This is the fifth article in a <a href="https://www.wevolver.com/article/higher-reliability-and-longer-lifetimes-with-advanced-battery-management-in-healthcare-energy-storage-systems" rel="noopener noreferrer" target="_blank" style="border: 5px solid green;">5-part series (GOOD)</a> exploring Energy Storage Systems (ESS). The series will provide insight into emerging technologies, covering topics including battery manufacturing, testing, and application.The articles were originally published in an e-magazine, and have been substantially edited by Wevolver to update them and make them available on the Wevolver platform. This series is sponsored by Mouser, an online distributor of electronic components. Through their sponsorship,&nbsp;<a href="about%3Ablank" target="_blank" style="border: 5px solid green;">Mouser Electronics (GOOD)</a>&nbsp;supports the growth of a sustainable future fuelled by renewable and distributed generation technologies.<h2 data-element-id="headingsMap-4-0">Introduction</h2>Driven by tighter CO2 regulations and more eco-conscious consumers (BAD), the pace of migration to electric vehicles continues to accelerate. This is despite the high cost of the batteries, which stubbornly remain approximately half of the vehicle's overall cost.While many factors determine the cost of the battery, one area in which manufacturers can make significant headway in reducing cost is during the final stages of manufacturing. Specifically, during the battery formation and testing which can account for up to 20 percent of an EV’s battery cost.Battery formation and testing is a time-consuming process involving multiple charges and discharges that activate a battery’s chemistry. It can take up to two full days. This necessary procedure readies the battery for use and is critical to ensure its reliability and quality. Because of how slow the process is, it is a significant bottleneck that prevents battery manufacturing from achieving greater throughput, which would lower the overall cost to produce batteries. Partnerships between EV battery manufacturers and suppliers with formation and test systems expertise are allowing them to increase their attention to reducing the time and cost involved at this crucial stage of manufacturing while still maintaining the precision required for advanced battery chemistries.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzMTU0NTA3NTM3LW5pc3Nhbi1tZmctOTAwLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 788px;" class="fr-fic fr-dib"><h2 data-element-id="headingsMap-5-0">Faster Throughput Equals Lower Battery Cost</h2>To decrease the cost of batteries, manufacturers need to take a holistic approach that starts by leveraging suppliers’ system-level expertise to reduce the overall battery test circuit footprint while increasing the number of channels. It’s important to note that both must be done while maintaining the accuracy, precision, reliability, and speed of their battery formation and test measurements to ensure safety, performance, and reliability requirements are met.This is not easy to do. For the front end, the power supplies driving the battery charging circuits need to be tightly controlled. Going deeper, battery formation (BAD) and testing requires close monitoring of current and voltage profiles used during battery cycling to prevent overcharging and undercharging. This ensures safety during tests, while also maximizing battery longevity, which greatly lowers overall cost of ownership for the end user.For these critical battery measurements, very high-quality instrumentation amplifiers (in-amps) and associated shunt resistors are needed to measure battery charge/discharge current to better than ±0.05% accuracy, even under harsh factory conditions. The same level of accuracy applies to the difference amplifiers used to monitor the voltage over the entire thermal operational range.&nbsp;There are several ways to incorporate these components into a full solution, but it is a significant challenge to maximize performance and minimize the system footprint. ADI trys to combat this with the integration of the analog front end, power control, and monitoring circuits in a single IC, the <a href="about%3Ablank" target="_blank" style="border: 5px solid green;">AD8452 (GOOD)</a>. These ICs can include battery reversal prevention, overvoltage protection switches, and smart controls to prevent overcharge of batteries, and they can reduce the system footprint by 50 percent. This suite of capabilities allows battery manufacturers to incorporate more capabilities into test systems that will simultaneously make more efficient use of factory floor space. Moreover, they allow manufacturers to design systems with more functionality and more robust testing procedures.