In today's world, where batteries power numerous devices and electric vehicles (EVs) have become increasingly popular, ensuring optimal battery performance and safety is of most importance. This is where Battery Management Systems (BMS) come into play. In this blog post, we will discuss the fundamentals of BMS, exploring its building blocks, working principles, and key functions.
What is a Battery Management System (BMS)?
A Battery Management System (BMS) is an electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, and balancing it.
BMSs are used in a wide variety of applications, including electric vehicles, solar panels, and power tools. They are essential for ensuring the safety, performance, and longevity of battery-powered systems.
BMS Building Blocks
The BMS comprises four primary functional blocks:
Cut-off FETs:
The Cut-off FETs serve as an isolation mechanism between the battery and the charger. They facilitate the connection of the high-side and low-side of the battery pack. The high-side activates the MOSFET using the charge pump driver, while the low-side activates the MOSFET without the need for a charge pump driver. Integrating Cut-off FETs reduces the overall cost of the BMS and eliminates the use of high voltage devices, thus saving significant die area.
Fuel Gauge Monitor:
The Fuel Gauge Monitor aids in tracking the charge entering and leaving the battery pack. The charge flow is calculated by multiplying the current and time. Various methods can be employed to measure current flow, but the most efficient and cost-effective approach involves measuring the voltage across a sense resistor using a 16-bit ADC with low offset and a high common-mode rating. Employing a higher ADC allows for a broader dynamic range and faster operation.
Cell Voltage Sensors:
Cell voltage monitoring is a standard function of the Battery Management System. It helps determine the battery's health by ensuring that all cells in the battery operate at standard voltage levels during charging and discharging. This approach enhances safety and improves the battery's lifespan.
Temperature Monitoring:
With advancing technology, batteries are designed to supply high currents while maintaining a constant voltage. However, high current flow can lead to rapid temperature increases, potentially resulting in accidental explosions. To prevent this, the BMS continuously monitors the battery's temperature and regulates it to the rated value. This feature is valuable as it alerts users to start/stop charging or discharging when the temperature exceeds the specified threshold.
Additional building blocks found in BMS are:
Battery Authentication: Prevents the connection of BMS electronics to third-party battery packs, ensuring compatibility and safety.
Real-time Clock (RTC): Used in black-box applications for accurate timekeeping.
Memory: Utilized in black-box applications for storing data.
Daisy Chain: Make seamless connection between stacked devices, streamlining communication and control.
BMS Working
The BMS works by constantly monitoring the battery's state and taking steps to protect it from damage. For example, if the battery's voltage gets too high, the BMS will shut off the charger. If the battery's temperature gets too high, the BMS will reduce the charging current.
The BMS also collects data about the battery's performance, such as its state of charge (SOC), state of health (SOH), and remaining useful life. This data can be used to optimize the battery's performance and extend its lifespan.
The operation of a battery management system (BMS) relies on the complexity of the onboard electronic components.
The BMS's microcontroller constantly measures the real-time cell voltage and current, utilizing this information to control the switching of MOSFETs. The BMS employs a single bus for both charging and discharging operations.
Initially, both the charging and discharging FETs remain off, resulting in no current flow.
The BMS's microcontroller detects the voltage at the input and activates the charging MOSFET, initiating the battery charging process.
If there is no voltage present at the input pin, the BMS determines that a load is connected and activates the discharging FET.
Two common cell balancing techniques are employed in BMS:
Passive Cell Balancing: This method employs bypass resistors to discharge excess voltage from cells and equalize their charge levels.
Active Cell Balancing: In this approach, the excess charge from one cell is transferred to another cell with a lower charge to equalize them. It utilizes charge-storing capacitors and inductors.
By employing these cell balancing techniques, the BMS ensures that all cells operate within safe and optimal voltage ranges, enhancing overall battery performance and longevity.
The above image depicts the connection of a 3S BMS to a battery pack comprising three cells that are linked in series.The designated voltage for storage mode is 3.7V, while the full charge voltage is 4.2V. Consequently, the charging voltage for the three-cell battery pack totals 12.6V.
The power supply is connected to the battery pack through heavy gauge wires, whereas smaller wires are employed as cell balancing wires, carrying lower currents.A BMS is equipped with a specified capacity rating, enabling it to gauge and correlate the available electric charge with the capacity of the battery.
As the battery's capacity diminishes, there is a corresponding decrease in the amount of electric charge it can hold.
Functions of a Battery Management System (BMS):
Ensuring Safety: Given the higher density found in lithium-ion battery packs, there is an elevated risk of fire hazards. Hence, it is crucial to operate batteries within their specified limits.A BMS takes care of this task by preventing overcharging and over discharging of the battery pack, thus prolonging its lifespan.It also provides safeguards such as protection against short circuits, overcurrent during charging and discharging, and prevention of reverse charging, among other features.Modern BMS systems come equipped with Bluetooth and UART communications for enhanced functionality.
Optimizing Battery Performance: To ensure optimal battery performance, it is essential for batteries to operate within the designated range of maximum and minimum values, including current, voltage, and temperature.As previously mentioned, a BMS assists batteries in maintaining operation within these critical rated parameters.In the case of battery packs, it facilitates equal charging and discharging of individual cells, significantly enhancing the overall performance of the battery pack.Moreover, an efficient Battery Management System not only improves performance but also extends the lifespan of the battery packs.
Health Monitoring and Diagnostic Capabilities: The charge level of a battery directly affects its charging and discharging duration. A BMS possesses the capability to calculate and indicate the remaining charge available in the battery.By comparing battery parameters with their rated values, a BMS can identify any anomalies and take corrective measures to enhance the battery's health and performance.
Benefits of BMS
BMSs offer a number of benefits, including:
Increased safety: BMSs can help to prevent battery fires and explosions by protecting the battery from damage.
Extended battery life: BMSs can help to extend the battery's lifespan by preventing it from being overcharged or over-discharged.
Improved Battery performance: BMSs can help to improve the battery's performance by optimizing its charging and discharging cycles.
Reduced maintenance costs: BMSs can help to reduce maintenance costs by preventing battery failures.
Conclusion
BMSs are an essential component of any battery-powered system. They offer a number of benefits, including increased safety, extended battery life, improved performance, and reduced maintenance costs.
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