Electric vehicles (EVs) are becoming a regular player in road transportation systems, whether you’re talking about privately owned vehicles, public transportation, or vehicles used in the corporate setting. Constantly evolving technologies are proving to make EVs more appealing and cost-effective for both public and private consumers.
As history has shown us before, a step-change in technology often drives the safety standards associated with the new technology. However, they will lag as we learn how to safely approach the development and maintenance of the new technological trend. Is that the case for the new rechargeable energy storage system (RESS)? A basic overview of typical RESS components may help electrical professionals understand how these components may impact approaches to mitigating the hazards and help answer the question: Are current safety and design standards sufficient to address the electrical hazards associated with RESSs?
First, it’s important to point out that there are other hazards associated with RESSs, but for this article, we are singling out potential electrical hazards. The battery packs used for RESSs in EVs, hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) are large and complex. These systems provide electrical power to the drive train and other components in the vehicle through the controlled release of current at a design voltage. However, due to their high stored electric energy capacity, any uncontrolled release creates a dangerous situation.
The possible release of smoke, toxic fumes, fire, explosion, and/or hazardous electrical forces poses risk for exposure to personnel. Manufacturers have invested time and resources in designing physical barriers, mechanical safety systems, and other control measures to mitigate the potential for severe physical abuse, such as puncturing, crushing, or excessive heating, which can cause uncontrolled energy release. EV battery pack manufacturers design their products to deliver specified performance characteristics safely under anticipated usage conditions. Given the ever-changing chemistry of battery cell designs, understanding potential failure modes are still developing to ensure RESSs are reliably safe in the various environments to which they will potentially be exposed.
EV battery pack basics
While EV battery packs can vary significantly by manufacturer and application, some characteristics are common to many applications. The basic function of the EV battery pack is centered around several mechanical and electrical component systems. All EV battery packs include a collection of battery cells. These can have different sizes, shapes, and chemical make-up, depending on the application and preference of the battery pack manufacturer. One common trait is that the battery pack will always incorporate many individual battery cells connected in series and parallel to achieve the desired voltage and current output rating needed for the specific application. Depending on the requirements of the application, this can vary from a few cells to several hundred individual cells.
Another common feature in EV, HEV, and PHEV battery packs is the main fuse, which limits the current of the battery pack under short-circuit fault conditions. A “service disconnect” or a “service plug,” which provides a means to split the battery stack into multiple electrically isolated sections, is another way to limit the potential release of the full capacity of the battery pack. When performing service or maintenance on or around the battery pack, the service disconnect reduces the electrical hazard potential while the main terminals of the battery pack are exposed.
In addition, battery packs typically include contactors used to control the distribution of electrical energy to the output terminals. Typically, two or more contactors are included to control the connection of the high current supply to the electrical drive motor(s). A battery management system (BMS) is used to regulate the performance and safety of the battery pack during operational discharging and charging. The BMS controls all of the power distribution and includes sensors to monitor temperature, voltage, current, and many other characteristics the manufacturer deems important to the performance and safety of the battery pack and vehicle operation.
When it comes to the design of RESSs used in EVs, HEVs, and PHEVs, development and testing have gone into methods to ensure the systems are safe for general public use while delivering specified performance characteristics under anticipated usage conditions. Much like general permanently installed electrical systems, the RESS is designed according to industry standards to prevent the general public from being exposed to electrical hazards under normal circumstances. An example of one of these standards is ANSI/CAN/UL/ULC 2580:2021, Standard for Safety, Batteries for Use In Electric Vehicles. However, there are scenarios when interaction with the batteries may fall outside these normal circumstances, such as maintenance, repair, replacement, or accidental damage. Therefore, we must consider how to protect personnel from the electric energy stored within these systems.
Evaluating the hazard
When I am asked to provide an opinion on how to protect personnel from unique or unusual scenarios, the first thing I do is break down the hazards into their most basic forms. Electrical energy is very diverse and can present itself in unusual and novel ways, such as new large energy storage devices. However, at the end of the day, it is still just “electrons and holes,” as one of my mentors used to tell me.
As we have covered, RESSs have batteries that store electrical energy in battery cell arrays. The available electrical energy in a cell is limited by its chemistry. Specifically for lithium cells, their electromechanical material properties limit available electrical potential to 3V to 4V per cell. EVs, HEVs, and PHEVs typically have requirements that range from a few hundred volts to more than 1,000V in the battery arrays as technology continues to evolve. These individual cells must be arranged in series to reach the required voltage and then connected in parallel to achieve the electrical current and power requirements. The RESS delivers high voltage to the drive system and a range of lower voltages to other vehicle systems.
For the sake of this discussion, let’s assume we have exposure to a maximum direct-current (DC) voltage of 1,000V. Voltage is the more straightforward variable of the electrical hazard. Regardless of the scenarios we evaluate, the voltage will remain constant for the connected circuit. The amount of current a person could be exposed to may vary depending on the scenario in which they’re interacting with the batteries, various subsystems, and associated charging systems. There are several safety features built into the battery pack that will effectively limit the amount of current being released. However, if maintenance or repair requires dismantling the battery pack or some of its subsystems, each task needs to be evaluated to determine how these activities will expose personnel to electrical hazards as part of an overall risk assessment.
