Passenger car rearview mirror defrosting and heating systems rely on continuously powered normally open automotive relays, with single-operation times reaching over ten minutes. This contrasts sharply with short-duration pulse loads like horns and wipers. Prolonged continuous energization causes continuous thermal stress losses to the relay's core components. The core carrier of the circuit's current-carrying channel is the Bi-Metal Silver Alloy Contact. While its basic current-carrying capacity matches the rearview mirror's operating current of several amperes, the heat buildup from continuous steady-state energization continuously alters the contact interface's operating environment, resulting in a loss pattern distinct from that under frequent on/off loads.
The primary losses from continuous energization stem from contact heat accumulation and accelerated surface corrosion. Although the rearview mirror heating load power is only tens of watts, seemingly within the contact's rated capacity light load range, there is no power-off cooling buffer period. Bi-metal Silver Contact Points inherently possess milliohm-level contact resistance. Prolonged current flow generates Joule heat, and the high temperatures of the vehicle door and engine compartment, combined with the contact's own temperature rise, easily push the contact temperature close to the metal's softening threshold. High temperatures significantly accelerate the oxidation and sulfidation reactions on the silver alloy surface, causing the contact resistance to increase unidirectionally year by year, creating a potential for overheating and power degradation in subsequent circuits.
The rearview mirror heating relay operates very infrequently, with virtually no arc erosion during switching, causing the contacts to lose their self-cleaning ability and further exacerbating wear. The slowly forming oxide passivation film on the surface of the Bimetal Relay Contact Point cannot be broken off by the arc, causing the film to continuously thicken and increase the circuit voltage drop. When the contact resistance reaches a critical value, a voltage divider structure forms between the contact and the heating film, causing abnormal temperature rise at the relay terminals. This directly results in insufficient rearview mirror heating power, and long-term use will exacerbate localized overheating and deformation of the contacts.
The coil is another component prone to aging under long-term energization. Continuous energization generates stable copper loss heat, and the enameled wire insulation layer endures high-temperature thermal stress over time. The lifespan of conventional low-grade insulation materials decreases exponentially with increasing temperature. In summer, the internal temperature of a car door can reach over 70°C. The combined effect of coil temperature rise and ambient high temperature easily leads to insulation aging and inter-turn short circuits. High temperatures also increase copper wire resistance, reducing the coil's electromagnetic attraction. Under bumpy road conditions, the contacts are prone to slight separation, indirectly exacerbating localized arcing losses in the Bi-Metal Electrical Contact Points.
For relays with long-term energized loads, the aging rate of components can be reduced through multi-dimensional optimization. During the selection phase, prioritize air-sealed structures to isolate air and slow contact corrosion; upgrade the coil with high-heat-resistant insulation materials; and install it away from high-temperature heat sources such as the engine. The circuit design can employ a low-voltage holding scheme after pull-in to reduce continuous coil heating. Simultaneously, sufficient contact current margin is reserved, and an RC snubber circuit is added to suppress micro-arcs, comprehensively delaying the oxidation and degradation of the Silver Alloy Electrical Contacts and extending the overall service life of the relay.
In continuous, constant-load scenarios such as automotive rearview mirror heating, relay wear is a chronic, cumulative damage over a long period, with the thermal aging processes of the contacts and coil influencing each other. A thorough understanding of the oxidation and temperature rise characteristics of the Silver Electronic Contact under steady-state energization and the thermal decay characteristics of the coil insulation is crucial to mitigating high-temperature wear risks during component selection and vehicle electrical layout, thereby improving the operational stability and lifespan of automotive relays in long-term energized circuits.
Conventional contacts on the market struggle to handle the long-term high-temperature, steady-state current-carrying conditions in automotive applications. Our self-developed and mass-produced Bi-Metal Silver Alloy Contact features optimized alloy ratios and composite processes, significantly improving high-temperature resistance and resistance to oxidation and sulfidation. It maintains stable contact resistance over the long term, making it suitable for long-term energized relays such as those for rearview mirror heating. This effectively mitigates common failure issues like contact heat accumulation and passivation film thickening, greatly extending the overall lifespan of the relay and meeting the bulk supply requirements of various passenger vehicle electrical circuits. For automotive relay contact solutions, please feel free to contact us to inquire about sample testing and bulk purchasing.
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