How Does a Battery Management System (BMS) Work?

 

We all use cells and batteries in our daily lives. Some are rechargeable, and others are not. A non-rechargeable battery, or the primary battery, must be replaced after it is completely drained (Ex: batteries used for the wall clock, children's toys and TV remotes). On the other hand, rechargeable batteries or secondary batteries can be recharged with some external source on a regular basis. Popular appliances like mobile phones, laptops, and digital cameras run on rechargeable batteries. 

It is important to notice that while charging and discharging take place, a certain amount of heat is generated depending on the capacity of the battery. It can be negligible in smaller applications like mobile phones, but an intelligent backup system is required in larger applications like EVs. This phenomenon has led to the development of an electronic system called Battery Management System to ensure that the thermal limit of the battery pack is not exceeded.

 

What is a Battery Management System?

BMS is a technology developed to foresee the battery's State of Charging (SOC) and State of Health (SOH). SOC is the available energy that can be converted to work at that specific time. SOH is a factor that indicates the life cycle and durability of the battery. 

In simple terms, BMS technology is deployed in applications that demand a huge amount of energy to ensure a smooth operation of the battery. Before learning about BMS, you need to understand a battery pack. 

When high voltage is required, we cannot rely on a single cell to generate it. Only a series or parallel connection of cells can meet the requirements. Many cells arranged together constitute a module, and several modules and a Battery Management System form a battery pack. For example, a Tesla Model S Plaid battery pack consists of 7,920 lithium-ion cells installed in five modules, and its capacity is 99 kWh.

Now let us dive into different parameters controlled by a BMS system.

 

1. State of Charging (SOC)

An effective BMS system tracks individual batteries' charging and discharging status and distributes the current accordingly. It ensures that no cell is overcharged or discharged below its lower limit and makes it function within the Safe Operating Area (SOA). It will ensure that the voltage limit is never exceeded.

The BMS would perform some key calculations for estimating the cell's charge and discharge current limits. It calculates its operating time, energy discharges in the previous cycle and the total number of charging and discharging cycles. With the help of these calculations, it predicts the SOC, which is like a fuel indicator of electric vehicles. 

 

2. State of Health (SOH)

All rechargeable batteries can undergo only a finite number of charging and discharging cycles called cycle life. The cycle life can be optimised by effectively monitoring the charge of the battery while charging and discharging. Under proper circumstances and maintenance, a battery could last a long time.

 

 

3. Thermal Management

Thermal management is the most important function carried out by the Battery Management System. It always checks the temperature and cools the battery when needed. Cooling of batteries is not only vital in avoiding thermal runaway but also in optimising efficiency. Thermal management systems are designed considering battery size, peak voltage value, cost, and geographic location. Every battery has a specified operating temperature at which it can work with maximum efficiency. An increase in the battery's temperature can reduce its efficiency by up to 50%. 

 

A battery pack could use air or liquid coolant to maintain within the permissible temperature range. The efficiency of air coolant is relatively lower than that of liquid coolant. Air coolant systems are often passive and need additional components like an air filter and fan, which increases the system's weight. Liquid coolant has a higher cooling potential, and batteries are immersed in the liquid.

 

The Battery Management System controls all these parameters through effective monitoring. It gathers all data related to battery temperature, the flow of current in and out of the cell, the flow of coolant, speed of vehicle and state of power. Whenever the battery gets heated, it signals the pumping unit to deliver more coolant. In the same way, whenever the voltage requirement is increased, it sends requests to lower the current limits. Thus the battery management system helps to ensure the safety and life of the battery. 

 

The recent buzz around EVs has pushed BMS engineers to design safe and efficient battery packs. It has also increased job opportunities for battery engineers. The salary of a BMS engineer in India for a fresher with less than one year of experience reaches up to INR 7.3 LPA. It is the right time for you to enrol in a BMS course and learn what is BMS in detail.

 

To get started, you can also unlock access to our free certificate course on EV, 'How does an Electric Vehicle work?'.