High-Voltage Relay Selection And Application Analysis

As the new energy vehicle industry continues to evolve towards higher voltage and higher power, high-voltage DC relays have become one of the core control components in electric vehicle high-voltage systems. The proper selection of high-voltage relays directly affects the safety, reliability, and long-term stable operation of the entire vehicle's high-voltage system. Especially in critical circuits such as the power battery, motor controller, and charging/discharging system, high-voltage relays bear the important responsibilities of on/off control, system protection, and fault isolation; their performance limits have become a significant constraint on the design of the vehicle's high-voltage architecture.

 

In a typical electric vehicle high-voltage system, 5 to 8 high-voltage DC relays are usually required, covering various functional roles such as main relays, pre-charge relays, emergency charge relays, conventional charging relays, and high-voltage system auxiliary relays. Different relays have significant differences in operating frequency, electrical stress, and safety level within the system, which determines the different focuses in their selection and structural design. The internal electrical contacts, electric contacts, and high-current contacts of the relay are the key foundation for determining its performance upper limit.

 

 

A relay is an automatic control element that connects, disconnects, or switches circuits based on changes in input signals. It is widely used in automatic control, remote control, and signal isolation systems. Its fundamental characteristic is controlling high-current, high-power circuits with small current or low power, achieving effective isolation between the control and the load.

 

In the high-voltage systems of new energy vehicles, the primary task of relays is to safely connect or disconnect high-voltage power, preventing irreversible damage to core components such as the battery and motor controller under abnormal conditions. The copper contacts and precious metal contact materials inside the relay bear the electrical stress from frequent switching, forming a crucial physical basis for the safe operation of the high-voltage system.

 

 

During the cold start process of electric vehicles, the motor controller typically contains a large DC bus capacitor. If the main relay closes directly without pre-charging, the high-voltage battery will instantly discharge into the empty capacitor, creating a huge inrush current that can easily lead to contact erosion, adhesion, or even structural failure.

 

Therefore, high-voltage systems generally employ a pre-charge scheme: a pre-charge relay and a current-limiting resistor form a soft-start circuit, allowing the capacitor voltage to gradually increase. When the bus voltage approaches a certain proportion of the battery voltage, the main relay closes and connects to the main circuit, significantly reducing the inrush current at the moment of closure. This process places higher demands on the surge resistance of the relay's internal contactor contacts and silver electrical contacts.

 

 

From a functional and load-bearing capacity perspective, automotive relays can be divided into signal relays and power relays, with almost all new energy vehicles using power relays. From a structural implementation perspective, they can be divided into electromagnetic relays and solid-state relays; from an operating voltage perspective, they can be divided into low-voltage relays and high-voltage relays.

 

In high-voltage DC systems, electromagnetic high-voltage relays remain the mainstream solution due to their high maturity, strong breaking capacity, and ability to meet safety requirements under complex operating conditions. The internal structures of the relay, such as silver contacts, silver contacts in breakers, and bimetallic contact-breakers, are important design foundations for their adaptability to high-voltage DC environments.

 

 

Electromagnetic high-voltage relays mainly consist of a coil, iron core, armature mechanism, contact system, and return spring. When the coil is energized, it generates a magnetic field, attracting the armature and causing the moving contact to close with the stationary contact, thus completing the circuit connection. When the coil is de-energized, the magnetic field disappears, and the armature resets under the action of the spring, breaking the circuit.

 

Based on the initial state of the contacts in the de-energized state, relays can be classified as normally open contacts and normally closed contacts.

 

High-voltage relays typically employ a normally open structure to ensure that the high-voltage circuit is automatically disconnected in the event of power failure or fault, improving the intrinsic safety level of the system. The contact materials are mostly made of noble materials to balance conductivity, wear resistance, and arc resistance.

 

 

Compared to ordinary relays, the biggest structural difference in high-voltage DC relays lies in their sealing and arc-extinguishing design. High-voltage contacts are typically encapsulated in a sealed cavity, isolated from the outside air, thereby significantly improving withstand voltage and suppressing arc propagation.

 

During contact switching, high-voltage relays typically avoid disconnection under load, reducing arc energy through system-level control strategies. Internally, it extensively utilizes silver contacts in circuit breakers, a magnetic blowout arc-extinguishing structure, and high-strength insulation materials to enhance overall breaking reliability.

 

 

New energy vehicles place significantly higher performance demands on high-voltage DC relays than traditional vehicles, primarily in terms of high voltage resistance, high current resistance, impact resistance, strong arc extinguishing, and reliable breaking capability.

 

Regarding high voltage resistance, the relay must withstand hundreds of volts of DC voltage for extended periods and reliably close and open under load. Regarding load capacity, the rated current of drive systems typically exceeds 200A, with peak currents exceeding 300A. The relay must possess sufficient current-carrying and heat dissipation capabilities within a limited volume.

 

Impact resistance is one of the core safety indicators of high-voltage relays. The capacitive surge current at the moment of closure is often several times or even tens of times the rated current. If the contact's impact resistance is insufficient, adhesion failure can easily occur, posing a serious safety risk. Therefore, relays often employ silver electrical contacts, magnetic blowout structures, and high-stability alloy systems to enhance impact resistance.

 

In terms of arc extinguishing and breaking performance, electric arcs are not easily extinguished naturally in high-voltage DC environments. Relays need to rapidly weaken arc energy through structural and material design to prevent contact burnout. Some high-performance relays also incorporate rotating electrical contacts or multi-contact parallel structures to extend their service life.

 

 

Selecting a high-voltage relay is a systematic project that requires gradual convergence based on the overall vehicle electrical architecture. In the initial design phase, the voltage and current capabilities of the control circuit, as well as the rated voltage, current, and contact type requirements of the controlled circuit, must be clearly defined. Based on this, a preliminary product range is selected according to the voltage and current ratings, and the final solution is determined by considering factors such as arc extinguishing methods, cavity structure, and terminal types.

 

At the specific parameter level, six key contact indicators need to be evaluated: electrical life, rated current, short-time current-carrying capacity, maximum breaking current, overload breaking capacity, and reverse breaking capacity. These indicators directly determine the relay's reliability performance throughout the vehicle's lifecycle.

 

Electrical life assessment should be combined with the vehicle's pre-charge strategy and usage scenarios to ensure that contact performance does not experience unacceptable degradation under repeated load-bearing closure conditions. Rated current must cover the long-term stable operating conditions of the vehicle, while short-term current must meet the high-load transient demands of acceleration, hill climbing, and other conditions.

 

Maximum breaking current and overload breaking capacity are important safety indicators for relays, directly related to the system's protection capability under extreme conditions such as short circuits or collisions. For relays with polarized designs, their reverse breaking capacity must also be evaluated, balancing cost and safety redundancy.

 

 

As high-voltage systems in new energy vehicles continue to develop towards higher power and higher energy density, the technical threshold and system value of high-voltage DC relays are constantly increasing. Their internal key components, such as Custom Electrical Contacts, Precision Cold Forming Contacts, and Trimetallic Relay Contacts, have become crucial foundations for determining the overall high-voltage safety performance of the vehicle.

 

In the future, with the accelerated large-scale application of electric vehicles and energy storage systems, high-voltage relays will continue to evolve in terms of reliability, lifespan, and safety redundancy. Their selection and application design will become a critical aspect of the overall vehicle high-voltage architecture that cannot be ignored.