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Sravan Kumar Keerthi Written by Sravan Kumar Keerthi, Principal Engineer- EV System Engineering
on 29 Mar 2022

The electric vehicle drivetrain offers new freedom in terms of electric vehicle architectures while leading to new challenges in terms of meeting all requirements. Since electric vehicles have an electric motor and a battery instead of a combustion engine and a fuel tank, the architecture becomes simple and controllable at the component level. Modifications to locate the battery pack safe zone in an EV require extensive adoptions to integrate the battery safely.

 

Battery Electric Vehicles

A Battery Electric Vehicle (BEV), pure electric vehicle, only-electric vehicle or all-electric vehicle is a type of Electric Vehicles (EV) that exclusively uses chemical energy stored in rechargeable battery packs, with no secondary source of propulsion. Battery electric vehicles thus have no internal combustion engine, fuel cell, or fuel tank. Some of the broad categories of vehicles that come under this category are trucks, cars, buses, motorcycles, bicycles, and forklifts.

Different types of batteries are used to power electric vehicles, and deciding which battery is best depends on its energy storage efficiency, production costs, constructive characteristics, safety, and lifespan. Lithium-ion batteries are the most utilized technology in electric cars. EVs run on high voltage lithium-Ion battery packs. Lithium-ion batteries have higher energy density (100-265wh/kg) compared to other battery chemistries. They pose a risk of fire under unusual circumstances. It is crucial to operate electric vehicles in pre-defined safety limits to ensure the safety of the user as well as the vehicle.

 

Battery Management System

A Battery Management System (BMS), which manages the electronics of a rechargeable battery, whether a cell or a battery pack, thus becomes a crucial factor in ensuring electric vehicle safety. It safeguards both the user and the battery by ensuring that the cell operates within its safe operating parameters. BMS monitors the State Of Health (SOH) of the battery, collects data, controls environmental factors that affect the cell, and balances them to ensure the same voltage across cells.

A battery pack with a BMS connected to an external communication data transfer system or a data bus is referred to as a smart battery pack. It may include additional features and functions such as fuel gauge integration, smart bus communication protocols, General Purpose Input Output (GPIO) options, cell balancing, wireless charging, embedded battery chargers, and protection circuitry, all aimed at providing information about the battery’s power status. This information can help the device conserve power intelligently.

A smart battery pack can manage its own charging, generate error reports, detect and notify the device of any low-charge condition, and predict how long the battery will last or its remaining run-time. It also provides information about the current, voltage, and temperature of the cell and continuously self-corrects any errors to maintain its prediction accuracy. Smart battery packs are usually designed for use in portable devices such as laptops and have embedded electronics that improve the battery’s reliability, safety, lifespan, and functionality. These features enable the development of end products that are user-friendly and more reliable. For instance, with embedded chargers, batteries can have longer life cycles as the chargers charge the batteries to optimal, ideal specifications within the temperature limits. Accurate fuel gauges allow users to confidently discharge batteries to their limits and not worry about damaging the cell. GPIO, which stands for General Purpose Input/Output (GPIO), is an interface used to connect electronic devices and microcontrollers such as diodes, sensors, displays, and so on.

Typical Battery Management System

Functions of the BMS

Fitting an EV with a BMS can improve safety. The battery management system performs the following four functions:

 

1. Monitoring battery parameters

This is the primary function of a BMS. It monitors the state of a cell as represented by parameters such as:

  • Voltage—indicates a cell’s total voltage, the battery’s combined voltage, maximum and minimum cell voltages, and so on.
  • Temperature—displays the average cell temperature, coolant intake and output temperatures, and the overall battery temperature.
  • The state of charge of the cell to show the battery’s charge level.
  • The cell’s state of health—shows the remaining battery capacity as a percentage of the original capacity.
  • The cell’s state of power——shows the amount of power available for a certain duration given the current usage, temperature, and other factors.
  • The cell’s state of safety——determined by keeping a collective eye on all the parameters and determining if using the cell poses any danger.
  • The flow of coolant and its speed.
  • The flow of current into and out of the cell.

