Definition of Battery Management Systems

What is a Battery Management System

Factory of Lithium-ion/Lifepo4/NMC Battery Circuit Protection Boards

Click Here

Definition

A Battery Management System (BMS) is a technology specifically designed to oversee a battery pack, which is a component of a battery cell that is electrically organized in a row x column matrix configuration to be able to deliver a target range of voltages and currents over a period of time in response to an expected load scenario.

The oversight provided by the BMS typically includes:
–Monitoring the batteryProviding battery protection
–Estimating the operational status of the battery
–Continuously optimizing battery performance
–Reporting operational status to external devices

In this context, the term “battery” refers to the entire battery pack; however, monitoring and control functions are specifically applied to individual cells, or modules within the overall battery pack assembly. Lithium-ion rechargeable batteries have the highest energy density and are the standard choice for battery packs in many consumer products, from laptops to electric vehicles. While they perform well, they can be quite unforgiving if operated outside of the normally stringent Safe Operating Area (SOA), with results ranging from impaired battery performance to outright dangerous consequences.The job description for a BMS is truly challenging, and its overall complexity and oversight can span many disciplines, such as electrical, digital, controls, thermal, and hydraulics.

How does a battery management system work?

There is no set of fixed or unique standards for battery management systems that must be adopted.
The scope of the technical design and the functionality implemented is usually related to:
–Battery pack cost, complexity and size battery application
–Certification requirements for any safety, longevity and warranty issues various government regulations
–Costs and penalties are paramount if functional safety measures are inadequate.
There are many BMS design features, battery pack protection management and capacity management are two basic functions. We will discuss here how these two functions work. There are two key areas of battery pack protection management: electrical protection, which means not allowing the battery to be damaged by use outside of its SOA, and thermal protection, which involves passive and/or active temperature control to maintain the battery pack or bring the battery pack into its SOA.

Electrical Management Protection: Current

Monitoring battery pack current and cell or module voltage is the path to electrical protection. The electrical SOA of any battery is governed by both current and voltage. Figure 1 illustrates a typical Li-ion battery SOA where a well-designed BMS will protect the battery pack by preventing operation outside the manufacturer’s cell ratings. In many cases, further derating of cells within the SOA’s safety zone is possible in order to extend battery life.

Lithium-ion batteries have different current limits for charging and discharging, and both modes can handle higher peak currents, albeit for short periods of time. Battery manufacturers typically specify maximum continuous charge and discharge current limits, as well as peak charge and discharge current limits. A BMS that provides current protection will certainly apply the maximum continuous current. However, this may be to account for sudden changes in load conditions; for example, sudden acceleration of an electric vehicle.The BMS can decide to either reduce the available current or interrupt the battery pack current altogether by integrating the current and peak current monitoring after the time of the increment. This gives the BMS near-instantaneous sensitivity to extreme current spikes, such as short-circuit conditions that don’t draw the attention of any resident fuses, but can also tolerate high peak requirements as long as they are not excessive.

Electrical Management Protection: Voltage

Lithium-ion batteries must operate within a certain voltage range. These SOA boundaries will ultimately be determined by the intrinsic chemistry of the selected Li-ion battery and the temperature of the battery at any given time. Additionally, these SOA voltage limits are typically further constrained to optimize battery life since any battery pack will experience a large amount of current cycling, discharging due to load demands, and charging from various energy sources.The BMS must know what these limits are and will direct decisions based on proximity to these thresholds. For example, the BMS may require a gradual reduction in charging current when approaching a high voltage limit, or may require a complete termination of charging current if the limit is reached. However, this limit is typically accompanied by additional intrinsic voltage hysteresis considerations to prevent control chatter about the shutdown threshold. On the other hand, when approaching the low voltage limit, the BMS will require the critical active offending load to reduce its current demand. In the case of an electric vehicle, this can be accomplished by reducing the allowable torque available to the traction motor. Of course, the BMS must prioritize driver safety considerations while protecting the battery pack against permanent damage.

Thermal Management Protection: Temperature

On the face of it, lithium-ion batteries may have a wide operating temperature range, but at low temperatures the overall battery capacity decreases due to a significant reduction in the rate of chemical reaction. In terms of low-temperature performance, they do perform much better than lead-acid or nickel-metal hydride batteries; however, temperature management is a prudent necessity, as charging below 0°C (32°F) is physically problematic. During sub-freezing charging, lithium metal plating may occur on the anode. This is permanent damage that not only results in reduced capacity, but is more likely to fail if the battery is subjected to vibration or other stressful conditions.The BMS can control the temperature of the battery pack by heating and cooling.