<iframe src="//players.brightcove.net/706011717001/HJgoScClQ_default/index.html?videoId=5815766383001" allowfullscreen="" frameborder="0" class="fr-draggable" style="width: 789px; height: 459px;" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true"></iframe>Efficient power conversion is another opportunity to drive further system performance. By using advanced switching architectures, test systems can minimize power consumption by enabling bidirectional energy exchange with the grid. Efficient power conversion also reduces the need for heat management equipment, which can add to the system’s overall cost and power consumption. The net result is a reduction in wasted energy and manufacturing cost. Enabling these capabilities requires an appreciation of system features, such as isolated gate drivers, that support the faster switching needs of newer silicon carbide and gallium nitride power switching technologies.The benefits of working closely with suppliers who have system-level expertise and a broad portfolio of products goes beyond having access to more sophisticated components and building blocks. It also gives battery manufacturers access to reference designs for system architectures that can be more easily adopted, making time to market three to four times faster than if a battery manufacturer were to develop a formation and test system from scratch.<h2 data-element-id="headingsMap-7-0">Conclusion</h2>The global demand for EVs will&nbsp;increase at a Compound Annual Growth Rate of 21 percent out to 20211. The need for close partnerships between battery manufacturers and suppliers couldn’t be greater. Suppliers need to provide reliable, proven solutions that enable manufacturers’ systems to achieve new levels of efficiency. The best suppliers can help manufacturers bring these new capabilities to market even faster, and the results will allow battery and electric vehicle production to flourish.This article was originally written by Vikas Choudhary for Mouser and substantially edited by the Wevolver team. It's the final article of a 5-part series exploring Energy Storage Systems (ESS). Future articles will explore Communication Solutions for Essential Monitoring of Solar PV and Energy storage, electric vehicle production futures, and battery monitoring systems for lithium-ion solutions.&nbsp;<a href="https://www.wevolver.com/article/higher-reliability-and-longer-lifetimes-with-advanced-battery-management-in-healthcare-energy-storage-systems" rel="noopener noreferrer" target="_blank" style="border: 5px solid green;">Article one (GOOD)</a> outlined the Energy Storage Systems in healthcare applications. &nbsp;<a href="https://www.wevolver.com/article/icoupler-isolated-communication-solutions-for-essential-monitoring-of-solar-pv-and-energy-storage" rel="noopener noreferrer" target="_blank" style="border: 5px solid green;">Article two (GOOD)</a> explained communication systems for energy storage.&nbsp;&nbsp;<a href="https://www.wevolver.com/article/battery-stack-monitor-maximizes-performance-of-li-ion-batteries-in-hybrid-and-electric-vehicles" rel="noopener noreferrer" target="_blank" style="border: 5px solid green;">Article three (GOOD)</a>&nbsp;gave an overview of battery stack monitoring systems for li-on batteries.<a href="https://www.wevolver.com/article/critical-design-considerations-in-estimating-the-state-of-lithium-ion-batteries" rel="noopener noreferrer" target="_blank" style="border: 5px solid green;">Article four (GOOD)</a>&nbsp;provided a deep analysis of the critical design considerations in estimating the state of lithium-ion batteries.<hr><h2 data-element-id="headingsMap-8-0">About the sponsor: Mouser</h2>Mouser Electronics is a worldwide leading authorized distributor of semiconductors and electronic components for over 800 industry-leading manufacturers. They specialize in the rapid introduction of new products and technologies for design engineers and buyers. Their extensive product offering includes semiconductors, interconnects, passives, and electromechanical components.<h2 data-element-id="headingsMap-9-0"><img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzMTU0NjQzMDQ1LW1vdXNlciBsb2dvIGZvb3Rlci5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" style="width: 788px;" class="fr-fic fr-dib"> References</h2>1 <a href="about%3Ablank" target="_blank" style="border: 5px solid green;">Electric Vehicle Market by Propulsion (BEV, PHEV, FCEV), Vehicle (PC,CV), Charging Station (Normal, Super, Inductive), Charging Infrastructure(Normal, Type-2-AC, CHAdeMO, CCS, Tesla SC), Power Output, Installation,and Region—Global Forecast to 2025. (GOOD)</a> MarketsandMarkets, June 2018

Article 5 of our Energy Storage Series: Reducing cost is during the final stages of battery manufacturing can assist the migration to electric vehicles.

Scaling Electric Vehicle Production Requires Advanced Battery Formation and Test Systems

Article 3 of our Energy Storage Solutions series: A state-of-the-art Battery Monitoring System allows you to extract the highest quantity of charge from your battery pack and  manage the charge and discharge cycles in a safer way extending battery life.

