Answering one of the most common EV charging questions

If you’ve ever plugged your EV into a 7.4kW AC charger and seen just 6.4kW (or even less) on the display, you’re not alone. This issue is not a fault—it’s the result of several real-world electrical and system factors. Let’s break them down.

🔋 Nominal vs. Actual Power: Why 7.4kW Isn’t Always 7.4kW

A 7.4kW AC charging station (common in Europe: 230V × 32A) delivers power directly to your EV’s onboard charger (OBC). The charging station itself doesn’t control the actual power transferred. Instead, actual charging power is based on:

  • The OBC’s maximum current intake (typically 32A)
  • The real-time grid voltage (ideally 230V)
  • The OBC’s energy conversion efficiency (usually ~95%)

So, if the voltage drops, or your car limits intake, your actual charging rate can fall well below 7.4kW — even as low as 3.4kW.

⚠️ Key Reasons Why Charging Power Drops

1. 🔥 Overtemperature Derating by the Charger

  • Loose internal terminals create heat through resistance
  • Heat reaches internal sensors via copper busbars
  • The charger reduces CP/PP signal duty cycle → OBC drops charging power

2. ⚙️ Grid Voltage Drop & Cable Losses

  • OBC limits current to 32A max
  • If actual voltage is 210V, then: 210V × 32A × 95% = 6.38kW
  • Cable loss examples:
    • Copper cable (100m): ~19V drop → ~6.1kW
    • Aluminum cable (100m): ~30V drop → ~5.7kW

Formula:

  • ΔU = I × R, where R = ρ × L / S
  • Copper: ρ = 0.0174, L = 100m, S = 6mm² → R = 0.29Ω, ΔU ≈ 18.6V
  • Aluminum: ρ = 0.0283 → R = 0.47Ω, ΔU ≈ 30.1V

Even short runs with loose breaker terminals can cause noticeable drops.

3. ⚡ Battery SOC & Voltage Behavior

  • OBC outputs constant power within voltage range (e.g., 300–450V)
  • If battery voltage is 280V, then: 280V × 22A = 6.16kW
  • High SOC or very low SOC = possible derating by the car

4. ❄️ Auxiliary Loads Stealing Power

  • Cold climates? Battery heaters activate pre-charging
  • Heaters may use 1–3kW → net charge power reduced

5. 📉 Vehicle’s Onboard Charger Limitations

  • Some cars only have a 3.6kW OBC
  • Even with a 7.4kW charger, vehicle limits the intake

6. 🔧 OBC Thermal Protection

  • If the OBC overheats internally (e.g., MOSFETs, coils), it will reduce power
  • Caused by airflow blockage, water pump failure, etc.

7. 🚗 Vehicle State or User Settings

  • Charging current manually set lower (e.g., via app)
  • Car not locked → some brands apply safety-based derating
  • A/C or windows left open during charging

8. 🔌 Socket or Connector Overheating

  • Gun head contact wear or dirt increases resistance
  • Excess heat triggers sensor → power cut
  • Repeated plug cycles lead to fatigue

9. 🔒 Inlet Lock Sensor Fault

  • Some EVs reduce power if the gun lock isn’t secure or malfunctions

✅ Conclusion: What You Can Do

A 7.4kW charger promises nominal power—but real-world performance varies. It’s usually due to grid voltage, cable quality, OBC design, or thermal limits.

For consistently high charging efficiency: Explore our high-performance AC chargers or contact our technical team for tailored EV charging solutions.