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... What makes an ideal battery management system? The first thing that should come to mind is safety ... There are many videos and articles out there that show lithium-based batteries venting and/or bursting into flames when pushed past their operating limits ... A BMS solution must prevent the battery pack from entering into an unsafe condition ... But safety is only the beginning of the story ... Users also want to maximize the life of their battery pack, and try to maintain the capacity of the battery pack as it ages ... This is where the idea of an “ideal” BMS solution can start to be defined. ... The ideal BMS solution provides accurate fuel gauging, and cell balancing to ensure that a battery back is completely empty or completely full ... There are two main design challenges to create the ideal BMS solution: ... Designing the internal battery pack topology to allow for monitoring of each cell. ... Including a mechanism for the BMS to balance the cells. ... There are three main topologies for cells in a battery pack: ... Battery Pack Topologies ... The simplest topology that allows for individual cell monitoring is a single string of series-connected battery cells ... A more complicated topology is necessary when the capacity of a battery pack needs to increase but the overall voltage needs to remain the same ... This requires a parallel connection of battery cells ... If the parallel connection occurs at the per-cell level, an approximation is made that each cell in the parallel string contributes equally to the overall health of the parallel string ... This approximation has the undesirable consequence of reducing the accuracy of the BMS ...  A parallel connection at the total pack voltage is better, as it allows the BMS to read the state of each individual cell ... It also opens the door for limp-home modes of operation ... (More on that later!) ... Just as there are multiple cell topologies, there are also multiple mechanisms for cell balancing ... Cell balancing brings each cell into alignment and ensures that each cell within the battery pack has equal state of charge (SoC) ... The Society of Automotive Engineers released a whitepaper in 2001 that describes different cell balancing solutions shown below: ... Cell balancing methods ... The simplest method to modify the level of charge of a battery cell is to apply a load to that specific cell ... This will lower the level of charge of highly charged cells to match the average level of charge ... This balancing method will provide the ideal BMS with a mechanism to ensure that the battery pack can be fully charged ... However, it doesn’t necessarily allow for the battery to be completely discharged in a useful way ... An active cell balancing method such as charge shuttling using a switched capacitor or energy conversion using a transformer allow cells nearing their end of charge level to be charged with excess energy contained in other cells still in their range of operation ... Refer to Nuvation’s recent article about cell balancing for further detail. ... Thus, to overcome the two previously mentioned BMS design challenges, it’s necessary to: ... Use a series-connected cell topology or use parallel-connected strings of series-connected cells to ensure accurate cell monitoring. ... Implement an active balancing method to allow increasing and decreasing SoC of each cell to ensure balance is achieved and maintained from completely full to completely empty. ... Resource: A Review of Cell Equalization Methods for Lithium Ion and Lithium Polymer Battery Systems.

... If you have worked with or looked at battery systems, you have most likely heard of a battery management system or BMS ... The term BMS refers to a wide variety of electronic devices that monitor and protect the battery in some way ... A battery management system is a battery monitoring device that can take actions to protect the battery from certain usage or other conditions that could damage or shorten the life of the cells. ... So why do BMS prices range from $10 to several thousand dollars if they all do the same thing? An appropriate parallel would be to ask why motorized transportation systems vary so greatly in price, with a motorized skateboard at one end of that spectrum, and a transport truck at the other ... Let’s take a closer look at how this analogy plays out with battery management systems. ... Small Devices ... An example at the small end of BMS requirements is what is needed to protect a battery pack for a small device like a cordless drill ... The typical cordless drill contains around 5 or 6 cells in series with the total cell cost of about $30 ... Clearly the BMS must be very inexpensive to maintain market viability; one is typically under $10 and functionality is limited to most basic protection ... This BMS would be more accurately referred to as a battery protector.  These systems monitor cell voltage and pack current and open one or both back-to-back FETs in the event of voltage or current going out of range ... These range limits are fixed at time of manufacture ... Over-temperature protection is also sometimes included ... These systems often do not include balancing and are standalone, with no communications to other systems. ... Moving up a level would be electric bikes, which typically use 36V battery packs ... A lithium-ion battery pack for this application is about 0.5 kWh and costs about $200 to $300 ... Unlike the cordless drill, a longer battery lifespan is important to the customer, and the bike’s total price point is aligned with that expectation, so cell balancing becomes mandatory ... The BMS must also be able to connect to some form of “gas gauge” display that shows the user how much power is left in the battery ... In addition to this functionality a diagnostic port for service