Battery Stack Monitor Maximizes Performance of Li-Ion Batteries in Hybrid and Electric Vehicles

This is the fourth article in a<a href="https://www.wevolver.com/article/higher-reliability-and-longer-lifetimes-with-advanced-battery-management-in-healthcare-energy-storage-systems" rel="noopener noreferrer" target="_blank">&nbsp;5-part series</a> exploring Energy Storage Systems (ESS). The series will provide insight into emerging technologies, covering topics including battery manufacturing, testing, and application.The articles were originally published in an e-magazine co-authored by Martin Murnane and Adel Ghazel and have been substantially edited by Wevolver to update them and make them available on the Wevolver platform. This series is sponsored by Mouser, an online distributor of electronic components. Through their sponsorship, <a href="about%3Ablank" target="_blank">Mouser Electronics</a> supports the growth of a sustainable future fuelled by renewable and distributed generation technologies.<h2 data-element-id="headingsMap-4-0">Introduction</h2>Lithium-ion batteries get employed in many vital applications, including energy storage (ESS), electric vehicles (EV), and EV chargers. In these applications, it is crucial to measure the state of charge (SOC) of the cells, which is defined as the available capacity (in Ah) and expressed as a percentage of its rated capacity. The SOC parameter can enable one to assess the potential energy of a battery. It is critically important to estimate the state of health (SOH) of a battery, which gives a measure of the battery’s lifetime, i.e. the battery’s ability to store and deliver electrical energy compared with a new battery. This article reviews the algorithms utilized for SOC and SOH estimation based on the coulomb counting method, the voltage method, and the Kalman filter method.<h2 data-element-id="headingsMap-5-0">Battery SOC Measurement Principle</h2>Accurate SOC estimation is one of the main tasks of battery management systems. It helps improve the system performance and reliability, and lengthen the lifetime of the battery (SOH). Precise SOC estimation of the battery can avoid unpredicted system interruption and prevent the batteries from being overcharged and over discharged, both of which can cause permanent damage to the battery's internal structure. &nbsp;However, since battery discharge and charge involve complex chemical and physical processes, accurate estimations of the SOC are not apparent under various operation conditions.The general approach for measuring SOC is to accurately measure both the coulombs and current flowing in and out of the cell stack under all operating conditions and the individual cell voltages of each cell in the stack. This data gets employed with previously loaded cell-pack data for the exact cells getting monitored to develop an accurate SOC estimate. The additional data required for such a calculation includes the cell temperature, whether the cell is charging or discharging when the measurements get made, the cell age, and other relevant cell data obtained from the cell manufacturer. Sometimes it is possible to get characterization data from the manufacturer of how their Li-ion cells perform under various operating conditions. Once a SOC has been determined, it is up to the system to keep the SOC updated during subsequent operation, essentially counting the coulombs that flow in and out of the cells. This approach's accuracy can be derailed by not knowing the initial SOC to an accurate enough state and by other factors, such as self-discharge of the cells and leakage effects.<h2 data-element-id="headingsMap-6-0">Technical Specifications</h2>This article encompasses the design and development of a coulomb counting evaluation platform to get utilized for SOC and SOH measurement for a typical energy storage module, which in this case is a 48V module, typically comprising 12 to 16 Li-ion cells. The Battery Management System (BMS) periodically measures each cell's voltage value and the battery pack’s current voltage, utilizing appropriate ADCs and sensors, and will run the SOC estimation algorithm in real-time. This algorithm &nbsp;uses measured voltage and current values and some other data collected by temperature sensors.<h2 data-element-id="headingsMap-7-0">SOC and SOH Estimation Methods Overview</h2>Regarding SOC and SOH estimation methods, three approaches mainly find employment: (1) a coulomb counting method, (2) voltage method, and (3) Kalman filter method.<h3 data-element-id="headingsMap-8-0">Coulomb Counting Method</h3>The coulomb counting method, also known as ampere-hour counting and current integration, is the most common technique for calculating the SOC. This method employs battery current readings mathematically integrated over the usage period to calculate SOC values given by:<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDY4MTU1MjU5LTEgbS5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" style="width: 300px;" class="fr-fic fr-dib">Where SOC(t0) is the initial SOC, Crated is the rated capacity, Ib is the battery current, and Iloss is the current consumed by the loss reactions.The coulomb counting method then calculates the remaining capacity by merely accumulating the charge transferred in or out of the battery. This method's accuracy is linked &nbsp;primarily to a precise measurement of the battery current and accurate estimation of the initial SOC. With a previously known capacity, which might be memorized or initially estimated by the operating conditions, the SOC of a battery can be calculated by integrating the charging and discharging currents over the operating periods. However, the releasable charge is always less than the stored charge in the charging and discharging cycle. In other words, there are losses during charging and discharging. These losses, also with the self-discharging, cause accumulating errors. For more precise SOC estimation, these factors should get consideration. Additionally, the SOC should get recalibrated regularly, and the declination of the releasable capacity should get considered for a more precise estimate.