 

2. Managing thermal temperatures

Temperature is the biggest factor affecting a battery. The battery’s thermal management system keeps an eye on and controls the temperature of the battery. These systems can either be passive or active, and the cooling medium can either be a non-corrosive liquid, air, or some form of phase change. Using air as a coolant is the simplest way to control battery temperatures.

Air cooling systems are often passive as they rely on the convection of the surrounding air or use a fan to induce airflow. However, the main drawback is the system’s inefficiency. Significant power is used to run the cooling system as compared to a liquid-based one. Also, in larger systems such as car batteries, the additional components needed for air-based systems such as filters can increase the weight of the car, further affecting the battery’s efficiency.

Liquid-cooled systems have a higher cooling potential than air because they are more thermally conductive. The batteries are submerged in coolant, or the coolant can freely flow into the BMS without affecting the battery. However, this indirect form of thermal cooling can create large temperature differences across the BMS due to the length of the cooling channels. But they can be reduced by pumping the coolant faster, so a tradeoff is created between the pumping speed and thermal consistency.

 

3. Making key calculations

A BMS calculates various battery values based on parameters such as maximum charge and discharge current to determine the cell’s charge and the discharge current limits. These include:

  • The energy in kilowatt-hour(s) (kWh) delivered since the last charge cycle
  • The internal impedance of a battery to measure the cell’s open-circuit voltage
  • Charge in Ampere per hour (Ah) delivered or contained in a cell (called the Coulomb counter), to determine the cell’s efficiency
  • Total energy delivered and operating time since the battery started being used
  • Total number of charging-discharging cycles the battery has gone through

 

4. Facilitating internal and external communication

A BMS has controllers that communicate internally with the hardware at a cellular level and externally with connected devices. These external communications differ in complexity, depending on the connected device. This communication is often through a centralized controller, and it can be done using several methods, including:

  • Different types of serial communications
  • CAN bus communicators, often used in vehicles
  • DC-BUS communications, which are serial communications over power lines
  • Various types of wireless communication including radio, pagers, cellphones, and so on.

Only a high-level voltage BMS has internal communication; low-level centralized ones simply measure cell voltage by resistance divide. A distributed or modular BMS must utilize a low-level internal cell controller for modular architecture or implement controller-to-controller communication for a distributed architecture. However, such communication is difficult, especially in high voltage systems, due to the voltage shift between cells. What this means is that the ground signal in one cell may be hundreds of volts higher than that of the next cell.

This issue can be addressed using software protocols or using hardware communication for volt-shifting systems. There are two methods of hardware communication—using an optical-isolator or wireless communication. Another factor hampering internal communication is the restriction of the maximum number of cells that can be used in a specific BMS architectural layout. For instance, for modular hardware, the maximum number of nodes is 255. Another restriction affecting high voltage systems is the seeking time (for reading voltage/current) of all cells, which limits bus speeds and causes loss of some hardware options.

 

Optimal Energy Utilization

Battery management systems keep the battery safe, reliable, and increase the senility without entering a damaging state. Different monitoring techniques are used to maintain the state of the battery, voltage, current, and ambient temperature. The BMS communicates with the onboard charger to monitor and control the charging of the battery pack. It also helps maximize the range of the vehicle by optimally using the amount of energy stored in it. It is a crucial component in electric vehicles to ensure that batteries do not get overcharged or over discharged, thus avoiding damage to the battery and harm to occupants.

The battery is a fundamental component of the electric vehicle, which represents a step forward toward sustainable mobility. The battery management system is a critical component of electric and hybrid electric vehicles. Its chief purpose is to ensure safe and reliable battery operation. As an engineering services provider, Cyient works closely with industry experts through our focus areas of megatrends—Sustainable Energy Solutions and Electrification.

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