personnel is also desirable ... This BMS is around $30 to $50. ... Residential Energy Storage Systems ... The next level up from these smaller systems to a low-voltage residential energy storage system raises the stakes yet again, this time by an order of magnitude in certain respects ... Residential “behind-the-meter” energy storage systems are often in the 48V range, provide 7 kWh-20 kWh, and cost about $5K-$20K ... At this stage balancing performance and monitoring battery parameters must be well implemented and also support the needs of installation and service companies to be able to perform remote monitoring and event logging to manage their customers’ installations (since most home owners are not technically able to manage their own systems) ... Service truck rolls are expensive and the availability of remotely accessible information can eliminate many unnecessary calls ... This usually means an ethernet interface is required as well as software support such as a browser-based GUI that can be run remotely, and event log pushing to facilitate priority notifications in the event of faults or pending faults ... A BMS for this application is typically in the $250 to $500 range. ... Mobile Energy Storage ... Large mobile power systems for things like electric trucks or boats, mobile energy storage for emergency power, or power for the living space of a mobile home take us to the next level up in BMS design ... There is a wide range of battery pack pricing in this example, from a few thousand dollars for mobile home auxiliary power to $500K for a mobile emergency energy system that can fill a long-haul trailer ... Mobile systems usually require a CAN interface (which is most common in vehicles) and also use inverters specifically designed for these types of applications ... In addition, when these systems return to home base or their service center it is usually desirable to connect them to a service and monitoring computer to download events, operational parameters, and data logs for maintenance purposes. ... This is best done through an Ethernet connection, which requires software support from the BMS ... A BMS for this application can easily range from $300 to $10,000 depending on the nominal voltage of the battery stack and quantity of parallel stacks ... For example, a 48V system needs to monitor and balance 14 to 16 cells depending on the type of lithium battery cells used, whereas a 1200V system could be monitoring 320 cells; large ISO container size systems usually have multiple stacks (also referred to as \u2018strings’) in parallel in order to scale up to large enough energy levels ... This means 10 to 20 times 320 cells to monitor, plus an overall management system to aggregate all the stacks into a single system representation. ... Grid-Attached Energy Storage ... Grid-attached storage is the final level up and is the most demanding of all applications ... These are usually megawatt-scale systems in the 800-1200V range, with 20 or more stacks in parallel ... In addition to all BMS functions already described, these systems need to respond with agility to the changing demands of the grid ... They do this using a variety of algorithms that manage various types of demand responses throughout the day.  As a result, there is the need for back-end data analytics engines to gain access to real-time data about all operating parameters in the battery system that the BMS monitors ... This means access to a constant flow of data regarding cell voltages, temperatures and all other parameters ... This requires a system that can communicate all this data over Ethernet without impacting the primary protective algorithms of the BMS, while also driving user interface displays and responding to demands from the site controller to provide data with which it manages the inverter - all in real time ... These systems usually require Modbus TCP protocol support for communications with the site controller and inverter, which are increasingly implementing the MESA standard. ... The other reality of these large systems is that the inverters used in these applications generate large amounts of common mode noise usually in the 2 kHz to 4 kHz range, which is the fundamental switching frequency of the inverter ... A BMS with 320 cell taps is effectively sampling that common mode noise in 320 different locations relative to the system ground and communication channels ... This requires extreme care and filtering to avoid inadvertently causing some of the common mode being converted to differential, which can cause communications and battery measurement errors ... It places high demands in general around the common mode rejection ratio of the BMS.  ... The other aspect of grid energy storage is that it has become a testing ground for many new battery chemistries that operate at different voltages ... Nuvation Energy has been encountering nominal voltages ranging from 1.1 V to 1.4 V in zinc-based chemistries, 2 V for lead-acid, 2.4 V lithium titanate and 3.2 V to 3.8 V for some lithium cells ... All this means that the ideal BMS must be highly configurable to serve such a wide range of battery cells ... Nuvation Energy's battery management system, for example, has over 1000 configuration registers which allow their customers to tune the BMS for their unique chemistry and operating environment ... It also allows customizable features such as fans, alarms, and status lights to be controlled based on programmable configuration registers. ... *Originally published in EDN.

Energy Storage System Visibility ... In the 3-part tutorial "How to Build a Better energy Storage System", Nuvation Energy CEO Michael Worry and two of our Senior Hardware Designers explore the pain points that energy storage system developers and operators can encounter in the field, and how to avoid designing these problems into your next ESS. ... .