<h3 data-element-id="headingsMap-9-0">Voltage Method</h3>The SOC of a battery, (its remaining capacity) can be determined using a discharge test under controlled conditions. The voltage method converts a reading of the battery voltage to the equivalent SOC value using the known discharge curve (voltage vs. SOC) of the battery. However, the voltage is more significantly affected by the battery current because of the battery’s electrochemical kinetics and temperature. It is possible to make this method more accurate by compensating the voltage reading by a correction term proportional to the battery current and by using a lookup.<h3 data-element-id="headingsMap-10-0">Kalman Filter Method</h3>The Kalman filter is an algorithm to estimate the inner states of any dynamic system—it can also &nbsp;determine the SOC of a battery. Kalman filters were introduced in 1960 to provide a recursive solution to optimal linear filtering for both state observation and prediction problems. Compared to other estimation approaches, the Kalman filter automatically provides dynamic error bounds on its state estimates. By modeling the battery system to include the wanted unknown quantities (such as SOC) in its state description, the Kalman filter estimates their values and gives error bounds on the estimates. It then becomes a model-based state estimation technique that employs an error correction mechanism to provide real-time predictions of the SOC. It can get extended to increase the capability of real-time SOH estimation using the extended Kalman filter. Notably, the extended Kalman filter is applied when the battery system is nonlinear, and a linearization step is needed. Kalman filtering is an online and a dynamic method, and it requires a suitable model for the battery and precise identification of its parameters. &nbsp;It also needs a large computing capacity and an accurate initialization.Other methods for SOC estimation include &nbsp;impedance spectroscopy, based upon cell impedance measurements, employing an impedance analyzer in real-time for both charge and discharge. Although this technique can get used for Li-ion cells SOC and SOH estimation, it was omitted in this article since it is based on external measurements utilizing instrumentation. The methods based on the electrolytes’ physical properties and artificial neural networks are not applicable for Li-ion batteries.<h2 data-element-id="headingsMap-11-0">Technical Principle</h2>Next, we’ll dive into the technical aspect of the topic, however for a more detailed description and equations, refer to the full article, <a href="about%3Ablank#TextLinks-12" target="_blank">A Closer Look at SOC\SOH Estimation Techniques</a>. The releasable capacity (Creleasable) of an operating battery is the released capacity when it gets completely discharged. Accordingly, the SOC is defined as the percentage of the releasable capacity relative to the battery rated capacity (Crated), given by the manufacturer.&nbsp;<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDY4MTY5MDY0LTJtLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 788px;" class="fr-fic fr-dib">A fully charged battery has the maximal releasable capacity (Cmax), which can be different from the rated capacity. In general, Cmax is, to some extent, different from Crated for a newly used battery and will decline with the used time. It can get utilized for evaluating the SOH of a battery.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDcwMDQ1NDc5LTNtLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 300px;" class="fr-fic fr-dib">When a battery is discharging, the depth of discharge (DOD) can get expressed as the percentage of the capacity that has gotten discharged relative to Crated,<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDcwMDc0MzUzLTRtLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 300px;" class="fr-fic fr-dib">Where Creleased is the capacity discharged by any amount of current. With a measured charging and discharging current (Ib), the difference of the DOD in an operating period (Ʈ) can be calculated by<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDcwNDIyNDgyLTVtLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 300px;" class="fr-fic fr-dib">where Ib is positive for charging and negative for discharging. As time elapsed, the DOD is accumulated<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDcwMjM3Mzc4LTZtLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 300px;" class="fr-fic fr-dib">To improve the accuracy of estimation, the operating efficiency denoted