December 2012

NTC temperature sensors for e-mobility

Bodyguards for batteries and drives

No picture available

Excessive temperatures can be dangerous for the batteries and drives used in electric and hybrid vehicles. Rugged EPCOS NTC temperature sensors monitor their heat to increase their reliability and safety.

The environment of vehicle batteries is subject to fluctuating temperatures, mechanical stresses, vibrations, shock and moisture. The batteries are the critical components in electric vehicles (EVs) and hybrid electric vehicles (HEVs) with respect to reliability and safety, and must be continuously monitored. In particular, moisture combined with an electric field can lead to irreversible destruction of a sensor with insufficient or absent protection (electromigration). To ensure that temperature sensors operate reliably even under harsh conditions, they must be protected by a rugged and moisture-resistant housing. The various automotive manufacturers and suppliers specify differing requirements on the geometry, characteristics, response behavior, dielectric strength and terminals of the sensors. It is thus vital to develop sensors that can be adapted to the diverse requirements in a modular way.

Reliable monitoring of battery temperatures
The batteries of EVs and HEVs supply an optimum energy output at precisely defined operating temperatures. Reliable monitoring and regulation of battery temperatures prevents the batteries from overheating and thus maximizes their operating life.

A large number of NTC temperature sensors are used in EV or HEV batteries to measure their cell temperatures. A newly developed special screw-type EPCOS temperature sensor (Figure 1a) is optimized for the high demands made on parameters such as moisture, bedewing and mechanical stress, and is validated for use in series-production vehicles. This sensor consists of a rugged and moisture-resistant housing in which the NTC element is embedded. The development of the materials and the process definition were important for implementing this sensor housing. The sensor is easily mounted and secured via a metal ring tongue.

The standard version of the screw-on sensor is specified to measure the battery temperature with a resistance of 10 kΩ at 25 °C. This resistance as well as the characteristic of the NTC temperature sensor can be adapted to suit customer requirements.

Figure 1: EPCOS NTC temperature sensors

a) EPCOS NTC sensor for measuring battery temperatures
The rugged plastic housing of the sensor element offers reliable protection against moisture and mechanical stress. Secure mounting is ensured via the metal ring tongue.

b) EPCOS NTC sensor for measuring coolant temperature

The NTC tube-mounted sensor can be attached quickly and reliably onto tubing of various diameters.

c) EPCOS NTC plug-in sensor for measuring coolant temperature
A special media-resistant plastic material surrounds the NTC element and ensures absolute sealing of the coolant circuit thanks to its application-specific sleeve geometry.

d) EPCOS sensor for stator temperature measurement

The high dielectric strength of the new sensor prevents flashovers between the drive and measurement circuits.

Monitoring coolants essential
Absolute operational reliability of the batteries of electric vehicles is a key criterion for automotive manufacturers. Overloading by charging or discharging processes can damage the battery cells or – in the worst case – start a fire. Depending on the design, this can be a serious risk. The temperature of the battery coolant is a critical parameter in this case, as this medium conducts power dissipation away as heat via a heat exchanger. Measuring the coolant temperature gives reliable feedback about the operating state of the batteries. A new EPCOS temperature sensor (Figure 1b) is now available to measure the coolant temperature reliably. It can be mounted directly onto the tubing of the cooling system.

A special feature of this newly developed EPCOS tube-mounted sensor is the combination of the sensor with its fixing element. This allows the sensor to be mounted on tubing while ensuring a reliable and vibration-resistant connection between sensor and pipe. The sensor can be mounted and removed quickly and easily to and from the tubing of the cooling system. Because of the sensor’s modular design, it is suitable for differing tube diameters.

To ensure the long-term stability of the sensor system in the cooling circuit, special attention was paid to the materials used. This assures compatibility with respect to the electrode potential of the metals used. A small metal sleeve that is in contact with the metal tubing of the cooling system permits short response times in combination with the thermal and electrical properties of the NTC element molded into it.

Special focus was also placed on the resistance of the sensor system to frost as well as to high temperatures with simultaneously high air humidity. These ambient influences cause condensation in the vehicle and thus on the sensor when temperatures drop. This problem can be reliably countered by the EPCOS design based on the plastic coatings already used in other sensors. The connectors also are protected against moisture. The design enables simple and reliable handling during mounting. By the same token, the sensor system can be removed from the tubing and remounted onto it again as often as required to facilitate any maintenance work.

Sensor with media contact
In addition to the tube-mounted sensors in which the temperature sensor is not exposed to any direct contact with the coolant, there is also a need for sensor solutions that permit the temperature of the battery coolant to be measured directly. The special challenge in this case is to develop a sensor which is resistant to the coolant and ensures reliable sealing of the coolant circuit despite the presence of mechanical stresses. These requirements are met by the new EPCOS battery coolant temperature sensor (Figure 1c).

The sensor system combines a media-resistant plastic material with a rugged NTC element allowing high-precision measurement. The battery coolant temperature sensor can be integrated easily and reliably into the cooling circuit. The temperature data thus obtained enables reliable monitoring of the battery status and regulation of the system temperature.

Preventing stator overheating
Together with the batteries, the electric drive motor is the other basic unit of EVs and HEVs. To ensure both long motor life and optimum performance, the temperature of the electric motor must be continuously monitored. It is particularly important here to determine the temperature of the stator windings in the range above 140 °C as accurately as possible. This is the only way of utilizing the full potential of the motor without incurring a risk of overheating. The main demands made on the sensor for stator temperature measurement are simple mounting, protection against any mechanical stresses that may occur, both during mounting and operation, and high dielectric strength. The latter prevents flashovers from the drive current circuit to the measuring current circuit. A new EPCOS sensor was developed specifically to meet these more rigorous requirements (Figure 1d).

This new sensor offers the high dielectric strength that is needed to embed it directly in the coil former of the stator. Its high temperature resistance of up to 200 °C and short response time are achieved with a molded NTC sensor element. The sensor is very robust with regard to mechanical stresses and is easy to mount.

Like almost all TDK and EPCOS sensors, it can also be customized with respect to its characteristics, cable lengths and connector variants.