Battery Management for Large-Scale Energy Storage (Part 1) ... Part 1 of 4: ... ... Battery Management and Large-Scale Energy Storage ... Battery Monitoring vs ... Battery Management ... Communication Between the BMS and the PCS ... While all battery management systems (BMS) share certain roles and responsibilities in an energy storage system (ESS), they do not all include the same features and functions that a BMS can contribute to the operation of an ESS ... This article will explore the general roles and responsibilities of all battery management systems, and will also reference Nuvation Energy’s battery management system when exploring specific functions and features that may or may not be included in all battery management systems but which are important to the management of any energy storage system. ... This article is a primer for energy storage industry professionals who would like to gain a better understanding of battery management in large-scale applications ... After reading it you will be able to hold your own in most conversations about battery management, whether you are chatting with energy storage colleagues at a conference, or taking part in a planning meeting about the design and build of your next energy storage system ... It will not answer all your questions, but we hope it empowers you to ask new questions that advance the long-term viability of your energy storage system. ... Multiple devices coordinate with each other in an energy storage system to operate the batteries within their nominal operating parameters ... The management of these parameters: ... Enables the battery to perform the tasks required by the energy storage application. ... Protects the battery from becoming damaged during use. ... Ensures system safety. ... Topics we will cover include: ... The role of the BMS in the energy storage system ... Communicating with energy controllers ... Cell balancing ... State of Charge (SoC) and Depth of Discharge (DoD) ... Warnings, Faults, and User-Defined Thresholds ... Battery Management for Lead-Acid Batteries ... This article is generally applicable to most battery chemistries, including lithium-ion, lead-acid, zinc-based, nickel-based etc., and will specify when a discussion point applies to only a subset of battery chemistries. ... Battery management systems and battery monitoring systems both use sensors connected to cells in a battery module to collect temperature, voltage, and current data ... They also apply algorithms to this data to determine how much energy is available for use in the battery at any given time, and can share this data with other ESS control systems including safety systems, energy management systems, and energy storage applications ... These consumers of battery data use this information to perform their tasks and to ensure that their operation does not result in safety issues or damage to the batteries. ... In this article we’ll use the term “BMS” to refer only to battery management systems ... The key difference between battery monitoring and battery management is that while both systems can provide data to other ESS components, a battery monitoring system is a passive data collection device whereas a battery management system takes direct action to protect the battery ... For example, a battery management system will disconnect the battery to prevent a safety-related situation from escalating, and performs cell balancing (we’ll talk more about balancing later) during the charge cycle. ... At the battery stack level, when integrated into a Stack Switchgear device, Nuvation Energy’s BMS makes decisions about when it is safe to connect a battery stack to the rest of the energy storage system, and can automatically perform that connection ... At Nuvation Energy the term \u2018Stack Switchgear’ refers to our battery stack control system ... It includes battery management modules, fuses, bus bars, contactors, current shunts, networking hardware and other components that work together to manage the cells, connect and disconnect a battery stack to and from the DC bus, and communicate with other ESS control systems. ... With a few exceptions, most battery chemistries require a BMS to support their day to day operation ... All batteries can become damaged when abused, and a BMS helps prevent such damage ... The term \u2018abuse’ refers to the operation of the batteries outside of their nominal parameters ... While \u2018damage’ can take many forms, the most common type is to the active material in the cell, which results in the battery having less power capability, and retaining less energy (i.e: having a lower capacity) at a full charge. ... Battery monitoring systems rarely provide as much algorithm-derived data as battery management systems ... For example, one Nuvation Energy island grid (i.e: the utility grid of a small tropical island) client replaced a lead-acid battery manufacturer’s monitoring system with Nuvation’s battery management system because the monitoring system derived SoC using only stack voltage sensor data ... This is not a very accurate way to derive the SoC of a battery while current is flowing through it, because the battery voltage changes with varying current ... This ESS was utilizing large-format 2.2 Volt lead-acid cells, which were very expensive due to their large size ... The system was being deployed in a remote location where the shipping of replacement batteries (or anything else) was quite challenging ... So this system needed to work as reliably as possible, for as long as possible, and incur as few service calls as possible ... Nuvation’s BMS was the preferred option for ensuring reliable SoC data because it derives SoC using a combination of per-cell and overall stack voltage data in addition to coulomb counting ... Also, the stack-level SoC data it communicates to the PCS includes information that