as ŋ is considered and the DOD expression becomes,<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDcwNDUxNjYxLTdtLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 300px;" class="fr-fic fr-dib">With ŋ equal to ŋc during the charging stage and equal to ŋd during the discharging stage. Without considering the operating efficiency and the battery aging, the SOC can get expressed as<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDcwNDc0NjI1LThtLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 300px;" class="fr-fic fr-dib">Considering the SOH, the SOC is estimated as<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzNDcwNDg1NDQ2LTltLmpwZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=" style="width: 300px;" class="fr-fic fr-dib">The estimation process is based on monitoring the battery voltage (Vb) and Ib. The battery operation mode can be understood from the amount and the direction of the operating current. The DOD is adding up the drained charge in the discharging mode and counting down with the accumulated charge into the battery for the charging mode. After correction with the charging and discharging efficiency, a more accurate estimation can get realized. The SOC can then get estimated by subtracting the DOD quantity from the SOH one. When the battery is open-circuited with zero current, the SOC is directly obtained from the relationship between the OCV and SOC.Note that the SOH can get reevaluated when the battery is either exhausted or fully charged, and manufacturers specify the battery operating current and voltage. The battery may be considered drained when the loaded voltage (Vb) becomes less than the lower limit (Vmin) during the discharging. In this case, the battery can no longer be used and should get recharged. At the same time, a recalibration to the SOH can get made by reevaluating the SOH value by the accumulative DOD at the exhausted state. On the other hand, the used battery may be considered fully charged if (Vb) reaches the upper limit (Vmax), and (Ib) declines to the lower limit (Imin) during charging. A new SOH is obtained by accumulating the sum of the total charge put into the battery and is then equal to SOC. In practice, the fully charged and exhausted states occur occasionally. The accuracy of the SOH evaluation can be improved when the battery is frequently fully charged and discharged.Thanks to the simple calculation and the uncomplicated hardware requirements, the enhanced coulomb counting algorithm can be easily implemented in all portable devices, as well as electric vehicles. Additionally, the estimation error can be reduced to one percent (1%) at the operating cycle next to the reevaluation of the SOH.This article was originally written by Martin Murnane for Mouser and substantially edited by the Wevolver team. It's the fourth article of a 5-part series exploring Energy Storage Systems (ESS). Future articles will explore Communication Solutions for Essential Monitoring of Solar PV and Energy storage, electric vehicles production futures, and battery monitoring systems for lithium-ion solutions.&nbsp;<a href="https://www.wevolver.com/article/higher-reliability-and-longer-lifetimes-with-advanced-battery-management-in-healthcare-energy-storage-systems" rel="noopener noreferrer" target="_blank">Article one</a> outlined the Energy Storage Systems in healthcare applications. &nbsp;<a href="https://www.wevolver.com/article/icoupler-isolated-communication-solutions-for-essential-monitoring-of-solar-pv-and-energy-storage" rel="noopener noreferrer" target="_blank">Article two</a> explained communication systems for energy storage. &nbsp;<a href="https://www.wevolver.com/article/battery-stack-monitor-maximizes-performance-of-li-ion-batteries-in-hybrid-and-electric-vehicles" rel="noopener noreferrer" target="_blank">Article three</a>&nbsp;gave an overview of battery stack monitoring systems for li-on batteries.<a href="https://www.wevolver.com/article/critical-design-considerations-in-estimating-the-state-of-lithium-ion-batteries" rel="noopener noreferrer" target="_blank">Article four</a>&nbsp;provided a deep analysis of the critical design considerations in estimating the state of lithium-ion batteries.<a href="https://www.wevolver.com/article/scaling-electric-vehicle-production-requires-advanced-battery-formation-and-test-systems" rel="noopener noreferrer" target="_blank">Article five</a>&nbsp;looked at solutions for scaling electric vehicle production.&nbsp;<hr><h2 data-element-id="headingsMap-12-0">About the sponsor: Mouser</h2><a href="https://eu.mouser.com/" rel="noopener noreferrer" target="_blank">Mouser Electronics</a>&nbsp;is a worldwide leading authorized distributor of semiconductors and electronic components for over 800 industry-leading manufacturers. They specialize in the rapid introduction of new products and technologies for design engineers and buyers. Their extensive product offering includes semiconductors, interconnects, passives, and electromechanical components.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNjEzMTUxNjUzMzY1LW1vdXNlciBsb2dvIGZvb3Rlci5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" style="width: 788px;" class="fr-fic fr-dib">

Article 4 of our Energy Storage Series: Accurate state of health and state of charge estimations is essential for safe and efficient battery use.

