a BMS typically includes:
- Monitoring the entire battery pack
- Providing battery protection
- Estimating the battery's operational state
- Continuously optimizing battery performance
- Reporting operational status to external devices

however, monitoring and control functions specifically apply to individual cells or groups within the battery pack, known as modules. Lithium-ion rechargeable batteries, known for their high energy density, are the standard choice for battery packs in a wide range of consumer goods from laptops to electric vehicles. While they perform excellently, lithium-ion batteries can be unforgiving if operated outside their typically stringent Safety Operating Area (SOA), with consequences ranging from degraded battery performance to outright dangerous outcomes. The job description of a BMS undoubtedly presents challenges, with its overall complexity and supervision scope potentially involving multiple disciplines such as electrical, digital, control, thermal, and hydraulic engineering.

How Does a Battery Management System Work?
There is no fixed or unique standard that a Battery Management System must adhere to. The technical design scope and features implemented are typically related to:
- The battery pack's cost, complexity, and size
- The application of the battery and any concerns regarding safety, lifespan, and warranty
- Certification requirements of various governmental regulations, where the costs and penalties are crucial if functional safety measures are not in place

BMS designs encompass many functionalities, with battery pack protection management and capacity management being two fundamental aspects. This article discusses how these two functionalities work. Battery pack protection management covers two key areas: electrical protection, which means preventing damage to the battery by not allowing it to be used outside of its SOA, and thermal protection, which involves passive and/or active temperature control to maintain or bring the battery pack into the SOA.
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Types of Battery Management Systems
Battery Management Systems vary from simple to complex and can adopt different technologies to fulfill their primary directive of “caring for the battery.” However, these systems can be categorized based on their topology, which relates to how they are installed and operate across the battery or modules within the entire battery pack.

The Importance of Battery Management Systems
In BMS, functional safety is paramount. Preventing any battery or module under supervision control from exceeding specified SOA limits for voltage, current, and temperature during charging and discharging operations is crucial. Exceeding these limits for any length of time can not only affect the potentially expensive battery pack but may also result in dangerous thermal runaway conditions. Furthermore, for the protection of lithium-ion batteries and functional safety, lower voltage threshold limits are strictly monitored. If lithium-ion batteries remain in such a low voltage state, copper dendrites may eventually grow on the anode, which can increase the self-discharge rate and pose potential safety issues. The high energy density of lithium-ion power systems comes with the cost of having virtually no room for errors in battery management. Thanks to BMS and improvements in lithium-ion batteries, it is one of the most successful and safest battery chemistries available today.

Benefits of a Battery Management System
A complete battery energy storage system, often referred to as BESS, can be composed of dozens, hundreds, or even thousands of lithium-ion batteries strategically assembled together, depending on the application. The rated voltage of these systems may be less than 100V but can go up to 800V, with battery pack supply currents ranging up to 300A or more. Any mismanagement of high-voltage battery packs could lead to life-threatening catastrophic disasters. Therefore, a BMS is crucial for ensuring safe operation. The benefits of a BMS can be summarized as follows:

- Functional Safety:
Particularly prudent and necessary for large-sized lithium-ion battery packs. However, it's well-known that even smaller formats used in laptops can catch fire and cause immense destruction. There's virtually no room for battery management errors regarding the personal safety of users of products containing lithium-ion power systems.
- Lifespan and Reliability: Battery pack protection management, both electrical and thermal, ensures that all batteries are used within the declared SOA requirements. This delicate supervision ensures the safe use of batteries through rapid charging and discharging cycles, inevitably producing a stable system capable of providing years of reliable service.
- Performance and Range: BMS battery pack capacity management, where inter-battery balancing is employed to equalize the SOC across adjacent cells in the battery pack component, allows for optimal battery capacity. Without this BMS functionality to account for variations in self-discharge, charging/discharging cycles, temperature effects, and general aging, the battery pack could ultimately become useless.
- Diagnostics, Data Collection, and External Communication:
The supervisory task includes continuous monitoring of all battery units, where data logging itself can be used for diagnostics but is usually employed for computing tasks to predict the SOC across all batteries in the component. This information is used for balancing algorithms but can also be forwarded to external devices and displays to indicate the available residing energy, estimate expected range or lifespan based on current usage, and provide the health status of the battery pack.
- Reduced Costs and Warranty:
Introducing a BMS into a BESS adds cost, as battery packs are expensive and have potential hazards. The more complex the system, the higher the safety requirements, thus more BMS supervision needed. However, the protection and preventative maintenance offered by a BMS in terms of functional safety, lifespan and reliability, performance and range, diagnostics, etc., ensure that it will lower overall costs, including those associated with warranty.

Simulation is a valuable ally in BMS design, especially when applied to explore and resolve design challenges in hardware development, prototyping, and testing. With accurate lithium-ion battery models, the simulated model of the BMS architecture is recognized as an executable specification for virtual prototypes. Moreover, simulation allows for the painless investigation of variants of BMS supervisory functions against different battery and environmental operating scenarios. Implementation issues can be discovered and investigated early, allowing for verification of performance and functional safety improvements before actual hardware prototype implementation. This reduces development time and helps ensure that the first hardware prototype is robust. Moreover, many authentication tests can be conducted on the BMS and battery pack, including worst-case scenarios, when applied in embedded systems applications.

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