Can EVs Overheat?

Compared to the internal combustion engine (ICE), the electric vehicle (EV) has many different sub-systems. The main focus of EV development is the vehicle's range, which is dependent on making the batteries longer lasting and sub-systems more efficient. There are design considerations for protecting the electric motor and for makingit more powerful and self-sufficient. Design decisions are also involved in how to charge the system reliably and safely. All of these design considerations includethermal management challenges that can be overcome by using NTC thermistors for temperature sensing.

Start-ups versus incumbent automakers

There is a renewed focus onzero-emission EVs globally led by the US emission environmental standards, but propelled by China. China’s manufacturing base has brought the need for EV to an existential level driven by high pollution levels in its major cities. U.S. emissions policies are driven by Corporate Average Fuel Economy (CAFE) standards. These standards dictate the average fuel economy of an automakers fleet. The US has finally reached a tipping point where the ICE vehicle can no longer meet the tightening required mileage. This has been accelerated further by the consumer shift from sedans to trucks and SUVs, which make up nearly 70% of light vehicles sold in NAFTA. These vehicles are heavier and have lower mpg averages. The big three automakers in theU.S. have been reluctant to fully commit to electric powertrains, but with success of Tesla and diesel violations from VW, a race has begun to electrify vehicles across their entire fleet.

The most off-putting issue to drivers wanting to change to all-electric vehicles is range anxiety and the ability to charge their vehicle easily. Having a limit of 200 miles perbattery charge does pose questions ofthe vehicle's usability or, flipping the coin, makes assumptions aboutthe driver's lifestyle. Popping to the local mall or going for short journeys is fine, but if you want to drive upstate, range may be an issue. This problem becomes more difficult for consumers in cold weather climates where battery efficiency can decrease the vehicle's range by up to 50%. Fortunately, new battery technologies and powertrains are making this less of a limiting factor.

Why IsTemperature Measurement Critical in EVs?

In layman's terms, the electric motor in an EV is a big wheel of windings with a magnetic core. Electric overload can occur if the supply voltage drops, resulting in the motor drawing more current into the windings to maintain its torque. If overload occurs, the motor will overheat and, if left unchecked, will ultimately fail. Over half of insulation failures in motors arise due to overheating, which will lead to insufficient isolation; for every 10°C rise in temperature, insulation life halves. This degradation can cause leakages and short circuits, and eventually motor failure. A replacement of an electric motor is costly, and if the vehicle shuts down unexpectedly while driving, it stops abruptly, which could cause a crash. Withan ICE, in the unlikely event that it runs out of oil, as sensors already monitor oil level, the engine will seize, but the drivershould have time to maneuver the vehicle into a safe stopping position.

For an ICE car in a typical driving scenario, if there is a fuel leak, it may cause a fire under the hood, with the worst-case scenario beingthat the fuel tank will explode. If an EV battery overheats and starts to burn, it cannot be extinguished by standard fire extinguishers; it becomes a self-sustaining chemical fire. After an accident, any hidden damage to the modules or individual cells within the battery pack can cause overheating. Over-charging of the battery pack during re-charging can also cause thermal runaway. In the past, this thermal runaway has been a cause for concern for both the passengers and emergency services attending the incident. If the car is parked at home, this could cause significant fire damage to the property or, worse still, harm human life.  

Therefore, it is vitally important to measure and monitor temperature within the EV sub-systems, both accurately and reliably, but what technologies are available? As with any new technology, it will require new solutions for its implementation, which impacts how people use the technology in their daily lives; it may take an adjustment period for users to adapt accordingly.

Click image to enlarge

Figure 1: The five main sub-systems of an electric vehicle

Temperature sensor technologies

There are quite a few different types of temperature sensor technologies available on the market, each with its characteristics. Resistance thermal detector (RTD) sensors are excellent for measuring extensive temperature ranges. Because they use pure metals, they can be expensive and require a transmitter, which further adds to the cost– formidably so for the automotive industry. On the other hand, thermistors measure temperature with the same, if not better, accuracy at a fraction of the cost.

PTC versus NTC thermistors

Positive temperature coefficient (PTC) thermistors increase in resistance as the temperature rises. When the pre-defined temperature limit of the thermistor is met, the resistance spikes and cuts power to the circuit. Acting like a fuse or circuit breaker, they are commonly used as temperature limit sensors in shut-off or safety circuits.

Click image to enlarge

Figure 2: The NTC screw-on battery sensor from TDK has long-term moisture resistance

In contrast, negative temperature coefficient (NTC) thermistors reduce in resistance as the temperature rises. As the resistance-temperature (R-T) relationship is a flattened curve, it is excellent for highly accurate and stable temperature measurement.  However, the choice of which type of NTC to specify depends on a few conditions – temperature range, resistance range, accuracy, environment, response time, and size.

Epoxy resin coated and glass-encapsulated elements measure temperatures typically between -55°C and +155°C. The epoxy resin types have a rugged construction to protect against humid environments and come with high accuracy (±0.1°C). For applications requiring extremely fast response times, in the order of milliseconds, glass-encapsulated elements would be a more appropriate choice. They are also more compact than epoxy resin NTC thermistors with diameters down to 0.8mm and can measure elevated temperatures up to +300°C.

It is also essential to match the NTC thermistor's temperature to that of the component causing the temperature change. Therefore, they are available in conventional leaded styles and incorporated in screw- or clip-type housings for attachment to heat sinks or cable harnesses and as chip versions for surface mounting.

