You get into your electric vehicle, turn it on, and the cluster shows you the number of miles you can travel. Based on this range, you decide the pit stops you will take to reach your destination, but have you ever wondered how your vehicle calculates the distance it can travel?

Well, the Battery Management System or the BMS keeps an eye on the battery pack that powers your electric vehicle and estimates the range for you. Moreover, the system monitors the health of the battery pack and ensures that it's safe to use.

Understanding Battery Packs and Lithium-Ion Cells

Before we get into Battery Management Systems, it's essential to understand how battery packs are made.

A battery pack on an electric vehicle is made of Lithium-ion cells, and these cells are connected to each other to create a battery pack module. These modules are further connected to other modules to create a battery pack. This modular design helps in efficiently managing the battery pack and improves serviceability. Due to this design architecture, the battery pack manufacturer can replace a faulty module rather than replacing the whole battery pack.

In terms of advantages, Lithium-ion cells offer several features such as a high power-to-weight ratio, high energy efficiency, low self-discharge characteristics, and good high-temperature performance. Due to these characteristics, Lithium-ion cells are the go-to choice for electric vehicles, but these batteries are not flawless, and solid-state battery technology is trying to solve the problems that come with Lithium-ion batteries.

image of a Nissan Leaf battery pack
Image Credit: Tennen Gas/Wikimedia Commons

Another thing to note here is that Lithium-ion cells can only offer the advantages mentioned above if they are operated within specified limits. Below is a brief overview of these operational limits.

  • Voltage specifications: The battery pack on an electric vehicle is made of several Lithium-ion cells. To put things into perspective, the Tesla Roadster came with 6,831 cells, and each of these cells needs to operate within a set voltage range. For most cells, this range is between 3.0 and 4.1 volts. If the cells are used outside these ranges, the life of the battery pack and the performance it offers deteriorates.
  • Temperature limits: In addition to the voltage limits, the temperature of Lithium-ion batteries also needs to be monitored. For most cells, this range is between -4 and 131 degrees Fahrenheit (-20 and 55 degrees Celsius). If the cells are operated outside these temperature ranges, the performance and life of the battery pack can go down drastically.
  • Current draw: The amount of current drawn from the cells must also be monitored. If the amount of current drawn from the cells is outside the prescribed limits, the life of the cells degrades exponentially.
  • Charging current: The battery pack also needs to be monitored during charging. This is because high amounts of current are pumped into the battery pack in a short duration of time, and this usually occurs during fast charging using level 3 chargers. Due to this high current flow in the battery pack, the cells can overcharge, causing them to heat, degrading the life and performance of the cells.

As several parameters are required to be monitored for the optimum performance of a battery pack, it needs a Battery Management System. This management system is a computing device that monitors several characteristics of each cell and ensures that the battery pack operates within the specified limits.

What Happens if the Cells Don't Operate Within the Prescribed Limits?

If the cells in a battery pack are operated at a high temperature or too much current is drawn from them, a phenomenon known as thermal runaway can occur.

You see, a Lithium-ion battery provides energy through a series of chemical reactions. These reactions generate heat, and if the batteries are not operated in suitable ranges, the amount of heat generated by these reactions goes up exponentially.

An exploding Lithium-ion battery
Image Credit: Tavo Romann/Wikimedia Commons  

Due to this increase in heat generation, the cells can catch fire and cause a chain reaction in the battery pack. Therefore, it is essential to monitor the temperature of each cell to prevent thermal runaway.

How Does a Battery Management System Work, and What Does It Do?

The Battery Management System is a computer connected to several sensors. These sensors monitor the voltage, current, and temperature of each cell and send it to the BMS.

The Battery Management System then analyzes this data to ensure that each cell operates within the prescribed limits. If that is not the case, then it tries to solve the problem.

If the cells inside the battery pack are too hot, then the BMS manages the cooling system to reduce the battery pack's temperature.

In case of variations in cell voltage, the Battery Management System performs cell balancing. To balance the cells, it transfers energy from one cell to another to ensure that all cells operate at the same voltage level.

A chipset

In addition to the tasks mentioned above, the BMS takes logs of the data it receives to calculate the state of charge and health of the battery.

How Does a Battery Management System Calculate Range?

One of the sensors connected to the BMS measures the amount of current entering and exiting the battery pack. Based on this data, the Battery Management System estimates the amount of current the battery pack has and the amount of distance your vehicle can travel, keeping your range anxiety at bay.

Are Battery Management Systems Really Needed?

The Battery Management System on an electric vehicle monitors each cell in the battery pack closely. It ensures that the battery pack is safe to use and protects the car if the cells are not working correctly.

In addition, it estimates the range that the vehicle can travel and helps improve the battery pack's overall lifecycle. Therefore, a Battery Management System is a critical part of an electric vehicle, and a good battery management system can improve the life of an electric vehicle by several years.