Battery Management System (BMS)

25 AUGUST 2022 • 7 MIN READ

Paweł Głogowski



 Battery Management System

In our recent article about wireless charging, we’ve briefly touched upon some of the emerging technologies in the world of eMobility. One of them (admittedly, not-so-emerging as it's been going on for a while) was the Battery Management System used in EVs. It’s now time to shed some more light on what it is.

Surely, the batteries may be regarded as the core of any electric vehicle. Essentially, it is what gets the car going. It’s also what determines to a great extent how appealing an EV can get for the potential buyers. Thus, ensuring the battery runs smoothly, safely and efficiently is paramount in the EV formation process. Here’s where BMS kicks in.

What is a Battery Management System?

Battery Management System (BMS)) is a system that monitors and manages a rechargeable battery (or group of batteries), such as those found in electric vehicles, UPS systems, and solar energy storage systems.

The BMS is responsible for protecting the battery from operating outside its safe limits, maximizing its lifespan, and providing accurate information about the battery's state of charge, state of health, and current capacity. In short, its primary functions can be narrowed down to ensuring the battery remains safe and reliable.

A BMS typically consists of a control unit, sensors, and connection wires. The control unit may be a separate device or integrated into the battery pack. The sensors monitor various parameters of the battery, such as voltage, current, temperature, and state of charge. The connection wires carry power and data between the battery and the BMS.

The control unit uses the information from the sensors to determine when to charge or discharge the battery, how much power to allow in or out, and whether the battery is operating within its safe limits. If the battery is operating outside its safe limits, the BMS will take action to protect the battery, such as disconnecting it from the load or shutting off power altogether.

The BMS may also provide information about the battery to a user or system, such as the current state of charge, state of health, and remaining capacity. This information can be used to optimize the performance of the system in which the battery is used.

A BMS typically includes safety features to protect the battery, such as overcharge and over discharge protection, short circuit protection, and thermal runaway protection. These safety features help to prevent damage to the battery and ensure that it operates safely.

The BMS may also include features to maximize the lifespan of the battery, such as balancing. Balancing equalizes the voltage of the cells in a battery pack, which helps to prevent capacity loss and extends the lifespan of the battery.

Battery management systems are used in a variety of applications, including electric vehicles, UPS systems, and solar energy storage systems. BMSs are an important part of these systems, as they help to protect the battery from damage, extend its lifespan, and provide accurate information about the battery's state.

How does BMS work?

With lithium-ion, an average battery module consists of packs of single battery cells. That is because a singular cell doesn’t hold enough capacity to power the bigger establishments. In such cases (where there are multiple modules connected to make up for a battery) monitoring and managing becomes rather complicated, as the singular cells can charge differently.

BMS collects data gathered from the batteries (in real-time) and based on the information it acts accordingly. Amongst the parameters collected  there are:

  • voltage
  • temperature
  • current

Once these are collected, the BMS system will process it. The outcomes of such processing is the below data:

  • State of charge (SOC)- an estimation of how much charge the battery currently has - while in use
  • State of health (SOH) - an estimation of the capacity/condition of the battery, it measures how well the battery is able to hold a charge
  • Issues - monitoring and addressing any recognized malfunctions

Based on the outputs, the BMS then takes action to protect the battery, such as turning off charging when the SOC is full or shutting down the system when a cell is over temperature.

There are two main types of BMSs. The first is a centralized BMS, which uses one control unit to manage all of the battery cells in the system. The second type of BMS is a distributed BMS, which uses multiple control units to manage the battery cells in the system.

The main advantage of using a centralized BMS is that it is simpler and cheaper to design and build. The main disadvantage of using a centralized BMS is that if the control unit fails, the entire system will fail.

The main advantage of using a distributed BMS is that it is more resilient and can continue to operate even if one or more of the control units fails. The main disadvantage of using a distributed BMS is that it is more complex and expensive to design and build.

Which type of BMS is best for a given application depends on the specific requirements of the application. For example, if cost is the primary concern, then a centralized BMS may be the best choice. If reliability is the primary concern, then a distributed BMS may be the best choice.

BMS in the electric vehicles

In terms of the use of BMS in automotive, it doesn’t differ from any other means. It helps to ensure that the battery pack lasts for as long as possible and performs at its best. A well-designed BMS can make the difference between a battery pack that lasts for years and one that needs to be replaced frequently. This can have a significant impact on the overall cost of ownership of an electric vehicle.

Amongst the vital aspects that get covered by BMS in EVs, there are:

  • Thermal management - much like any other batteries, the ones installed in EVs can overheat. With thermal management, the censor units control the temperature and, if it get too high, they trigger the cooling mechanisms
  • Discharge and charge control - the primary function of any BMS is to protect the cells within the battery pack from being over-discharged or overcharged. To do this, the BMS will cut off the discharge path when the cell voltage drops below a certain threshold, and will also cut off the charging path when the cell voltage reaches a higher threshold.
  • Battery balancing - In order to ensure that all the cells in the battery pack are at the same voltage, the BMS will occasionally "balance" them by equalizing the voltages of all the cells. This is usually done during charging, when the cell voltages are highest.
  • Communication and Safety monitoring - Some BMS systems also have a communication interface that allows them to be connected to a computer or other devices. This can be used for firmware updates, data logging, or even remote control of the BMS. Additionally, BMS systems have built-in safety features that can monitor the system for faults and take appropriate action if a problem is detected.
  • SOH and SOC - as explained earlier