enables the PCS to respond to individual cells at risk. ... Communication Between the BMS and PCS ... A key device with which the BMS shares data is the power conversion system (PCS) ... The primary task of the PCS is to manage the charging and discharging of the battery ... The PCS uses BMS data to regulate current within the battery cells and ensure that its operation does not damage the batteries ... Other ways the PCS leverages battery data from the BMS include: ... When the BMS disconnects a battery stack in response to a battery fault (e.g: overvoltage, over-discharge), Nuvation Energy’s will communicate the reduction in total ESS capacity to the PCS ... Alternately, when Nuvation Energy’s Stack Switchgear connects a battery stack to the DC bus, the BMS will communicate the capacity increase to the PCS ... This data sharing allows the smart PCS and higher-level energy controllers to factor into their decision-making the amount of energy that is available in the ESS at any given time battery management systemt system will communicate the reduction in total ESS capacity to the PCS ... This data sharing allows the smart PCS and higher-level energy controllers to factor into their decision-making the amount of energy that is available in the ESS at any given time. ... The PCS obtains current limit thresholds from the BMS ... Nuvation’s battery management system lowers the current limits as cell temperatures and voltages approach their rated thresholds ... As long as the PCS is adjusting current based on data obtained from the BMS, it will avoid causing battery voltage and temperature overages that would occur from the application of excessive current. ... The PCS receives aggregated battery data from the BMS, which it uses to ensure that it is managing the energy flowing in and out of the battery in a manner that does not result in battery damage or safety issues ... The way that data is aggregated is critical to the prevention of battery damage ... If for example, the BMS shared only the “average” of all cell temperatures and voltages, then that data would obscure individual cell temperatures and voltages that are exceeding their nominal thresholds ... The PCS might then take no action to mitigate risk to the battery, because it was not informed by the BMS that such a risk was present ... However, power conversion systems are not designed to receive individual cell-level data from the BMS ... The BMS must therefore provide aggregate battery performance data that also identifies cell-level risk to the battery ... Nuvation Energy’s BMS achieves this by continually running a series of complex algorithms that generates an aggregate data set which includes information that enables the PCS to recognize the presence of cell-level risks and take actions to mitigate them. ... The quality of these very sophisticated algorithms directly impacts how well or poorly the battery management system performs this sensor data processing task ... When BMS-derived data is incorrect, batteries can become damaged due to the erroneous reporting of safe-state conditions to the PCS ... Alternately, flawed BMS algorithms can erroneously report the existence of a risk condition, which can result in the disconnection of a battery system for safety reasons when such a condition is not actually present. ... Up Next ... Part 2 of 4: ... Open Wire Detection ... Energizing and De-Energizing the Contactors ... Thermal Runaway Mitigation ... Part 2 .

Battery Management for Large-Scale Energy Storage (Part 2) ... Part 2 of 4: ... ... Open Wire Detection ... Energizing and De-Energizing the Contactors ... Thermal Runaway Mitigation ... The accuracy of battery data and the sharing of that data across BMS modules are of critical importance in ensuring precise battery management. ... There can be hundreds or even thousands of sense wires in a large-scale energy storage system, including sense connections between cells within a battery module ... A loose or unconnected sense wire between the cells in a module, between the batteries and BMS, or among the many wires that connect BMS modules to each other, could result in the BMS operating with incorrect data and sharing that faulty data with other ESS components ... This can lead to battery damage and safety issues ... For example: ... The PCS could overcharge or over-discharge cells. ... The PCS could provide or draw a damaging level of current during the charge or discharge process. ... Thermal management systems could fail to initiate in a timely fashion in response to overheating cells. ... The BMS may fail to disconnect the battery when safety thresholds have been exceeded. ... When designing energy storage systems, it is recommended to include a mechanism for ensuring that the BMS is receiving data from correctly connected sense wires, and a way of verifying that voltage, current, and temperature data from every cell are propagating correctly across all battery management modules. ... Nuvation Energy battery management systems include a feature called Open Wire Detection which detects damaged, loose, disconnected, or incorrectly torqued sense wires ... This includes identifying connection quality issues in sense wires between cells within the many battery modules in the energy storage system ... Nuvation’s BMS also includes a self-diagnostic function that auto-initiates during the BMS start-up sequence and: ... Verifies that all sensor data is propagating correctly across all battery management modules. ... Reports any errors by generating a system Fault through the BMS Operator Interface. ... Suspends the BMS startup sequence until any errors have been resolved. ... Automatically resumes the startup sequence from where it left off after issues have been resolved. ... Nuvation Energy battery management system modules \u200bundergoing electromagnetic interference susceptibility