Critical Design Considerations in Estimating the State of Lithium-ion Batteries

<h2 dir="ltr" data-element-id="headingsMap-4-0">Introduction</h2>Once a luxury used primarily by early adopters, electric vehicles (EVs) are becoming increasingly commonplace due to rising awareness about carbon emissions and their impact on climate change, supportive government policies, volatile oil prices, and growing cultural acceptance of green energy use. Global EV stock is projected to reach 243 million by 2030 - over 30 times higher than today’s level [<a href="https://www.iea.org/reports/global-ev-outlook-2020" target="_blank">1</a>]. Most major car manufacturers are planning either a complete shift or a transition to EVs [<a href="https://www.nature.com/articles/d41586-018-07585-6?source=post_page-----e0f2661bf61----------------------" target="_blank">2</a>].As demand for electric vehicles increases, so does demand for improved performance, including decreased cost and charge time, increased range and battery life, and improved safety. The cost of EV batteries continues to fall and EV battery technology is advancing rapidly in the form of increasing capacity and energy density [<a href="https://www.iea.org/reports/global-ev-outlook-2020" target="_blank">1</a>,<a href="https://www.nature.com/articles/d41586-018-07585-6?source=post_page-----e0f2661bf61----------------------" target="_blank">&nbsp;2</a>]. However, increases in energy density are accompanied by a growing challenge - safety. In particular, lithium ion batteries - the power source of choice for EVs thanks in part to their high energy density - are susceptible to overheating, and even explosion, and safety incidents have been widely reported [<a href="https://www.mdpi.com/2076-3417/9/12/2483/htm" target="_blank">3</a>,<a href="https://link.springer.com/article/10.1007/s10694-019-00944-3" target="_blank">&nbsp;4</a>]. As EV batteries become more powerful and complex, ever more sophisticated battery management systems are needed to ensure the safe, reliable, and cost-efficient operation of electric vehicles.<h2 dir="ltr" data-element-id="headingsMap-5-0">What is a battery management system?&nbsp;</h2>Battery management systems (BMS) are electronic control circuits that closely and accurately monitor and regulate the charging and discharging of rechargeable batteries, serving as the “brains” of an EV battery system. The BMS maintains batteries within their safe operating area and ensures optimal use of the residual energy in batteries, thereby maximizing their safety, reliability, durability, and performance.<h3 dir="ltr" data-element-id="headingsMap-6-0">Functions of battery management systems</h3>Battery management systems are central to the safe and efficient operation of electric vehicles and perform a broad array of functions, including:Battery monitoring&nbsp;The BMS monitors the state of an EV battery system through a variety of direct and indirect measures. Current, voltage, and surface temperatures of battery cells are directly measured using sensors. These are in turn used to estimate the battery pack State of Charge (SOC), which indicates the charge level of the battery; the State of Health (SOH), which represents the remaining capacity of the battery pack and indicates when it should be replaced; the internal temperature of battery cells; and other parameters. Accurate voltage monitoring is of particular importance, since voltage deviations due to overcharging, over-discharging, or high power pulses can lead to significantly reduced battery life and safety issues.Battery protection&nbsp;The BMS is also equipped to prevent operations outside the safe operating area of and damage to the battery system, for example due to overcharging, over-discharging, or high temperatures. The action of the BMS can be physical (such as emergency shut down of the battery) or informational (by triggering an alarm or otherwise reporting trouble to the driver).For example, the BMS controls the charger to protect the battery from overheating as well as ensure optimal charge of the battery, thereby prolonging its life and improving capacity utilization. The BMS is also responsible for thermal management of the battery. Battery temperature has a significant impact on battery life and performance, and the BMS controls the pump, fans, coolant (where relevant), and heater to maintain the temperature of the battery pack within its optimum operating range.&nbsp;Battery optimization&nbsp;The BMS also optimizes battery performance via a process called cell balancing. Within an EV battery pack, no two cells are identical, due to manufacturing differences as well as variation in their on-vehicle environment. Weaker cells charge and discharge faster than other cells. Because charging and discharging stop as soon as any cell hits programmed upper and lower capacity limits, the weaker cells cause energy within the other cells to go unused while discharging and limit the capacity of the battery pack while charging. This also results in an increase in the number of battery charge and discharge cycles, which shortens the lifetime of the battery.Cell balancing helps resolve these issues and ensure optimum performance of the battery by equalizing the individual cells’ state of charge. Two types of cell balancing can be used – passive and active. In passive balancing a bleed resistor is used to dissipate energy from cells with excess charge until all cells have a similar state of charge. While this approach is lower cost, it is also less efficient since excess energy is lost as heat. Active cell balancing is a more complex technique that redistributes charge between cells during the charge and discharge cycles. While it is more efficient, since energy is transferred to where it is needed instead of being lost as heat, it also requires additional components at higher cost. Either method extends battery life and improves safety by preventing damage to battery cells due to overcharging or over-discharging.High precision cell voltage and temperature measurements and robust communication links between cells and to battery controllers can further contribute to optimizing battery performance. By collecting better measurements and communicating them faster, the BMS can be more responsive to battery conditions and enable more efficient use of the battery.