The thermal management achieved is entirely dependent on the size and cost of the battery pack as well as performance objectives, design criteria for the BMS, and the product cell, which may include consideration of the targeted geographic area. Regardless of the heater type, it is often more efficient to draw energy from an external AC power source or an alternative resident battery used to operate the heater when needed. However, if the electric heater draws a moderate amount of current, energy from the primary battery bank can be siphoned off to heat itself. If a thermo-hydraulic system is implemented, an electric heater is used to heat the coolant, which is pumped and distributed throughout the battery pack components.BMS design engineers undoubtedly have the know-how of the design industry to drip thermal energy into the battery pack. For example, it is possible to turn on various power electronics within the BMS that are dedicated to capacity management. It’s not as effective as direct heat, but it can be utilized anyway. Cooling is especially important to minimize performance loss in lithium-ion battery packs. For example, perhaps a given battery operates best at 20°C; if the pack temperature rises to 30°C, its performance efficiency may be reduced by as much as 20%. If the battery pack is continuously charged and recharged at 45°C (113°F), the performance loss may rise to as much as 50%. Battery life may also be prematurely aged and degraded if continuously exposed to excessive heat, especially during rapid charging and discharging cycles. Cooling is usually achieved by either passive or active methods, and both technologies can be used. Passive cooling relies on the movement of airflow to cool the battery. In the case of an electric vehicle, this means it’s just moving down the road. However, it can be more complex than it seems, as air velocity sensors can be integrated to strategically and automatically adjust the deflector air dam to maximize airflow. Implementing an active TEMPERATURE-CONTROLLED fan can help at low speeds or when the vehicle is stopped, but this only keeps the battery pack in equilibrium with the ambient temperature. In hot weather, this may raise the initial battery pack temperature. Thermo-hydraulic active cooling can be designed as a supplemental system, typically using a glycol coolant with a specific mixing ratio that is circulated by an electric motor-driven pump through tubes/hoses, a distribution manifold, a cross-flow heat exchanger (radiator), and cooling panels, which reside on the battery pack components.The BMS monitors the temperature of the entire battery pack and opens and closes a variety of valves in order to maintain the overall battery temperature within a narrow temperature range to ensure optimal battery performance.

Capacity Management

Maximizing battery pack capacity is arguably one of the most important battery performance features offered by a BMS. Without this maintenance, battery packs can eventually become useless. The root of the problem is that battery pack “stacks” (arrays of cells connected in series) are not exactly equal and inherently have slightly different leakage or self-discharge rates. Leakage is not a manufacturer’s defect, but a characteristic of the cell chemistry, although it can be statistically affected by small manufacturing process variations. Initially, a battery pack may have well-matched cells, but over time the similarity between the cells will further diminish, not only due to self-discharge, but also by charging/discharging cycles, elevated temperatures, and general calendar aging. Knowing this, and looking back at the previous discussion, Li-Ion batteries perform well, but can be quite unforgiving if run outside of a strict SOA. We previously learned about the electrical protection required because lithium-ion batteries don’t handle overcharging well. Once fully charged, they can no longer accept current, and any extra energy is converted at high temperatures and the voltage can rise rapidly, possibly to dangerous levels. This is not a healthy situation for batteries and if it continues, it can lead to permanent damage and unsafe operating conditions. Battery Pack Series Battery arrays determine the overall battery pack voltage, and mismatches between neighboring cells can create a dilemma when attempting to charge any battery pack. Figure 3 shows why this is the case. If there is a perfectly balanced set of cells, all is well because each cell will charge in an equal manner and the charging current can be cut off when the 4.0 upper voltage cutoff threshold is reached. However, in the unbalanced case, the top cell will reach its charging limit early and will need to terminate the charging current in the branch before the other bottom cells are fully charged.

