Introduction
Battery Management System (BMS) is the technology that has been dedicated to the oversight of a battery pack which is an assembly of Battery Cells electrically connected in a row x column matrix configuration, for delivering the targeted range of voltage and current.
In simple words, BMS can be thought of as the brain of the Battery Pack which monitors and controls the charging and discharging to maintain the performance of the rechargeable batteries. Most importantly it keeps the battery operating inside the safety margins.
The Battery Characteristics which are monitored/overseen by the BMS are:
- Detection of Battery Type
- State of Charge and State of Discharge (Voltages Monitoring)
- Safe-Operating Areas (SOA) of Batteries
- The capacity of the Battery Pack
- Battery Life and overall health of the Batteries (State of Health)
- Power Consumption
- Remaining Operational Time
- Charge Cycles
If BMS detects any abnormality/problem in the operational parameters (like the voltage, temperature, etc.) it immediately triggers an input to the alarm and disconnects the battery pack from the load/charger.
What Are Battery Packs?
Battery packs are generally made up of Lithium-ion cells and these cells are connected for creating a battery pack module. Several modules connected together form a battery pack. The modular design of the battery packs helps in efficiently managing the battery pack and improving the serviceability which results in replacing the faulty module instead of the whole battery pack.
Lithium-ion cells offer several features like a high power-to-weight ratio, high energy efficiency, low self-discharge characteristics and good temperature performance. Due to the previously mentioned characteristics, Lithium-ion cells have been used everywhere from Mobile phones to EVs.
The biggest drawback of Lithium-ion cells can only offer the above-mentioned features when it is being used/operated under limited conditions. The operational limits are as follows:
Voltage Specifications
- As the battery packs in EVs are made-up of several Lithium-ion cells and each of the cells needs to be operated in a set voltage range (i.e. 3.0 to 4.1V).
- If the cells are operated outside the set voltage range, the life, as well as the performance offered deteriorates of the battery pack.
- Just to put things into perspective, the Tesla Roadster had come with 6,831 Lithium-ion cells.
Temperature Limits
- The range of the temperature for Lithium-ion cells is around -20 °C to 60 °C (-4 °F to 136 °F), hence monitoring the temperature of cells is important.
- If the cells are being operated/used outside the temperature range, it results in drastic degradation of the cells (life and performance of the battery pack).
Current Draw
- The amount of current drawn from the cells needs to be closely monitored because the life of the battery pack degrades exponentially if used/operated outside the prescribed limits
Charging Current
- The high amount of current pumped into the battery pack in a short period results in over-charging of Lithium-ion cells which causes them to heat and degrade the life as well as the performance of the cells.
- Hence due to the above factor, monitoring the battery pack while charging is highly important.
Due to above mentioned operational limits, BMS plays an important role.
Let us have a look at a scenario if we don’t operate the Lithium-ion batteries within the prescribed limits:
- When a battery pack is operated at high temperatures or too much current is drawn from them, there is a phenomenon called Thermal Runaway can take place.
- As we all are aware that Battery Pack made up of Lithium-ion cells provides energy through a series of chemical reactions, and when these reactions occur, heat is generated if the battery pack is not operated in suitable conditions, the amount of heat generated can go up exponentially which might cause the cells to catch fire and cause a chain-reaction in the battery pack.
How does Battery Management System (BMS) Work?
BMS has several sensors connected to it for monitoring the voltage, current and temperature of each cell and then analyzes the data (received from the sensors) to ensure that each cell is operating within the prescribed limits. If it finds that there is some problem, it will try to solve the problem.
For example,
- If the cells inside the battery pack are too hot, BMS will be managing the cooling system attached to the battery pack for reducing the temperature of the battery pack.
- If BMS observes variations in cell voltages, it would be performing cell balancing, i.e., transferring the energy from one cell to another to ensure that all the cells are operating at the same voltage levels.
In addition to the above tasks, BMS also takes the log of the data it receives and then calculates the state of charge and battery health.
BMS also has sensors which measure the amount of charge entering as well as exiting the battery pack and based on this data, BMS estimates the amount of current battery pack might have.
Building Blocks of Battery Management System
There are four main functional blocks,
- Cut-off FETs
- Fuel Gauge Monitor
- Cell voltage monitor
- Temperature Monitor
Cut-off FETs
A FET driver acts as an isolation between the battery and the charger. It is used to connect the high side and low side of the battery pack.
- High-side – Activates NMOSFET using the charge pump driver
- Low-side – Activates NMOSFET without charge pump driver
These integrated Cut-off FETs reduce the overall cost of the BMS. It is also eliminating the use of high-voltage devices that could consume a large die area.
Fuel Gauge Monitor
This helps in keeping track of the charge entering and exiting the battery pack. The charge flowing is calculated by multiplying current and time.
Although several methods are used to measure the current flow, the most efficient and cost-effective solution is to measure the voltage of the sense resistor using a 16-bit ADC with low offset and high common-mode rating.
Higher ADC is beneficial to obtain an extensive dynamic range at more speed.
Cell Voltage Sensors
Cell voltage monitoring can be called a standard function of the Battery Management System. It is useful in determining the health of the battery.
All cells in a battery should operate at standard voltage levels during charging and discharging for safety and improving the life cycle.
Temperature Monitoring
As technology is evolving, batteries are made to supply high currents in the meantime keeping the voltage constant.
