What is a BMS and how do battery management systems work?

You get into your electric vehicle, switch it on and the cluster tells you how many kilometers you can travel. Based on this range, you decide which pit stops you will make to reach your destination, but have you ever wondered how your vehicle calculates the distance it can travel?

Well, the Battery Management System or BMS keeps an eye on the battery powering your electric vehicle and estimates the range for you. In addition, the system monitors the condition of the battery and ensures that it can be used safely.

Understanding lithium-ion batteries and cells

Before discussing battery management systems, it is essential to understand how batteries are made.

A battery on an electric vehicle is made up of lithium-ion cells, and these cells are connected together to create a battery module. These modules are further connected to other modules to create a battery. This modular design helps manage the battery efficiently and improves serviceability. With this design architecture, the battery pack manufacturer can replace a faulty module rather than replacing the entire battery pack.

In terms of advantages, lithium-ion cells offer several characteristics such as high power-to-weight ratio, high energy efficiency, low self-discharge characteristics and good high temperature performance. Because of 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 with lithium-ion batteries.

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

  • Voltage specifications: The battery pack of an electric vehicle consists of several Lithium-ion cells. To put things into perspective, the Tesla Roadster comes with 6,831 cells, and each of those cells must operate within a set voltage range. For most cells, this range is between 3.0 and 4.1 volts. If cells are used outside of these ranges, battery life and performance will deteriorate.
  • Temperature limits: In addition to voltage limits, the temperature of lithium-ion batteries must also be monitored. For most cells, this range is between -4 and 131 degrees Fahrenheit (-20 and 55 degrees Celsius). If cells are operated outside of these temperature ranges, battery performance and life may decrease significantly.
  • Current draw: The amount of current drawn from the cells should 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 should also be monitored during charging. This is because large amounts of current are pumped into the battery in a short time, and this usually occurs when fast charging using level 3 chargers. Due to this high current flow into the battery , cells can overcharge, causing them to heat up, degrading cell life and performance.

As several parameters need to be monitored for 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 is operating within specified limits.

What happens if the cells do not operate within the prescribed limits?

If a battery’s cells are operating at high temperature or if too much current is drawn from them, a phenomenon known as thermal runaway can occur.

You see, a lithium-ion battery provides power through a series of chemical reactions. These reactions generate heat, and if the batteries are not operating within appropriate ranges, the amount of heat generated by these reactions increases exponentially.

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

How does a battery management system work and what is it used for?

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

The battery management system then analyzes this data to ensure that each cell is operating within prescribed limits. If not, it tries to fix the problem.

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

In the event of cell voltage variations, the battery management system performs cell balancing. To balance the cells, it transfers energy from cell to cell to ensure that all cells operate at the same voltage level.

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

How does a battery management system calculate runtime?

One of the sensors connected to the BMS measures the amount of current flowing in and out of the battery. Based on this data, the battery management system estimates how much current the battery has and how far your vehicle can travel, keeping your anxiety at bay.

Are battery management systems really necessary?

An electric vehicle’s battery management system closely monitors each battery cell. It ensures the battery is safe to use and protects the car if the cells are malfunctioning.

In addition, it estimates the range the vehicle can travel and helps improve the overall battery life cycle. Therefore, a battery management system is an essential part of an electric vehicle, and a good battery management system can improve the life of an electric vehicle by years.

Maria D. Ervin