A BMS is something that steps in and saves the day, or in this case a battery pack. To show how this works, a key definition needs to be explained. The state of charge (SOC) of a battery or module at a given time is directly proportional to the amount of power available relative to the total amount of power available when fully charged. Thus, a battery at 50% SOC means it is 50% charged, which is analogous to the figure of merit on a fuel gauge.BMS capacity management is all about balancing the changes in SOC in each stack of battery pack components. Since SOC is not a directly measurable quantity, it can be estimated by a variety of techniques.The balancing programs themselves are usually divided into two main categories, passive and active. There are many variations on the theme and each type has pros and cons. It is up to the BMS design engineer to decide which is the best choice for a given battery pack and its application. Passive balancing is the easiest to implement and the simplest way to explain the general balancing concept. The passive approach allows each cell in the stack to have the same charge capacity as the weakest cell. Using a relatively low current, it delivers a small amount of energy from the high SOC cell during the charge cycle so that all cells are charged to the maximum SOC.Figure 4 illustrates how the BMS accomplishes this. It monitors each cell and utilizes transistor switches and appropriately sized discharge resistors in parallel with each cell. When the BMS senses that a given cell is approaching its charging limit, it directs excess current around it to the next cell below in a top-down fashion.

Before and after the balancing process endpoints are shown in Figure 5. In summary, the BMS balances the battery pack by allowing the cells or modules in the stack to see a charge current different from the pack current in one of the following ways:Removing charge from the most charged battery, which provides room for additional charging current to prevent overcharging and allows less charged batteries to receive more charging current redirecting some or nearly all of the charging current around the most charged battery, thus allowing less charged batteries to receive charging current for a longer period of time

Types of Battery Management Systems

Battery management systems range from the simple to the complex, and can employ a variety of different technologies to fulfill the primary directive of “taking care of the battery”; however, these systems can be categorized according to their topology, which relates to how they install and operate the cells or modules on the battery pack.

Centralized Battery Management System Architecture

There is a centralized BMS in the battery pack assembly. all the battery packs are directly connected to the centralized BMS. the structure of the centralized BMS is shown in Fig. 6. The centralized BMS has some advantages. It is more compact, and since there is only one BMS, it tends to be the most economical. However, the centralized BMS also has disadvantages. Since all the batteries are connected directly to the BMS, the BMS requires a large number of ports to connect all the battery banks. This means a large number of wires, cables, connectors, etc. in a large battery bank, which complicates troubleshooting and maintenance.

Modular Battery Management System Topology

Similar to a centralized implementation, the BMS is divided into several duplicate modules, each with a dedicated bundle of wires and connected to adjacent distribution sections of the battery pack. See Fig. 7. In some cases, these BMS sub-modules may be located under the supervision of the main BMS module, whose function is to monitor the status of the sub-modules and communicate with peripheral devices. Troubleshooting and maintenance are easier due to the repetitive modularity, and expansion to larger battery packs is simple. The disadvantages are a slightly higher overall cost and the potential for duplication of unused functionality, depending on the application.

Primary/Subordinate BMS

However, conceptually similar to the modular topology, in this case the slaves are more limited to relaying measurement information and the master is dedicated to computation and control, as well as external communications. Thus, while like the modular type, the cost may be lower because the slave station’s functions tend to be simpler, the overhead may be lower, and there are fewer unused functions.

Distributed BMS Architecture

This is very different from other topologies where the electronic hardware and software are encapsulated in modules that are connected to the battery through bundles of additional wiring. A distributed BMS integrates all electronic hardware on a control board placed directly on the cell or module being monitored. This alleviates much of the wiring that connects to the several sensor and communication wires between neighboring BMS modules. As a result, each BMS is more independent and handles calculations and communications as needed. However, despite its apparent simplicity, this form of integration does make troubleshooting and maintenance potentially problematic because it is located deep within the shielded module assembly. Costs also tend to be higher due to more BMS in the overall battery pack structure.

The Importance of Battery Management Systems

Functional safety is paramount in a BMS. During charging and discharging operations, it is critical to prevent the voltage, current and temperature of any monitored cell or module from exceeding defined SOA limits. If limits are exceeded for an extended period of time, not only will potentially expensive battery packs be compromised, but dangerous thermal runaway conditions may occur. In addition, lower voltage threshold limits are strictly monitored to protect the lithium-ion battery and functional safety. If lithium-ion batteries are kept in this low-voltage state, copper dendrites may eventually grow on the anode, which could lead to increased self-discharge rates and raise possible safety concerns. The high energy density of Li-ion powered systems comes at the cost of little room for battery management error. Thanks to improvements in BMS and Li-ion, this is one of the most successful and safe battery chemistries available today.

