Analysis of DC Charging Pile Terminal Overheating and Copper Aluminum Transition Standards

1. Requirements and Scope

In the operation of high-power DC charging stations (120kW–360kW), site managers often encounter charred cable insulation, localized melting, or even equipment fires at the connection points. These failures typically occur where aluminum cables meet the copper busbars inside the charger.

Why do seemingly tight electrical connections fail and overheat under high-current loads? This article analyzes the physical and chemical triggers of terminal failure and provides a standardized installation framework to ensure long-term reliability and safety.

2. Root Cause Analysis Electrochemical and Thermal Mechanisms

Electrochemical Corrosion (The Primary Trigger)

Copper and Aluminum have significantly different standard electrode potentials:

• Copper (Cu) Potential: $+0.34V$
• Aluminum (Al) Potential: $-1.66V$

When these two metals are in direct contact in the presence of atmospheric moisture, they form a "galvanic cell." Aluminum, being the more reactive metal (the anode), undergoes rapid oxidation. This produces a layer of aluminum oxide (Al2O3Al2​O3​), which is a highly resistive insulator.

• Result: The contact resistance $R$ increases sharply. According to Joule's Law Q=I²Rt, the heat generated at 250A–400A charging currents leads to a thermal runaway.

Thermal Expansion and Creep

• Expansion Coefficient (Al): Approx 23.8×10⁻⁶/K
• Expansion Coefficient (Cu/Steel): Approx 17×10⁻⁶/K

During high-load charging, temperatures rise, and the aluminum expands more than the copper busbar or the steel bolt. This causes the aluminum to undergo plastic deformation (creep). When the current stops and the joint cools, the aluminum contracts, leaving a microscopic gap that further increases resistance and oxidation for the next cycle.

3. Selection Criteria Transition Solutions

To prevent galvanic corrosion, specialized transition hardware must be used.

4. Standard Installation Process

Regulatory Compliance

Per GB 50149-2010 (Code for construction and acceptance of busbar device in electrical equipment installation):

"When connecting copper with aluminum or aluminum alloy terminals, copper-aluminum transition joints must be used."

Step-by-Step Installation

• Oxide Removal: Sand the aluminum terminal surface with fine-grit paper and apply conductive grease within 60 seconds to prevent re-oxidation.
• Application of Conductive Grease: This compound lowers contact resistance and seals the joint against moisture and oxygen.
• Torque Calibration: Use a calibrated torque wrench to ensure optimal contact pressure.
  • Calculation: Target contact pressure P=F/S should be $10–15MPa$. For an M12 bolt, the recommended torque is $45–50N\cdot m$.

5. Commissioning and Verification

After installation, the quality must be verified through thermal imaging during the first 24 hours of operation.

• Infrared (IR) Thermography: Scan all bimetallic joints while the charger is at 80%–100% load.
• Acceptance Criteria: The temperature of the Cu-Al joint should not be more than $5K$ higher than the adjacent conductor. The absolute temperature must never exceed 80^∘C (176^∘F.

6. Expert Tips and Avoidance of Pitfalls

• Forced Calculation: Always verify the current-carrying capacity of the cable terminal.
  • Formula: I_max=J⋅S, where $J$ is the current density (approx 1.5A/mm2 for aluminum in high-temp environments).
• Avoid Pitfalls: Never use a standard aluminum lug (DL) on a copper busbar. The silver-colored coating on some lugs is often just tin, not a true bimetallic transition. Look for the friction weld line (a distinct seam between the copper head and aluminum body).
• Safety Warning: Never leave Cu-Al joints exposed in coastal or high-humidity environments without protective heat-shrink tubing or fireproof sealing.