Another variable that comes into play is the use of charging systems. General-use systems are designed to be safe for the operator to plug in, charge, and unplug. A more complex scenario comes into play when a manufacturer is performing research and development tasks that may pose an elevated risk to employees working around these test environments. Every scenario needs to be evaluated on a case-by-case basis.
Addressing the electrical hazard
Often when a scenario falls outside of what we are accustomed to, it will look foreign, complicated, or intimidating. When it comes to electrical safety, that is not necessarily a bad thing. That reaction is a great excuse to pause and think through the task at hand. Whether the hazard is in the form of a storage system or a distribution system, from the context of electrical hazard safety, it is still just a DC voltage source.
We protect ourselves from the RESS’ DC voltage sources just like we would any other DC voltage source. Our first consideration, using the hierarchy of risk control method, is to create an electrically safe work condition (NFPA 70E-2021, Sec. 110.3). The challenge of working directly with the storage system is that in many scenarios, it’s simply infeasible (NFPA 70E-2021, Sec. 110.4) to create an electrically safe work condition. The next option would be to perform a shock risk assessment and an arc flash risk assessment (NFPA 70E-2021, Sec. 130.4 and Sec. 130.5) to determine the extent of the electrical hazards and ways to be protected from such hazards. This will likely result in establishing shock boundaries, utilizing insulating gloves and/or other materials, and considering other mitigating techniques. The point is, once the scenario is broken down into fundamental hazards, mitigating measures start to become more apparent for how personnel can be protected from specific hazards.
Research and development scenarios
One common question is related to research and development (R&D) facilities. Due to unique aspects of an R&D facility’s tasks, there seems to be a common perception that there are hazards too complex to be mitigated. Unfortunately, there are no “one-size-fits-all” solutions to be offered. Each scenario needs to be evaluated on a case-by-case basis using methods of determining the basic hazards, going through the hierarchy of risk control methods to determine appropriate means of protection, and implementing protective measures.
When carefully thought through, a scenario has yet to be presented that prevents developing methods to ensure personnel can execute necessary tasks safely. Don’t let unique aspects of any given scenario intimidate you from breaking it down to the hazards that are very common and sometimes easily able to be mitigated. One word of caution: Just because it is a DC system, do not overlook the potential for an arc flash hazard. Have a knowledgeable professional look at the scenario to determine if a potential for enough energy to sustain an arc is present. If so, perform an arc flash evaluation to determine the level of protection necessary to protect personnel inside the arc flash boundary.
Thermal runaway
While many safety features are put into place to limit the probability and severity of a battery fire or thermal runaway event, the hazards are still present. Let’s address the basics of the fire and thermal runaway hazards in EV battery packs. Thermal runaway (in layman’s terms) is an abnormal event that causes a battery cell to no longer dissipate heat as quickly as it’s being generated.
The most common battery for consumer vehicles is the lithium-ion battery, which contains a flammable electrolyte that serves as the liquid membrane through which chemical ions pass between electrodes. If a cell short circuits, the flammable electrolyte may catch fire and burst through battery cell walls. As this has been known for quite some time, manufacturers design barriers to insulate the battery as well as portions of the battery pack to prevent thermal runaway and minimize the spread if it does occur.
Applying the same principles discussed earlier, the goal of the manufacturer is to develop mitigating techniques that enable the battery pack as a whole and individual cells to dissipate heat faster than is generated. While thermal runaway events tend to make headlines fairly frequently, several studies indicate when compared to the flammability of gasoline, Li-ion batteries pose a far lower risk of fire or explosion during or after a vehicle accident involving an EV.
Tying it all together
Continuous technological advancements in the performance, efficiency, and affordability of EVs have helped get hundreds of thousands of EVs, HEVs, and PHEVs into the hands of public and private consumers over the last decade. Various styles of EVs are now available from several manufacturers, offering drivers a wide variety of body styles, performance specs, and price points. While the capabilities of RESSs have been rapidly evolving, the safety testing required to ensure EVs, HEVs, and PHEVs are safe for use by the public is, for the most part, very similar to testing techniques used for safety testing of traditional combustion engines.
New standards are emerging, which are regularly reviewed and revised, as the industry continues to learn and understand the evolving technology. There is no reason for any elevated concern related to operating these vehicles. Maintenance and R&D personnel should evaluate specific scenarios by thinking about the basic hazards to which they are exposed, consider the risk of each hazard, and take the appropriate precautions to mitigate the hazards as required by the NFPA 70E. EVs and their electrically dense battery packs are an exciting technological trend that continues to advance and provide different and efficient transportation means but has yet to pose an electrical hazard risk that exceeds our ability to be protected.
Tommy Northcott is a professional engineer licensed in the state of Tennessee and a senior power engineer with Jacobs Technology, Inc., in Tullahoma, Tenn. He is also an NFPA 70E compliance subject matter expert, a principal member of the NFPA 70B Committee, electrical safety trainer, certified maintenance and reliability professional, and certified reliability leader. He can be reached at [email protected].