Battery pack performance is the next most important feature of a BMS, and this involves both electrical and thermal management. In order to electrically optimize the overall battery capacity, all cells in the battery pack need to be balanced, which means that the SOC of adjacent cells throughout the assembly is roughly equal. This is important because not only can optimal cell capacity be achieved, but it helps prevent general degradation and reduces potential hotspots for overcharging weak cells. Li-ion batteries should avoid discharging below the low voltage limit as this can lead to memory effects and significant capacity loss. Electrochemical processes are highly sensitive to temperature and batteries are no exception. When the ambient temperature drops, capacity and available battery energy decrease significantly. As a result, the BMS may use an external inline heater that sits on the liquid cooling system of the battery pack in an electric vehicle, or open a resident heater plate mounted below the battery pack module inside a helicopter or other aircraft. Additionally, since charging cold lithium-ion batteries is detrimental to battery life performance, it is important to raise the battery temperature sufficiently first. Most lithium-ion batteries cannot be rapidly charged below 5°C and should not be charged at all below 0°C. For optimal performance during typical operational use, BMS thermal management often ensures that the battery operates within a narrow Goldilocks operating region (e.g. 30-35°C). This protects performance, extends life, and fosters a healthy and reliable battery pack.

The Benefits of Battery Management Systems

A complete battery energy storage system, often referred to as a BESS, can consist of tens, hundreds or even thousands of lithium-ion batteries, depending on the application. These systems may be rated for voltages below 100V, but may be as high as 800V, with battery pack supply currents of up to 300A or more. Any mismanagement of a high-voltage battery pack can trigger a catastrophic, life-threatening disaster. Therefore, a BMS is absolutely critical to ensure safe operation.The benefits of a BMS can be summarized as follows.

Functional Safety: There is no doubt that this is especially prudent and necessary for large lithium-ion battery packs. However, even the smaller formats used in laptops can catch fire and cause significant damage. The personal safety of users of products using lithium-ion power systems leaves little room for battery management errors.
Lifetime and Reliability: Battery pack protection is managed, electrically and thermally, to ensure that all batteries are used within stated SOA requirements. This subtle oversight ensures that batteries are protected against overuse and rapid charge and discharge cycles, and inevitably leads to a stable system that has the potential to provide years of reliable service.
Performance and Range:BMS battery pack capacity management uses inter-cell balancing to equalize the SOC of adjacent cells in a battery pack assembly to achieve optimal battery capacity. Without this BMS functionality to account for variations in self-discharge, charge/discharge cycling, temperature effects, and general aging, the battery pack may eventually become useless.
Diagnostics, data collection and external communications: Supervisory tasks include continuous monitoring of all battery cells, where data logging can be used individually for diagnostics but is typically used for computational tasks to estimate the SOC of all cells in the assembly.This information is used for balancing algorithms but can be collectively forwarded to external devices and displays to indicate available resident energy, estimate expected range or range/lifespan based on current usage, and provide a state of health for the battery bank .
Reduced cost and warranty: Introducing a BMS into a BESS increases costs, and battery packs are expensive and potentially dangerous. The more complex the system, the higher the safety requirements and therefore the need for more BMS oversight. But the protection and preventive maintenance of the BMS in terms of functional safety, longevity and reliability, performance and range, and diagnostics ensures that it will reduce overall costs, including those associated with warranty.

Battery Management System and Kinglisheng

Simulation is an invaluable ally in BMS design, especially when applied to exploring and solving design challenges in hardware development, prototyping, and testing. With accurate lithium-ion battery models in play, simulation models of BMS architectures are recognized as executable specifications for virtual prototyping. In addition, simulation allows painless investigation of variants of BMS supervisory functions for different battery and environmental operating scenarios. Implementation issues can be identified and investigated very early, which allows performance and functional safety improvements to be verified prior to implementation on a real hardware prototype. This reduces development time and helps to ensure that the first hardware prototype will be robust. In addition, when performed in a physically realistic embedded system application, many identity verification tests can be performed on the BMS and battery pack, including worst-case scenarios. kinglisheng BMS can fully satisfy customers’ needs for various circuit protection system solutions for lithium-ion/nmc/lifepo4 batteries, such as power tools, electric 2-wheeled Three-wheeled vehicle, electric power assisted vehicle, outdoor energy storage and other should lithium-ion battery occasions, more than 10 years of deep plowing in the BMS industry, has a wealth of experience in problem solving and R & D and production, we have a large number of excellent engineers interested in the design and development of the BMS and battery packs, with advanced BMS R & D, testing, production equipment to ensure the overall reliability of the BMS.

Shenzhen Kinglisheng New Energy Eechnology .,Ltd