<h2 dir="ltr" data-element-id="headingsMap-7-0">Communication</h2>The BMS also facilitates communication of information to the driver, for example by triggering an alarm or reporting the state of charge, as well as to other on-board equipment, for example to request changes in vehicle operation in response to monitored battery pack conditions.<h3 dir="ltr" data-element-id="headingsMap-8-0">Design of battery management systems</h3>Electric vehicles are powered by hundreds or even thousands of individual battery cells. These cells are connected in modules in series and parallel and placed inside a steel casing to form the battery pack. Battery packs are typically very large, occupying the entire floor area of an EV, and are connected to the BMS and thermal management system. The BMS transmits information from the battery pack to the controller, which converts the DC voltage produced by the battery pack and drives the motor, and the rotational energy from the motor is transferred to the wheels by the transmission system (Figure 1).<img src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjQzMjU3MTc1LUlMTFVTVFJBVElPTiAyMi0xLTAxICgxKS5qcGciLCAiZWRpdHMiOiB7InJlc2l6ZSI6IHsid2lkdGgiOiA5NTAsICJmaXQiOiAiY292ZXIifX19">Figure 1: Schematic showing layout of BMS, battery pack, controller, transmission, etc. within a car&nbsp;The BMS itself is a combination of hardware and software (Figure 2). Hardware includes sensing microchips, transceivers, power management microchips, and microcontrollers. The sensors measure battery voltage, current, and surface temperature to monitor the condition of individual cells. Transceivers relay information between components. Power management microchips allow the connection and isolation of the battery pack between the charger and load. Microcontrollers receive information from sensors and plug it into algorithms which model internal battery states, such as internal temperature, SOC, and SOH. Once these internal states have been defined, the microcontroller can make decisions, such as optimizing the charging and discharging of the battery or triggering alarm or safety modules.<img src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjQzMjk1MzYxLUlMTFVTVFJBVElPTiAyMS0wMSAoMSkuanBnIiwgImVkaXRzIjogeyJyZXNpemUiOiB7IndpZHRoIjogOTUwLCAiZml0IjogImNvdmVyIn19fQ==">&nbsp;Figure 2: Schematic showing layout and components of a simplified BMSBattery management systems can be implemented using a variety of architectures. The choice of architecture has a significant impact on the cost, development time, flexibility, and reliability of a BMS. Architectures can be broadly classified as centralized and decentralized (Figure 3). In centralized architectures, a single controller monitors, balances, and controls all battery cells and is connected to them through a large number of wires. This approach is relatively inexpensive but also challenging to expand. In decentralized architectures, which include distributed, master-slave, and modular architectures, the BMS functionality is not concentrated on a single circuit board. These approaches entail fewer wires and are more flexible, but also more expensive.<img src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjQzNDkzODAwLUlMTFVTVFJBVElPTiAyMi0wMSAoMSkuanBnIiwgImVkaXRzIjogeyJyZXNpemUiOiB7IndpZHRoIjogOTUwLCAiZml0IjogImNvdmVyIn19fQ==">&nbsp;Figure 3: Diagram comparing generic centralized BMS architecture to decentralized/distributed BMS architectureGenerally speaking, decentralized architectures are advantageous because they offer superior measurement precision, because shorter wires are less susceptible to noise; more reliable connections, because shorter wires are less prone to disconnection due to mechanical shocks and vibrations; scalability; and more robust communication between components.&nbsp;Achieving reliable, high performance communication is a key challenge faced by BMS designers. Besides opting for decentralized architectures, designers can improve communication between components (and therefore achieve more accurate, real-time measurements and control) by using microchips which consolidate a variety of functions and controllers with built-in diagnostic functions. Minimizing the number of components, as well as the size of the components which are used, offer the added benefit of giving designers more flexibility and allowing for optimal placement of components within the vehicle.Another challenge BMS can help designers tackle is security. Because EVs (including smart vehicles and autonomous vehicles) connect to the internet, these vehicles run the risk of cyberattacks. BMS security controllers can prevent unauthorized modification of the battery system, protecting the safety of the system as well as privacy and intellectual property.