NTC thermistors in action

The fiveuse cases below concentrate on the design and temperature sensing requirements of EV sub-systems and describe how NTC thermistor sensor design can meet them. 

Case 1 - Battery cells, modules and packs         

Cells perform the primary functions of a rechargeable battery. Multiple cells are grouped into a single mechanical and electrical module with a thermal interface and connector terminals. These modules are electrically connected with sensors and a controller to form a battery pack, which is connected to the powertrain.

Depending onthe battery chemistry, cell structure, and whether the cells and modules are connected in series or parallel, these high-voltage battery packs achieve optimum efficiency at precisely defined operating temperatures. Reliable monitoring and control of theirtemperature at multiple points prevent local overheating, improving battery life and increasing safety.

Screw-on NTC thermistors can easily attach to the battery's surface with metal eyelets. The standard screw-on series are compatible with the aluminum surfaces within the battery, measuring temperatures from -40 to +85°C (Figure 2). Their over molded design provides high humidity resistance.

The new spring-load series of NTC thermistors improves thermal contact and reduces the temperature offset on the battery's surface, providing an accurate temperature reading from -40 to +90°C. They are offered in flexible mountings such as spiral springs, flat springs, and plastic hooks to compensate for installation tolerances.

The B58703M series is compatible with copper surfaces to provide accurate (up to ±5K) busbar temperature readings from -40 to +150°C and peak voltages up to 2.5kVdc for 60s.

Case 2 - Monitoring coolant temperature 

Monitoring the temperature of the coolant indicates the operating state of the battery. Pipe-mounted sensors clip onto the coolant inlet and outlet and are easy to remove and re-mount during maintenance. There is no need for a fixed installation hole or additional fixing elements, such as screws and sealing components, eliminating the risk of coolant leakage.

TDK offers a wide range of clip-on sensors in various geometries and with adjustable electrical parameters (Figure 3). Available in plastic (-40 to +90°C) and metal (-40 to -150°C) versions, their design ensures secure thermal contact, even in high vibration environments and response times to below 2s.

Click image to enlarge

Figure 3: Clip-on sensors from TDK are suitable for coolant pipes of different diameters, are easy to mount and have fast response times

Case 3 – High voltage powertrain busbars                                                           

The busbar series of NTC thermistors measures temperatures in the range of -40 to +150°C, and, for short periods, up to +200°C (Figure 4). They are mounted on to defined hotspots on the electric motor's busbar, ensuring stable performance. The mounting clip design provides a robust connection and interlock under challenging vibration and mechanical load conditions. The LCP housing material increases thermal conductivity and high voltage performance, even in environments where automatic transmission fluid (ATF) oil may be present.

Case 4 - EV connectors and charging stations    

The temperature of connectors, charging stations and general high voltage pins have to be monitored. TDK’sscrew-on M703 (-55 to +155°C) series mounts on heatsinks and the B58703M series (-40 to +150°C) mounts on copper busbars via their metal eyelets.

TDK’snew K862 series has been developed to fulfill the requirements for a more compact design combined with a very high voltage strength of 4.0kV for 60s, thanks to its 2.5mm diameter ceramic housing. It reads extended temperatures from -40 to +180°C and is designed to withstand temperatures up to +200°C.

Click image to enlarge

Figure 4: The new TDK busbar sensor stands out thanks to its long-term stability and a high electrical resistance class

Case 5 – Stators within electric motors

For the design of NTC thermistors for stators in electric motors, TDK uses a modular approach. The ultra-small case series was developed for applications where ATF oil may be present and features a miniaturized sensor housing design. The soft case series has an elastic sensor housing, which enables it to be squeezed between the motor's windings. The material then adapts to its surrounding area, providing good vibration resistance. The soft case series has precise housing dimensions and is an excellent alternative to the standard sensors with the dual shrink tubes. 

Click image to enlarge

Figure 5: TDK B57703M NTC sensor has a high level of long-term stability

The key to achieving the best sensor performance is perfect integration in the application's interface. As a second step, the modular design enables interface-specific mounting to the stator without additional bonding or fixation. With this two-step modular approach, fast integration is achieved without any compromise on sensor performance.

In addition to these highlighted use cases, NTC thermistors are used elsewhere in EVs, including heated steering wheels, seats and side-view mirrors, as well as sophisticated climate control systems.

Future of EVs        

Today, EVs are poised to displaceICEs, with equivalent driving ranges. EVs are fun to drive - unlike an ICE, they are responsive due to their instant torque - and the running cost is around $9 for 200 miles. It is all about synchronicity; new battery technologies and powertrains are upping the range to more than 500 miles. There are also developments to use hydrogen fuel cells alongside batteries to create a hybrid system, or even swapping out the battery with a hydrogen fuel cell EV (FCEV).

Click image to enlarge

Figure 6: Thanks to its flat and compact design, the K862 series from TDK is very well suited for integration within an interface-specific connector system

One of the most critical factors for the further development of e-mobility is increasing energy efficiency. With all these new technologies, engineers will need to overcome yet further design challenges.

NTC thermistors from TDK are designed specifically for EV applications, enabling precise temperature measurements with long-term stability. With interface-specific mounting features for excellent performance, high-voltage strength, fast response time and robust design, all the temperature monitoring aspects of EV's sub-systems can be met.

TDK Electronics