Battery Management System (BMS)

Let’s start by talking about the evolution of battery usage in the past decades. It has been implemented in numerous devices and has revolutionized to reach better performance at the same time increase efficiency. The Royal Swedish Academy of Sciences awarded Nobel Prize in Chemistry, 2019 to John B. Goodenough, M. Stanley Whittingham, and Akira Yoshino for the development of lithium-ion batteries, batteries, which were developed in the ’90s. It has not lost its charm, rather the direct opposite has happened, the revolution it has created is tremendous and today from iPod shuffle to electric trucks everything carries lithium in its pocket. Recent customers of lithium are the electric vehicles, transportation running on the same principle as of a battery toy car.

Why EV manufacturers chose lithium batteries over other battery technology?

The beauty embodied in lithium battery packs is the energy they can carry in the required volume. The high energy density of lithium batteries makes it perfect for an electric vehicle, lighter and efficient. Charging is always a concern for any EV owner and with a faster charging rate of lithium, packs outshine other technologies. Lithium charges at a faster rate and reduces the refuelling time of the vehicle.

Moving on to the construction of lithium cell -the chemical structure, solid electrolyte, and stable encapsulation make it suitable for application prone to mechanical vibrations and impacts. The lithium cells are found performing at the same efficiency in both stationary and mobile applications. like every coin has two sides, Lithium brings a good amount of constraints and restrictions in its usage.

Why do we need BMS?

Thermal instability has always remained the greatest obstacle in a lithium cell’s design and application. The efficiency plummets considerably at higher operating temperatures. Though lithium brings higher energy density, faster-charging rate, low self-discharge, and longer life cycle but these factors are maintained only when it is provided by a manager. Overcharging is another prime concern with lithium batteries usage therefore charging needs to be closely & accurately monitored. memory effect is also a benefactor to its disadvantages.

So, what is meant by a manager? it definitely is not a physical entity, and here goes the answer: BMS. Lithium battery packs are comprised of multiple cells connected in series and parallel configuration, hence cell balancing is the important aspect for efficient power delivery. Imbalance cell causes internal loading of battery reducing the state of charge and increasing failure rate. A cell balancing mechanism is required for proper battery pack operation and higher cell life. A battery management system helps lithium batteries maintaining and enhancing their performance with protection against hostile conditions to prevent irreversible failure.

What is Battery Management System?

“To make a best lithium battery pack just manage its voltage, current & temperature and this is what a BMS does”

As the name is self-explanatory, a system designed to manage and control a battery unit for proper operation. Initial designs only incorporated protection components i.e. fuses, FET switches with analogue comparators sensing deviation, and pulling the complete battery unit down in case of abruptions. With due course of time, lithium batteries found their way into several electronic devices, and hence, simultaneously the need to design an efficient manager to take care of its working increased. A modern BMS comprises multiple blocks integrated to build a complete lithium battery unit:

1. Power Delivery Unit (PDU): Comprises of FET switch and gate driver circuit which control the outgoing and incoming power in the battery. Cell balancing with individual current bypass FETs is also part of the PDU. Few specific PDU also manages regenerative braking power
2. Sensing and measurement: Comprises of on-chip or external ADCs to compute cell voltages, junction temperature, and charge/discharge current. SAR-based ADC provides a high sampling rate and resolution. The modern BMS chip has SAR-ADC-based AFE integrated into it.
3. Communication block: Comprises of CAN transceiver, RS232 transceiver for communicating with parallel battery packs or with the vehicle control unit
4. Central processor: Manages the above blocks with an embedded algorithm deciding as per inputs and battery state. It also manages the external and internal communications of BMS.

What are the functions provided by BMS?

Battery Management System (BMS) performs three primary functions:

1. It protects the battery pack from being over-charged (cell voltages going too high) or over-discharged (cell voltages going too low) thereby extending the life of the battery pack. It does this by constantly monitoring every cell in the battery pack and calculating exactly how much current can safely go in and come out of the battery pack without damaging it. These calculated current limits are then sent to the source and load (motor controller, power inverter, etc), which are responsible for respecting these limits.
2. It calculates the State of Charge (the amount of energy remaining in the battery) by tracking how much energy goes in and out of the battery pack and by monitoring cell voltages.
3. It monitors the health and safety of the battery pack by constantly checking for shorts, loose connections, breakdowns in wire insulation, and weak or defective battery cells that need to be replaced.

The secondary functions that the BMS performs:

• Balances all the cells in the battery pack by intelligently bleeding off excess energy from cells that are charged more than others. This provides the maximum amount of usable energy (capacity) from the battery pack since the pack is only as strong as the weakest cell.
• Monitors the temperature of the battery pack and controls a battery fan to regulate the temperature of the pack. Additionally, it constantly monitors the output of the fan to make sure it is working properly.
• Provides real-time information and values to other devices such as motor controllers, chargers, displays, and data loggers using several different methods.
• Stores error codes and comprehensive diagnostic information to aid in fixing problems with the battery pack should any issues arise.

What are the basic components of a BMS?

Fuse: When a violent short circuit occurs, the battery cells need to be protected fast. Fuse is meant to be blown by the overvoltage control IC in case of overvoltages, driving it to the ground. The MCU can communicate the blown fuse’s condition, which is why the MCU power supply has to be before the fuse.

Current Sensing: Keeping a time reference and integrating the current over time, basically a sensor used to sense the current flow and report it to the MCU, we obtain the total energy entered or exited the battery, implementing a Coulomb counter. In other words, we can estimate the state of charge by using the following formula:

Thermistors: Temperature sensors, usually thermistors, are used both for temperature monitor and for safety intervention. It blows the fuse, when the temperature goes above the required rating without MCU intervention, leading to no time delay.

Balancer: Battery cells have given tolerances in their capacity and impedance. So, over cycles, a charge difference can accumulate among cells in series. If a weaker set of cells has less capacity, it will charge faster compared to others in series. The BMS has to therefore stop other cells from charging, or else the weaker cells will get overcharged.

Conversely, a cell can get discharged faster, risking that cells going under its minimum voltage. In this instance, a BMS without a balancer has to stop the power delivery earlier

In Figure 7, you can see a thermistor that controls the input of the overvoltage control IC. This artificially blows the SCP (the fuse shown in Figure 5) without MCU intervention.

In conclusion, a proper Battery Management System can lead to the optimal usage of a lithium-ion battery in any available electronic device. Its main features are utilized and amplified by BMS, also it acts as a catalyst for the work done by the battery by improving overall battery life and efficiency.