<h2 dir="ltr" data-element-id="headingsMap-9-0">What’s next for battery management systems?</h2>Demand for improved performance and societal changes are driving new developments in battery management systems. One trend which promises major improvements to the performance and cost of EVs is the advent of wireless BMS. In a typical BMS, components are connected by a large amount of wiring - up to several kilometers long - which adds weight to the EV and makes it less efficient. This wiring is also costly and prone to mechanical failure. Furthermore, the wiring can make it so complex and time consuming to replace an individual battery cell that it makes more sense to simply replace the entire pack. By eliminating the need for wiring and associated components, a wireless BMS helps decrease the weight and size of the BMS, thereby improving vehicle efficiency, extending driving range, and helping make vehicles easier and cheaper to produce and maintain. At the same time, wireless BMS could offer even more accurate and useful information about the condition of the battery cells.Another development is the increasing expectation that BMS components meet the most stringent safety standards. The safety of automotive components is assessed according to the ISO26262 standard, defined by the International Organization for Standardization (<a href="https://www.iso.org/standard/68383.html" target="_blank">5</a>). During the development of hardware and software components, all potential hazardous scenarios are evaluated for a particular component and classified based on the levels of severity, probability of occurrence, and driver controllability. The component is then assigned an Automotive Safety Integrity Level (ASIL), which indicates the level of safety required by the component to function normally without posing any threats to the vehicle. ISO 26262 defines four ASIL levels - A, B, C, and D - where ASIL D refers to the highest level of safety required. Vehicle manufacturers increasingly demand that BMS components are ASIL D compliant. At the same time vehicle designs are becoming increasingly complex, meaning that BMS suppliers need to be more rigorous than ever to meet safety requirements and ensure that EVs can operate safely.Finally, a rapidly evolving and urgently needed development for EVs is fast charging, especially for buses and shared mobility vehicles which need to operate continuously (<a href="https://www.annualreviews.org/doi/full/10.1146/annurev-control-053018-023643" target="_blank">6</a>). With the established AC charging, it can take several hours to fully charge an EV, whereas fast charging enables this within about 20 minutes. Batteries undergoing fast charging require more sophisticated battery management and much more accurate estimations of conditions inside battery cells to prevent overcharging and overheating.<h2 dir="ltr" data-element-id="headingsMap-10-0">Conclusion</h2>As electric vehicles become increasingly ubiquitous, so will demand for EVs to offer performance and cost which is comparable to that of traditional vehicles. By improving the safety and reliability of EVs, increasing the efficiency and range of EVs, and prolonging EV battery lifetimes, battery management systems will play a pivotal role in achieving comparable, affordable performance, thereby enabling society to reap the benefits of EVs on a large scale.<hr><h2 dir="ltr" data-element-id="headingsMap-11-0">About the sponsor: Infineon Technologies </h2>Infineon Technologies provides semiconductor products for use in a wide range of applications.&nbsp;Infineon Technologies is dedicated to developing integrated circuits and design resources for battery management systems that contribute to more efficient, longer-lasting, and reliable battery-powered applications. To learn more click&nbsp;<a href="https://www.infineon.com/" rel="noopener noreferrer" target="_blank">here.</a><img class="fr-fic fr-dib" src="https://images.wevolver.com/eyJidWNrZXQiOiAid2V2b2x2ZXItcHJvamVjdC1pbWFnZXMiLCAia2V5IjogImZyb2FsYS8xNTk3MjU2MTk0MzQyLUluZmluZW9uLWxvZ28tMDYucG5nIiwgImVkaXRzIjogeyJyZXNpemUiOiB7IndpZHRoIjogOTUwLCAiZml0IjogImNvdmVyIn19fQ==" style="width: 300px;"> <hr><h2 dir="ltr" data-element-id="headingsMap-12-0">References</h2><ol><li dir="ltr">Global EV Outlook 2020 – Analysis - IEA [Internet]. IEA. 2020. Available from: https://www.iea.org/reports/global-ev-outlook-2020</li><li dir="ltr">Figueres, C. Nature. [Online]. Available from: https://www.nature.com/articles/d41586-018-07585-6&nbsp;</li><li dir="ltr">Ouyang D, Chen M, Huang Q, Weng J, Wang Z, Wang J. A Review on the Thermal Hazards of the Lithium-Ion Battery and the Corresponding Countermeasures. Applied Sciences. 2019;9(12):2483. &nbsp;</li><li dir="ltr">A Review of Battery Fires in Electric Vehicles [Internet]. Springer. 2020. Available from: http://A Review of Battery Fires in Electric Vehicles &nbsp;</li><li dir="ltr">ISO 26262-1:2018 [Internet]. ISO. 2020. Available from: https://www.iso.org/standard/68383.html &nbsp;</li><li dir="ltr">Xinfan L. Modeling and Estimation for Advanced Battery Management [Internet]. Annual Reviews. 2020. Available from: https://www.annualreviews.org/doi/full/10.1146/annurev-control-053018-023643 &nbsp;</li></ol>

As EV batteries become more powerful and complex, ever more sophisticated battery management systems are needed to ensure the safe, reliable, and cost-efficient operation of electric vehicles.

battery management system (BMS)

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