Why do we need a Battery Management System (BMS)?
Introduction
Over years batteries have been a very crucial part of our life. Be it a watch,a mobile phone or even a trimmer they are widely used in various devices where supply of DC current is required. As the demand for electric vehicles increases and a wide range of battery packs being used there has been a need of a Battery Management System.
A BMS is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and balancing it.
Why do we need a Battery Management System (BMS)?
The Lithium-ion batteries have proved to be the battery of interest for Electric Vehicle manufacturers because of its high charge density and low weight. Even though these batteries pack in a lot of punch for its size they are highly unstable in nature. It is very important that these batteries should never be overcharged or under discharge at any circumstance which brings in the need to monitor its voltage and current.
This process gets a bit tougher since there are a lot of cells put together to form a battery pack in EV and every cell should be individually monitored for its safety and efficient operation which requires a special dedicated system called the Battery Management System. Also to get the maximum efficiency from a battery pack, we should completely charge and discharge all the cells at the same time at the same voltage which again calls in for a BMS.
In order to control battery performance and safety it is necessary to understand what needs to be controlled and why it needs controlling. This requires an in-depth understanding of the fundamental cell chemistries, performance characteristics and battery failure modes particularly Lithium battery failures. The battery can not simply be treated as a black box.
Battery management systems work in real-time to regulate many functions like:
- Cell monitoring
- Maintenance
- Regeneration
- Optimization
- Battery failure prediction and/or prevention
- Battery data collection/analysis and planning
BMS is an essential component of PHEVs and EVs to ensure proper battery performance and to protect the highly expensive automotive component.
Apart from the battery module, the essential elements in the BMS include the following:
Battery Monitoring Unit (BMU):
It uses a microprocessor-based unit to monitor the multiple parameters such as
- State of charge,
- Cell balancing
- Cell temperature
BMU compares the gathered data with the specifications and communicate to the BCU. It also interacts with other devices via the CAN bus.
Battery Control Unit (BCU):
It collects inputs from the BMU and combines any corrective measures needed to protect the battery or balance the cell or maintain the SOC. BCU is designed with power electronics components.
State-of-Charge (SOC) Determination:
You can think of SOC as the fuel indicator of the EV. It actually tells us thebattery capacity of the pack in percentage. Just like the one in our mobile phone. The voltage and charge/discharge current of the pack should always be monitored to predict the capacity of the battery. Once the voltage and current is measured there are a lot of algorithms that can be used to calculate the SOC of the Battery pack. The most commonly used method is the coulomb counting method.
State-of-Health (SOH) Determination:
The capacity of the battery not only depends on its voltage and current profile but also on its age and operating temperature. The SOH measurement tells us about the age and expected life cycle of the battery based on its usage history. This way we can know how much the mileage (distance covered after full charge) of the EV reduces as the battery ages and also we can know when the battery pack should be replaced. The SOH should also be calculated and kept in track by the BMS.
Building Blocks of a Battery Management System:
There are many different types of BMS available in the market, you can either design one on your own or even purchase the Integrated IC that is readily available. From a hardware structure perspective there are only three types of BMS based on its topology they are Centralized BMS, distributed BMS and Modular BMS. However the function of these BMS is all similar. A generic Battery Management system is illustrated below.
Battery state Estimation
The major computational power of a BMS is dedicated to estimate the Battery state. This includes the measurement of SOC and SOH. SOC can be calculated using the cell voltage, current, charging profile and discharging profile. SOH can be calculated by using the number of charge cycles and performance of the battery.
“How to measure the SOC of a Battery?”
There are many algorithms to measure the SOC of a battery, each having its own input values. The most commonly used method for SOC is called the Coulomb Counting a.k.a book keeping method. We will discuss more on that later. Apart from that there are many other advanced and more sophisticated algorithms that are listed below.
Basic Methods
- Coulomb Counting method
- Ampere-hour (Ah) method
- Open-Circuit Voltage (OCV) method
- Impedance / IR Measurement Method
Machine Learning Based Algorithms
- Neural Network Fuzzy Logic
- Support Vector Machine
Advanced Method
- State-Space Model Estimation using Kalman Filter
Battery Modeling
To use any of the above-discussed algorithms or to verify if your BMS is working as expected we need to develop a mathematical model for our battery pack.
“Why do we need Battery Modeling?”
A typical battery pack takes about 6 hours to get charge and another 6 hours to get discharged. The voltage and current profile of the cells will be different during charging and discharging based on the load, age, temperature and many such conditions. It is not practically possible to charge and discharge a battery in all required condition for the entire life cycle of the Battery pack to check if the BMS is working as expected.
The accuracy of the SOC and SOH also depends on the accuracy of the battery model; hence it should always provide high fidelity and robustness.
Thermal Management
Apart from measuring the voltage, current and temperature and calculating SOC, SOH etc the BMS has another important task of regulating the battery temperature. A battery pack would drain faster if operated in higher or lower temperatures. To prevent this cooling systems are used in the battery. The Tesla for example uses liquid cooling where a tube is passed through the battery pack to get in contact with all the cells. A coolant like water or Glycol is then passed through the tubes. The temperature of the coolant is controlled by the BMS based on the cell temperatures. Apart from this the batteries also use air or chemicals to maintain the required temperature.
Conclusion:
A BMS is considered not only as a monitoring system but also the main component of any Electric Vehicle. Today many silicon companies like Renesas, Texas Instruments etc. have their own series of BMS IC’s and Tool kits which could do the hardware pulling for you and you can use it without diving deep into all this. With every new EV in the market the BMS evolves to get much smarter and easy to use.
Job Opportunities
With a sudden increase in the demand of EV’s there has been a surge in the field of Jobs related to battery segments.
- PURE EV is looking for an Electrical Engineer- Battery Management System who will be responsible for developing a state of the art Battery Management System (BMS) for EVs with implementation of Kalman-Filtering (SOC estimation) as one of its major parts. The role will also include developing Arduino codes for various sensors and other electronic systems embedded in the BMS. Here is the link to apply:-
- Mercedes Benz is looking for Senior Engineer for software BTV of Battery management system. Here is the link to apply:-
- OLA Electric Mobility is looking for battery electrical components who have detailed understanding on various electrical components in Battery Pack and analysis. Here is the link to apply:-
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