BMS for EV

Battery Management System (BMS) for Electric Vehicles: Ensuring Safety and Reliability of Electric Vehicles

The automotive industry is serious about Electric Vehicles this time! Unlike the past escapades that fizzled out with the launch of a few EV models that were not received well.

Governments, OEMs, tier-1 suppliers, and every other stakeholder are vying to make electric vehicles a worthy replacement to IC engine automobiles.

While we often get totally caught up in the glitz, we ignore one of the most important components of an EV, the Battery Pack. Cost-wise, it constitutes almost 40% of the vehicle cost. Battery pack comprises the Lithium-ion cells that power the EV drivetrain and along with that, a smart solution called the EV Management system aka BMS.

A BMS can be found in almost every high-end electronic device that runs on battery, for example, your smartphone. How do you think the phone shows the battery percentage or the overall battery health? It’s the BMS at play that monitors every cell of the battery and uses its complex algorithm to calculate battery percentage, health etc. When we extrapolate the battery management aspect to an electric vehicle, the complexity gets several notches higher.

In this blog, we will learn more about battery management system for electric vehicles and the evolution that this smart solution has gone through over the years.

 

 

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 over charged 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.  

Battery Management system(BMS) Design Considerations:

There are lot of factors that are to be considered while designing a BMS. The complete considerations depend on the exact end application in which the BMS will be used. Apart from EV’s BMS are also used wherever a lithium battery pack is involved such as a solar panel array, windmills, power walls etc. Irrespective of the application a BMS design should consider all or many of the following factors.

Discharging Control: The primary function of a BMS is to maintain the lithium cells within the safe operating region. For example, a typical Lithium 18650 cell will have an under voltage rating of around 3V. It is the responsibility of the BMS to make sure that none of the cells in the pack get discharged below 3V.

Charging Control: Apart from the discharging the charging process should also be monitored by the BMS. Most batteries tend to get damaged or get reduced in lifespan when charged inappropriately. For lithium battery charger a 2-stage charger is used. The first stage is called the Constant Current (CC) during which the charger outputs a constant current to charge the battery. When the battery gets nearly full the second stage called the Constant Voltage (CV) stage is used during which a constant voltage is supplied to the battery at a very low current. The BMS should make sure both the voltage and current during charging does not exceed permeable limits so as to not over charge or fast charge the batteries.  The maximum permissible charging voltage and charging current can be found in the datasheet of the battery.

Thermal Control: The life and efficiency of a Lithium battery pack greatly depends on the operating temperature. The battery tends to discharge faster in hot climates compared with normal room temperatures. Adding to this the consumption of high current would further increase the temperature. This calls for a Thermal system (mostly oil) in a battery pack. This thermal system should only be able to decrease the temperature but should also be able to increase the temperature in cold climates if needed. The BMS is responsible for measuring the individual cell temperature and control the thermal system accordingly to maintain the overall temperature of the battery pack.

Powered from the Battery itself: The only power source available in the EV is the battery itself. So, a BMS should be designed to be powered by the same battery which it is supposed to protect and maintain. This might sound simple, but it does increase the difficulty of the design of the BMS.

Less Ideal Power: A BMS should be active and running even if the car is running or charging or in ideal mode. This makes the BMS circuit to be powered continuously and hence it is mandatory that the BMS consumes a very less power so as not to drain the battery much. When a EV is left uncharged for weeks or months the BMS and other circuitry tend to drain the battery by themselves and eventually requires to be cranked or charged before next use. This problem remains common with even popular cars like Tesla.

Accuracy: When a cell is being charged or discharged the voltage across it increases or decreases gradually. Unfortunately, the discharge curve (Voltage vs time) of a lithium battery has flat regions hence the change in voltage is very less. This change must be measured accurately to calculate the value of SOC or to use it for cell balancing. A well designed BMS could have accuracy as high as ±0.2mV but it should minimum have an accuracy of 1mV-2mV. Normally a 16-bit ADC is used in the process.

Processing Speed: The BMS of an EV must do a lot of number crunching to calculate the value of SOC, SOH etc. There are many algorithms to do this, and some even uses machine learning to get the task done. This makes the BMS a processing hungry device. Apart from this it also must measure the cell voltage across hundreds of cells and notice the subtle changes almost immediately.

Building Blocks of a BMS:

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.  

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BMS Data Acquisition:

 

“How to measure cell voltage in BMS?”

Since a typical EV has many cells connected together, it is a bit challenging to measure the individual cell voltage of a battery pack. But only if we know the individual cell voltage we can perform cell balancing and provide cell protection. To read the voltage value of a cell an ADC is used. But the complexity involved is high since the batteries are connected in series. Meaning the terminals across which the voltage is measured has to be changed every time. There are many ways to do this involving relay, muxes etc. Apart from this there is also some battery management IC like MAX14920 which can be used to measure individual cell voltages of multiple cells (12-16) connected in series.

“How to measure cell Temperature for BMS?”

Apart from cell temperature, sometimes the BMS also must measure the bus temperature and motor temperature since everything works on a high current. The most common element used to measure the temperature is called a NTC, which stands for Negative temperature Co-efficient (NTC). It is similar to a resistor but it changes (decreases) its resistance based on the temperature around it. By measuring the voltage across this device and by using a simple ohms law we can calculate the resistance and thus the temperature.

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. This is why Battery model is developed. This model can act as a virtual battery (Hardware in loop) during the developmental stage of the BMS.

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. A typical usage of battery model is shown below using the below image.

In an ideal Battery model, the input voltage should be equal to the output voltage and the error value should be zero. But in practical this scenario is hard to achieve since there are many parameters like temperature, age etc which can affect the system. There are many battery models available they can be broadly classified as Lumped-Parameter Model, Equivalent Circuit Model and Electro-chemical model out all three the Electro-chemical model is the most hard and most accurate model.

BMS – 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.

With this let us conclude the article here, there are still lots to know about BMS and how they work. 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.