Since a high current flow through batteries can cause sudden temperature rise and force them to explode accidentally. It needs to be avoided. For this reason, the BMS continuously monitors the temperature of the battery and regulates it to the rated value.
This feature is handy because if the temperature rises above the rated value, it will inform you to start/stop charging or discharging.
Other Building Blocks
Few more of the available blocks are,
- Battery Authentication – prevents the connection of BMS electronics to the third-party battery pack.
- Real-time Clock (RTC) – used in black-box application
- Memory – used in black-box application
- Daisy Chain – simplifies the connection between stacked devices
Functions of BMS
Safety
Due to the higher density of lithium-ion battery packs, there is an increased risk of catching fire. So as mentioned earlier, it is essential to operate batteries at the rated value.
A BMS does this work for you. It avoids over-charging and over-discharging of the battery pack to extend the battery life.
It also offers short-circuit protection, charging and discharging over current protection, anti-reverse charging protection etc.
Modern BMS is equipped with Bluetooth and UART communications.
Battery Performance Optimization
For a battery to perform at its best, it must operate somewhere between the maximum and minimum rated values, i.e. current, voltage, temperature etc.
As we learned earlier, a BMS helps batteries to operate within these critical rated values.
In the case of the battery packs, it helps in maintaining equal charging and discharging of the cells. This massively improves the performance of the battery pack.
Not only performance but an efficient Battery Management System is useful in improving the life of the battery packs.
Health Monitoring and Diagnostics
The charge level of a battery determines the charging and discharging time. A BMS is capable of calculating and indicating the charge available in a battery.
A BMS checks for the oddity in the battery parameter by comparing them with rated values. Also, it is capable of taking corrective actions to increase the health of the battery.
Importance of BMS
Functional safety is of the highest importance in a BMS. It is critical during charging and discharging operation, to prevent the voltage, current, and temperature of any cell or module. If limits are exceeded for a length of time, not only is a potentially expensive battery pack compromised, but dangerous thermal runaway conditions could ensue. If the Li-ion battery stays in this low-voltage state, copper dendrites could eventually grow on the anode, which can result in elevated self-discharge rates and raise possible safety concerns.
The performance of the battery pack is the next highest important feature of a BMS, and this involves electrical and thermal management. To electrically optimize the overall battery capacity, all the cells in the pack are required to be balanced, which implies that the SOC of adjacent cells throughout the assembly is approximately equivalent. This is exceptionally important because not only can optimal battery capacity be realized, but it helps prevent general degradation and reduces potential hotspots from overcharging weak cells. Lithium-ion batteries should avoid discharge below low voltage limits, as this can result in memory effects and significant capacity loss. For optimum performance during typical operational usage, BMS thermal management often ensures that a battery operates within a narrow Goldilocks region of operation (e.g., 30 – 35°C). This safeguards performance promotes more extended life and fosters a healthy, reliable battery pack.
Benefits Of Battery Management Systems
The entire battery energy storage system, often referred to as BESS, could be made up of tens, hundreds, or even thousands of lithium-ion cells strategically packed together, depending on the application. These systems may have a voltage rating of less than 100V but could be as high as 800V, with pack supply currents ranging as high as 300A or more. Any mismanagement of a high-voltage pack could trigger a life-threatening, catastrophic disaster. Consequently, BMSs are critical to ensure safe operation. The benefits of BMSs can be summarized as follows.
Functional Safety
Hands down, for large-format lithium-ion battery packs, this is particularly prudent and essential. But even smaller formats used in, say, laptops, have been known to catch fire and cause enormous damage. The personal safety of users of products that incorporate lithium-ion powered systems leaves little room for battery management error.
Life Span and Reliability
Battery pack protection management, electrical and thermal, ensures that all the cells are used within declared SOA requirements. This delicate oversight ensures the cells are taken care of against aggressive usage and fast charging and discharging cycling, and inevitably results in a stable system that will potentially provide many years of reliable service.
Performance and Range
BMS battery pack capacity management, where cell-to-cell balancing is employed to equalize the SOC of adjacent cells across the pack assembly, allows optimum battery capacity to be realized. Without this BMS feature to account for variations in self-discharge, charge/discharge cycling, temperature effects, and general ageing, a battery pack could eventually render itself useless.
Diagnostics, Data Collection, and External Communication
Oversight tasks include continuous monitoring of all battery cells, where data logging can be used by itself for diagnostics but is often purposed to the job for computation to estimate the SOC of all cells in the assembly. This information is leveraged for balancing algorithms but collectively can be relayed to external devices and displays to indicate the resident energy available, estimate expected range or range/lifetime based on current usage, and provide the state of health of the battery pack.
Cost and Warranty Reduction
The introduction of a BMS into a BESS adds costs, and battery packs are expensive and potentially hazardous. The more complicated the system, the higher the safety requirements, resulting in the need for more BMS oversight presence. But the protection and preventive maintenance of a BMS regarding functional safety, lifespan and reliability, performance and range, diagnostics, etc. guarantee that it will drive down overall costs, including those related to the warranty.
Conclusion
A BMS is one of the most basic components of an electrical energy storage system. Because BMS react to events both externally and internally, a safe BMS is essential to a successful electrical system. The BMS can be configured in several ways depending on the application, but the essential operational purpose and safety aspect remains the same- to protect the battery.