The Top 5: Why BMS Matters In a Lithium-Ion Battery

The move to decarbonise our grid is increasingly using Lithium-Ion batteries. Starting with the transportation sector, where Lithium-Ion batteries remain the option of choice to power EV’s. But even the Lithium battery is critically dependent on a sound Battery Management System (BMS) to deliver.

What Is the BMS?

The performance of electric vehicles depends on a lot of factors – cell voltage, battery life and health, safety, charging-discharging rates, etc. All these factors, one way or the other are linked to the rechargeable batteries in electric vehicles. Rechargeable battery packs are made of multiple cell modules arranged in a series and parallel. These battery packs produce several hundred volts of electricity. Various functions inside the car are dependent on them. That is why, it becomes a critical component of the vehicle that requires constant monitoring and control. This is where BMS comes into the picture.

A BMS ensures the complete tracking of all the functions performed by the battery, and so the vehicle. Hence, it is a system that manages lithium-ion battery packs through integrated firmware and hardware. When paired with telematics, it provides real-time data on the status and health of a forklift battery. A BMS design can become as complex as the purpose the battery seeks to serve.

Components of Battery Management System

A battery-management system (BMS) typically consists of several components. The most common components include cut-off field-effect transmitters (FETs), fuel-gauge monitors, cell voltage sensors, real-time clocks, temperature monitors, and microcontrollers (BMS algorithms).

FET is accountable for connection and isolation between load and charger of the battery pack while the fuel-gauge monitor keeps track of the charge entering and exiting the battery pack. Here, the charge flowing is calculated by multiplying current and time. Further, the cell voltage sensors carry out the function of cell voltage monitoring which is a standard function of the BMS. It is very crucial in determining the health of the battery.

The temperature monitoring is another important feature of BMS and the internal ADC voltage-powered thermistor performs this function. 0BMS also has a Real-time Clock (RTC) which acts as a black-box system for time-stamping and memory storage. RTC allows the user to know the battery pack’s behaviour and, thus, warns before any alarming event. There’s also one microcontroller with BMS algorithms that makes quick and effective decisions in real-time.

Topology of BMS

BMS can be categorized based on topology. Topology relates to how it is installed and operates upon the cells or modules across the battery pack. For electric or hybrid vehicles, the BMS is only a subsystem and cannot work as a standalone device. It must communicate with at least a charger (or charging infrastructure), a load, thermal management and emergency shutdown subsystems. Therefore, the BMS is tightly integrated with those subsystems.

Functions of BMS

While there are several reasons why Battery Management System matters, and with more advances in technology it may add new functions, here is a list of the most important functions of BMS in a Lithium-Ion Battery.

#1 Safety of Electric Vehicle

Without a doubt, a safety assurance is the most crucial function for anything that may go unsafe at some point. For an electric vehicle’s lithium-ion battery, the BMS captures crucial data such as voltage, temperature and current for its various functions, including the safety.

BMS ensures thermal management of the battery and monitors its temperature continuously. Apart from adjusting cooling, it triggers other safety mechanisms to cease operations and minimize the risk.

Thermal Runaway is a condition where the current flowing through the battery on charging or overcharging causes the cell temperature to rise. Conditions like this may seriously harm the lifespan or the capacity of the battery. Overcharging lithium-ion cells may also lead to thermal runaway and, in the worst case, an explosion. BMS controls the current supply as well to avoid overcharging by enforcing the limits of maximum charge or discharge current according to the temperature.

Protecting user from an electric shock, the BMS also makes sure the vehicle chassis is completely isolated from a high voltage battery pack.

#2 Cell Monitoring and Balancing

The battery cells need monitoring round the clock – charging or discharging. Any extraordinary situation needs to be identified and reported, in addition with triggering safety mechanism mentioned above. BMS does this function using integrated circuits with cell monitoring algorithms. A chain of command passes the voltage and temperature data to a cell management controller.

Apparently, these algorithms calculate the state of charge (SOC) and state of health (SOH). SOC helps to ensure that the battery is never over or undercharged. Playing the role of a fuel indicator of an electric vehicle, it indicates the energy remaining in the battery along with the distance the EV would cover before the battery needs a recharge. SOH, on the other hand, indicates the overall health of the battery. It provides an insight into the operating conditions of the battery. The information is crucial to project battery lifespan and maintenance schedule. Both SOC and SOH are generally in percentages.

#3 Power Optimization

This can be considered a direct outcome of the aforementioned cell monitoring system. The EV BMS tries to keep the SOC and SOH parameters within the limits as per specifications. It works like a guardian that determines the extent of current in the individual cells and communicates the same with the EV Supply Equipment (EVSE), or the charger. As the EV functions and discharges, the BMS also communicates with the motor controller to ensure that the voltage level does not get too low.

Thus, the vehicle shows an alert to the user to charge the battery pack. The BMS controls the recharging of the battery pack by energy generated through regenerative braking.

That’s not where the duty of Battery Management System ends. Individual cells of a battery pack develop differences in capacity with time. This amplifies the charge/discharge cycle for each of them creating non-uniformity and thus limits the amount of energy from the battery as a whole, and the extent of its charge. Cell balancing by BMS helps maintain the cell at equal voltage levels and maximize battery pack’s capacity utilization. BMS performs cell balancing by draining excess energy from cells that are more charged than others – through active and passive balancing techniques.

#4 Battery Charging Optimization

With time, battery cells deteriorate. BMS intelligently takes this deterioration in account which results in change on battery parameters such as voltage, current, etc. For instance, consider that a cell gets damaged by heat and starts getting charged at a lower voltage than the rest of the cells. BMS identifies this fault and optimizes the charging process for all cells now to charge on lower voltage. This reduces the stress on the overall battery pack and enhances its life overall.

#5 Communication

All the major functions of a Battery Management Systems such as monitoring, collecting information about cell parameters, etc. are possible because of the communication between them. BMS is the main entity responsible for communicating with other Electronic Control Units (ECUs) in an EV. It does this by exchanging data about the various battery parameters with the motor controller. This helps the controller perform its smooth-running function for the vehicle. It also takes care of the nature of charging, communicating via required current and output voltage information. In case of AC charging, BMS communicates with charger to monitor and control the process.

In case of DC charging, a communication link between the EVSE and the BMS gets established. To start and stop the charging process, it communicates output voltage and current levels to EVSE.

With so much counting on it, it is no wonder that the best battery manufacturers or assemblers develop their own BMS systems!