Driving Efficiency: Exploring the Advantages of Reed Relays in EVs

The growing popularity of electric and hybrid electric vehicles (EVs) has created several challenges for the transportation industry. While these vehicles aren’t new, mainstream acceptance will put more of these cars on the road, increasing efficiency and power performance needs and raising EV system voltage to several hundred volts, which requires high-voltage isolation.

 

Images used courtesy of Standex Electronics

 

High-Voltage Isolation Monitoring in Electric Vehicles

Unlike gasoline- or diesel-powered vehicles, hybrid and electric variations feature electrical hardware that allows them to function properly, including high-voltage batteries, DC/DC converters, inverters for driving electric motors, charger modules that connect to the 230 V/380 V power grid, and control units, which include the battery management system. All these systems and subsystems operate at higher voltages, usually between 400 and 800 V.

This electrical hardware, coupled with automotive safety norms, necessitates an internal measurement system, which all EVs have. Besides the standard monitoring functionality to ensure the best battery efficiency, this system must monitor leakage currents between the battery and the chassis. Switching or cutting this measurement system to the power system requires high isolation and reliable switching component functionality.

The vehicle’s electrical current can become a safety hazard without proper components to keep this isolation in check. This isolation is vulnerable to gradual or sudden deterioration, creating a dangerous situation. Changes can occur between the power system and the chassis, accumulating hazardous charges.

These factors make the continuous monitoring of high-voltage isolation essential in hybrid and electric vehicles. While different components can be used for isolation measurement, reed relays are exceptionally well-suited.

 

Images used courtesy of Standex Electronics

 

Reed Relay in Electric Vehicles

Electric vehicles require high, consistent, reliable isolation properties to function properly, especially because two electric networks must be interconnected and isolated.

Safety plays a key role, which is why the isolation voltage must sometimes be three to four times higher than the actual nominal voltage of the monitored circuit. Reed relays comply with automotive standards, such as IEC 60664-1 or ISO 6469-3.

Reed relays function well in an electric vehicle because of their composition. A standard reed relay comprises a reed switch with two soft magnetic metal alloy contacts called reed blades. These blades are contained in a hermetically sealed glass body, which is evacuated to create a vacuum or filled with inert gas to prevent the blades from oxidizing. This arrangement provides reed relays with the versatility to function in harsh environments. Reed relays can operate in ambient temperatures from -40°C to 105°C, with its internal temperature capable of safely reaching 125°C. This bodes well in hybrid and electric vehicles, which experience several environments over their lifetimes. Additionally, reed relays are robust enough to endure the dirt, water, humidity, oil, and grease they will encounter in these applications.

The reed relay’s copper coil creates a magnetic field when conducting current remotely, activating the magnetically conductive blades of the reed switch. The blades connect to complete the circuit and disengage once the magnetic properties aren’t active.

A sturdy, fully sealed housing further protects the components and the relay. The housing provides mechanical stability and resistance against shock temperature, stress, and other external influences. Depending on the application, the housing can be made of a potting resin, such as polyurethane, or over-molded with epoxy.

The reed relay housing, as well as the simple construction of the component, provides unrivaled longevity. Comparable technologies, such as a typical electromechanical relay, lack this protection layer, leaving them vulnerable to wear and debris. Further, electromechanical relays require mechanical parts to function, which can wear and corrode over time.

 

Reed Relay Performance Benefits

Reed relays provide benefits for insulation measurement in hybrid and electric vehicles. The component’s versatility allows it to perform its primary functions repeatedly and handle varying electrical loads and environments.

Unlike semiconductors, reed relays provide a linear capacitance, are not temperature-dependent, and the internal assembly doesn’t deteriorate with time.

Functionally, reed relays can accurately measure isolation voltage while having a leakage current in the picoampere range and fast-switching capabilities of up to 1 ms. This ensures millions of reliable operations, depending on the load. Should the isolation of the e-car deteriorate, the reed relay can help flag it effectively before it becomes a serious issue.

 

 

Images used courtesy of Standex Electronics

 

Regarding isolation measurement, reed relays can switch voltage levels from zero to 10 kVDC – up to 1000 VDC is sufficient for the e-car industry – providing not only a high dielectric strength between the coil and the contact but also high breakdown voltage across the open switch (up to 4.5kV). This voltage isolation pairs with a very high insulation resistance, which ensures minimal leakage currents in the picoamperes range.

Reed relays in electric vehicles are typically found in the battery management system or onboard charger and can take various forms depending on the developer’s needs. The relay is generally situated between the high-voltage battery and the ground. It acts as a separation to isolate the high-voltage circuit of the battery from the low-voltage circuit of the measurement microcontroller unit.

When the contacts are open, the high breakdown voltage provides perfect isolation between these two circuits to prevent interference. When the relay activates, the system uses the closed circuits to measure a sample current from one or both battery poles. The main computer unit then compares the sample with a referential voltage level, allowing the system to evaluate the battery’s health. Because the relay contact resistance is low—tens of milliohms—does not influence the measurement results.

 

Reed Relay Takeaways

As hybrid and electric vehicle demand swells, manufacturers have multiple options for high-isolation measurement. Reed relays provide the best array of features, such as functionality, longevity, and reliability. When incorporated into hybrid and electric vehicles, reed relays will stand up against the challenges and demands of these applications.

The benefit of reed relays lies in introducing new components that can maintain the existing switching performance and isolation properties while reducing the power consumption and size of the PCB layout area. With the latest technology and innovation, it is possible to respond to this trend in the high-voltage market for electric vehicles.