The Impact of Fast Charging on the EV Charging Chain

August 1, 2024

Fast DC charging of electric vehicles (EVs) could be key to alleviating the range anxiety that many believe is limiting EV adoption. With a robust charging infrastructure that enables battery charging in 20 minutes rather than 4 hours, EV charging moves much closer to the experience drivers are accustomed to when refueling an internal combustion engine (ICE) vehicle.

That infrastructure isn’t fully developed today but is growing and becoming accessible to more vehicles. Multiple EV manufacturers have recently adopted the Tesla EV charging connector and Tesla is currently leading the market in deploying fast charging ports. Research commissioned by Sensience forecasts that DC fast charging will account for more than 70 percent of public charging globally by 2030 (Figure 1).

While fast charging could unlock growth in the EV market, it also raises the bar for thermal management across the EV charging chain. DC fast chargers output between 50-350 kW of power, compared to the 7-19 kW typical for Level 2 chargers. Even at the higher voltages common with DC fast chargers, these ports generate significantly more heat than Level 2 charging ports. Effective thermal management, thus, becomes more critical than ever to prevent the temperatures that can shorten battery life, reduce charging efficiency, and introduce safety risks.

One component of effective thermal management that sometimes gets overlooked until late in the design process is the specification of the temperature sensors that enable battery and vehicle management systems to respond quickly and intelligently to temperature changes across the vehicle charging chain.

The Role of Temperature Sensors in the EV Charging Chain

Sensors are used across the charging chain. The most obvious application is in the battery itself where thermal management is of the highest priority. Sensor placement within the battery will be determined by battery design and size, but a single sensor will typically prove inadequate as it may not detect hot zones that can develop in cells or modules during fast charging. Ideally, you want the ability to directly monitor battery cell temperatures at multiple locations and have an understanding of the heat distribution within the overall pack.

The onboard charger represents a simpler but still critical environment for temperature sensors as sensor monitoring triggers the cooling systems that are required to manage heat during fast charging. Because there is less temperature variability across the component, fewer sensors are required.

The vehicle’s charging port is another important application for temperature sensors in the charging chain. Temperature sensors are used in both the vehicle’s charging inlet as well as the charging gun to monitor the electrical prong temperatures. This feedback provides a safety stop in case of any unintended phenomena caused by damage or contamination on the prongs. During fast charging, temperatures can be highest at the charging inlet, and special attention should be paid to the specifications of sensors used in this application.

Equally important are the sensors integrated with the cooling system that protects the battery, onboard charger and power electronics from overheating. The intelligent cooling systems integrated into EVs are only as effective as the sensors that support them.

Specifying Sensors for the EV Charging Chain

A high degree of accuracy is essential in any temperature sensing application but is particularly important in the EV charging chain due the narrow temperature window that batteries can be charged at their top speed. Low sensor accuracy can reduce charging efficiency because charging may have to be slowed to compensate for the sensor’s margin of error even if actual temperatures are within the desired range. Higher accuracy sensors reduce the degree to which vehicle systems have to account for sensor inaccuracy and may allow faster charging for longer periods of time.

Another critical characteristic of temperature sensors for EVs is the response time or the time between when temperature is measured and when it is communicated and can be acted on. Different sensor types have different response characteristics and within each type, response time can be optimized through sensor configuration.

For example, in negative temperature coefficient (NTC) thermistors, a metal body enables faster response than a plastic body due to its enhanced conductivity; however, the metal body does add to sensor costs and these costs need to be balanced against the value of faster response in a particular application. Where temperatures are not that high or are being managed by a cooling system, as is the case in key components of the EV charging chain, increasing costs to achieve the fastest possible response time may not be warranted. The exception is the charging inlet, where optimizing response time should be considered a higher priority.

The other factor to consider in sensor selection is serviceability — or lack thereof. Sensors are generally designed into the EV charging chain in a way that makes it impractical to replace them if a failure occurs. The whole component will need to be replaced if a sensor fails. That makes dependability the most important attribute for temperature sensors being used across the EV charging chain.

Fortunately, while the applications for temperature sensors are different in EVs compared to ICEs, the sensor technologies used are similar. Manufacturing processes for these technologies are mature and have benefitted from automation that enables consistent quality. Testing and calibration processes are also well understood and can help ensure predictable reliability and performance in this application.

Evaluating Sensor Technologies for the EV Charging Chain

Three temperature sensing technologies are typically considered for EVs: thermocouples, NTC thermistors, and resistance temperature detectors (RTDs).

Thermocouples generate a small voltage in response to a temperature change that is proportional to the change in temperature. These sensors are generally inexpensive but are not well suited for the EV charging chain. They can pose safety risks to test engineers in high voltage environments like EV batteries, their accuracy can be compromised by electrical noise, and their response time is slower than what is typically required.

In an NTC thermistor, resistance decreases as the temperature increases. This technology provides good accuracy in a compact size that enables integration into tight spaces. The response time and resistance-temperature curve of NTC thermistors can also be configured to meet a wide range of application requirements, including those of the different components within the charging chain. Consequently, they represent an ideal solution for these applications because they can be configured to meet design requirements efficiently and have been proven in automotive applications for decades.

In an RTD, resistance increases as temperature increases. These sensors generally deliver better accuracy and have a wider operating range than NTCs; however, they are also more expensive, and the advantages of this technology generally don’t deliver additional value in EV charging chain applications. With the constant pressure to reduce costs, designers may find they achieve the required performance at lower costs with NTC thermistors.

Designing in Accuracy and Reliability

With fast charging becoming more common, precise and reliable temperature sensing across the charging chain will remain critical even as batteries technologies evolve. The key to effective and cost-efficient integration of temperature sensors into new designs is to develop specifications and choose a supplier during prototype development. This enables more streamlined processes, helps ensure sensors meet application requirements cost effectively, and mitigates the risk that modifications will be required as the design moves from prototype to production.

This article was written by Phil Thibodeau, Product Manager, Transportation, Sensience (Westerwille, OH). For more information, visit here  .

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