Introduction to Battery Management System

INTRODUCTION

A Battery Management System is an analog and/or digital electronic hardware device complemented with specific software added to a battery system. To prevent battery failure and mitigate potential hazardous situations, there is a need for a supervising system that ensures that batteries function properly in the final application. This supervising system is referred to as a Battery Management System (BMS).

In electric vehicles, the Battery Management unit is a part of a system that stores and converts energy into motion and vice versa. Electrification will have a profound impact on the vehicle’s electrical system. As many systems are traditionally based on hydraulics which is now driven by electric motors in the world of battery-operated products and especially in electric vehicles, battery management is perhaps the most important aspect of the product.

Why is this, an article published in the WALL STREET JOURNAL in 2019 provides some insight which identifies the battery as quoted, the big obstacle on the road to electric vehicles?

The battery system is the most expensive component comprising the bill of materials. For this reason, the batteries must be able to impact performance and functionality in a big way! Commensurate with its relative cost. So, the care and feeding of the battery pack is a big focus to ensure that it delivers the performance and longevity that justifies its cost. Several factors go into the design of a battery pack as well as the battery management unit. Ideally, the battery pack should outlast the service life of the vehicle itself and it must do so safely and efficiently, accomplishing this; means monitoring and controlling certain operational parameters.

Why do we need a Battery Management System (BMS)?

• Sensing and high-voltage control

The sensors and high voltage controllers available in a Battery Management System (BMS) carry out work like measurement of voltage, current, temperature, switching devices, thermal management, and ground fault detection. A battery pack gauges each cell’s voltage. This indicates the relative balance of cells as overcharging them may lead to thermal runaway. One of the most pertinent parts of SOC and SOH estimation algorithms is cell voltage estimation.

Temperature plays a substantial role when it comes to the battery operating characteristics and degradation rates. Excessive heat is one of the major factors accountable for the degradation of a battery. Internal resistance present in a battery is responsible for increasing its temperature while it is undergoing the process of charging or discharging. Battery Management System (BMS) uses thermistors to sense the temperature inside the battery pack and its systematized working.

• Protection against Over Voltage, Under Voltage, Over Current, Short Circuit, and High Temperatures

Immense loss and dire consequences will be the outcomes if high-energy batteries uncontrollably release energy further leading to short circuits. The over-voltage protection starts by shorting the cell through a bleed resistor when the threshold level is reached.

The protection scheme will preclude the discharge of a cell or the entire battery when there is under voltage. These small steps can prevent the cell voltages from moving out of the battery threshold value. During a short circuit, the battery is cut off to ensure safe operation. Further, thermal shutdown occurs when the battery is not at its safe operating temperature.

•  Interface

The interface in a BMS is usually through the Controller Area Network (CAN) bus. In some manifestable cases, the I2C protocol is also used. I2C communication is the short form for inter-integrated circuits. I2C is a serial communication protocol, so data is transferred bit by bit along a single wire. It is highly synchronous, so the output of bits automatically synchronizes to the sampling of bits by a clock signal shared.

•  Performance management

Performance Management includes the state of charge (SOC) estimates and cell balancing/equalization. Cell equalization is a dynamic solution to this problem which takes into account the aging and operating conditions of the cells, the Battery Management System (BMS) may incorporate a cell balancing scheme to prevent individual cells from becoming overstressed. Battery Management System (BMS) uses cell balancing techniques to wangle cell balance or cell equalization.

Passive cell balancing uses the bypass resistors to discharge the excess voltage and equalize with other cells.

In the active cell balancing the excess charge of one cell transfers to another cell that has a low charge to equalize them. It uses charge storing capacitors and inductors. The efficiency of active cell balancing is relatively higher in contrast to passive cell balancing as it uses the distribution of energy rather than the dissipation of energy.

•  Diagnostics

A built-in current sensor on each battery cell allows early detection of internal short circuits. The Battery Management System (BMS) discovers any violations of current, voltage, or temperature limits. The capacity of the battery not only depends on its voltage and current profile but also its age and operating temperature. The certain SOH (state of health) estimation algorithms in the BMS enable us to surveil battery health due to normal degradation. The state of life (SOL) estimation algorithms approximately predict the remaining life of the battery.

Conclusion

The complexity of the electronic components available onboard determines the working of a battery management system.  1S, 2S…. to 20S BMS are various types of Battery Management Systems (BMS) available. Here the word “S” stands for the number of cells. It circumvents the over-charging and over-discharging of the battery pack. Moreover, it maintains charge levels within maximum and minimum allowed capacity to prevent sudden accidents which can have atrocious consequences.

Hence, a BMS is a highly crucial device to assure the safety of the battery and user.

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