Battery Management System Responsible for Reliable and Safe EV Driving
Edis Osmanbasic posted on April 17, 2019 |
Image courtesy of the NREL
Image courtesy of the NREL.

Electric vehicles (EVs) are becoming increasingly attractive because of the rising awareness about carbon emissions and their impact on climate change, government pressure, the cultural symbolism of using green energy sources, as well as constantly increasing oil prices.

A new study, titled “A brief review on key technologies in the battery management system of electric vehicles,” has been published detailing how energy can be properly managed within a battery system, in order to ensure the proper performance and safety of an EV battery. But before we get to that, we have to better understand the heart of the EV.

It’s well-known that EVs use Lithium-Ion (Li-Ion) batteries. They are chosen over other battery types for their good charge cycle rate, higher energy density, higher cell voltage and better self-discharge rate, among other things. The supplying of power for EVs may appear simple, but it’s important to keep in mind that a lot of power is involved in an EV, and it must be managed properly. In addition,an EV battery is an electrochemical device that needs to be carefully handled.

There are many issues with EV batteries that need to be properly maintained. Everyone remembers the issue with the Samsung Note and its spontaneous combustion, which can easily occur if the battery is not carefully managed. Improper battery operations—such as temperatures that are too high or too low, over-current, over-voltage, and over-charging/discharging—cause accelerated battery aging or even combustion and explosion.

The battery may ignite if the over voltage peaks are not appropriately monitored.The battery performance is directly dependent on the operation temperature, the age of the battery, charge and discharge rates and the state of charge (SoC). These factors are influenced by each other, e.g. the battery discharging process generates heat which can increase the discharge current.This process can cause thermal runaway with catastrophic consequences.

The SoC range of Li-Ion batteries needs to be restricted. This means the battery should not be charged at 100 percent and discharged to 0 percent of capacity because this process will degrade its capacity.Usually, the SoC range for Li-Ion batteries is from 20 percent to 80 percent. The voltage monitoring has to be accurate since the one percent change in SoC means just a few mV.

An advanced battery management system (BMS) should monitor each cell’s voltage and temperature, thus ensuring safe and reliable operation, balanced cells for long battery life and optimizing EV performance.

A new study titled “A brief review on key technologies in the battery management system of electric vehiclesdescribes key BMS technologies, including battery modeling, state estimation (SoC and state of health SoH) and battery charging approaches. Since many battery parameters, important for BMS, cannot be measured directly, the proper algorithms and estimation methods need to be defined to monitor important parameters, such as SoC, SoH, individual cell voltages, and internal temperature.The BMS needs to be carefully defined because it is a crucial fact in protecting batteries from damages. The BMS model requires the measuring of the battery voltage and the current and surface temperature which can be performed directly by using proper sensors. Then, battery models (estimation algorithms) are adopted to achieve estimated SoC and internal temperature states. After determining the battery’s electric and thermal behaviors, its charging process can be optimized. If any abnormal battery state situation occurs, the alarm and safety module will eliminate the influence of these factors on the algorithm. The figure below illustrates the key BMS technologies and their relations as well.

Figure 1 The relation of key technologies in the BMS
Figure 1 The relation of key technologies in the BMS.

Image courtesy of the author of the article mentioned in the text.

Developing a proper battery model is the starting point for BMS design, control and optimization. A battery model contains an electric model, thermal model, and coupled electric-thermal model.These models are used to estimate the battery’s SoC, SoH, internal temperature, and joint state. SoC is a vital parameter in monitoring the existing capacity state (remaining battery capacity).The internal resistance is a good parameter for determining SoC because aging degrades the battery capacity and increases the internal battery resistance. After estimating the battery’s internal states via battery modeling, the proper battery charging approach can be designed to protect the battery and provide reliable and efficient operation, thus prolonging the lifetime of the battery.

EVs are in massive use and their most important factors are reliability and safety. The battery system, as the main energy supply for EVs, looks like a simple integrated system. But, in fact, the battery system is a very sensitive component where inappropriate control can cause dangerous consequences and low performance of EVs. BMS is a complex method that enables optimal battery operation and EV performance, while providing protection and enabling safe driving.

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