testing at TUV facilities. ... An incorrectly torqued sense wire can deliver incorrect readings to the battery management system. ... Since most safety situations arise and escalate while energy is flowing through batteries, disconnecting the batteries from the flow of energy is a quick and highly effective way to de-escalate an emerging safety issue ... If batteries begin to operate outside of safe performance parameters, the BMS will disconnect the batteries by de-energizing the contactors connecting the batteries to the power path ... Contactors are physical switches in the path of the energy flow ... The BMS is able to open and close a contactor by de-energizing (opens the contactor) and energizing (closes the contactor) magnetic coils inside the contactor unit ... Nuvation Energy’s battery management system can provide power to the contactor coils directly, which strengthens its reliability as a safety system by enabling it to de-energize the contactors without assistance from any additional components. ... Uni-directional contactors must be installed in a direction that is aligned with the direction the current is flowing, otherwise their power rating during connection and disconnection can be as much as 50% lower ... Bi-directional contactors have the same power rating for both directions of current flow during connection and disconnection. ... https://www.youtube.com/embed/ekOeQoiqpho ... When either type of contactor is energized it has the same power rating in both directions ... One design approach when designing Stack Switchgear is to put a separate contactor at each end of the power path (i.e: one on the side of the positive terminal and one on the side of the negative terminal), and to first disconnect the contactor that is aligned with the direction as the flow of current (i.e: charging or discharging) ... One can also choose to implement only a single bi-directional contactor, but Nuvation Energy has made the choice of creating the redundant safety feature of designing in two separate contactors so that if one fails, the other can sever the connection. ... Nuvation Energy’s battery management system allows the alignment of the contactor to be programmed into the BMS ... This enables the BMS to intelligently disconnect the battery from the flow of energy by first opening the contactor that is aligned with the direction in which the current is flowing ... It will then open the second contactor to completely disconnect the battery stack from all potential power sources, for added safety. ... Thermal runaway is a condition that can occur in several battery types, including lithium-ion and some lead-acid batteries ... It is a condition where a rapid rise in the temperature of one or more battery cells causes a reaction within the cell that triggers further increases in temperature ... Causes of thermal runaway include a short circuit within or outside of the battery, overcharging, and the excessive application of current during charging ... Once initiated, this reaction can continue and cascade to other cells in the pack and eventually become uncontrollable ... This can lead to battery fires and even explosions. ... There are multiple thermal management mechanisms in an energy storage system ... These include batteries themselves, which are designed to dissipate heat and contain thermal reactions ... However, the action most likely to prevent a thermal runaway event is to recognize when individual cells are approaching operational limits and to disconnect the batteries from the power path ... A thermal runaway event can take place within a short period of time, but thermal runaway reactions tend to begin only after cell temperatures have risen well beyond the battery manufacturer’s nominal operating temperature ranges. ... Batteries are designed to dissipate heat and contain thermal reactions ... In this image Warner ESS and Tactical Fire Productions is conducting battery fire containment testing by deliberately setting the batteries on fire and ensuring they burn in a manner that can be managed by emergency responders ... Image courtesy of Warner ESS and Fire Tactical Productions. ... Damaged batteries however, can go into thermal runaway while being operated within their nominal parameters. One way to damage batteries is by cycling them below their rated operating temperature ... At low temperatures, a number of chemical reactions can occur within the battery which can cause irreversible damage to the cells ... For example, the image below shows the nominal temperature range of a typical lithium-ion battery ... During testing at the University of Waterloo, an LFP cell was deliberately operated at -30\u00b0C for several hours ... This extended operation below nominal temperatures damaged the cell ... In subsequent operation under nominal conditions a short circuit developed in the battery, due the the damage caused during low-temperature testing ... The short circuit triggered a thermal runaway event within 5 seconds of occurring, while the battery was being operated within nominal operating parameters. ... This lab test illustrates the importance of ensuring the battery is kept within its operating limits throughout its entire lifetime ... Although a battery might appear to be functioning normally, any cycling outside the nominal operating range can damage the battery internally ... The role of a battery management system is to ensure that the battery never exceeds its nominal temperature, voltage and current limits ... This will help preserve battery life and prevent thermal runaway events from occurring. ... Up Next ... Part 3 of 4: ... Warnings, Faults, & User-Defined Thresholds ... Protecting the Battery ... Power Conversion Systems (PCS) ... Energy Controllers ... Environmental Controls ... Communicating with Energy Controllers ... Cell Balancing ... Part 1 ... Part 3 .