Wireless EV Charging: Coil Alignment & Power Transfer Efficiency

Receiver Pad Position

Lateral misalignment 0.00 m

Air gap (ride height) 0.18 m

Coil System

Receiver pad size 1.00×

Coil quality factor Q 180

Power Transfer

Efficiency η0%

Coupling k

0.000

Figure of merit U

0.00

Input power

3.7 kW

Delivered power

0.0 kW

How it works

Dynamic wireless EV charging couples a ground-mounted transmitter coil to a receiver coil bolted under the vehicle through a resonant magnetic field. Treating each coil as a magnetic dipole of area A and moment aligned with its axis, the mutual inductance between two dipoles separated by a vector r = (x, z), distance D = √(x²+z²), is:

M(x,z) = μ₀A₁A₂N₁N₂ / (4πD³) · [3(z/D)² − 1]

k = M / √(L₁L₂)          (coupling coefficient)
U = k·√(Q₁Q₂)             (figure of merit)
η = U² / (1 + √(1+U²))²   (max efficiency at resonance)

This is the standard near-field dipole approximation used in wireless-power-transfer engineering to estimate how sensitive a charging pad is to misalignment — it is exact only when the coil separation is large relative to coil radius, but it correctly captures every qualitative feature seen on real hardware:

Air gap ↑ — coupling falls off roughly as 1/D³, the dominant reason ride height matters so much for WPT.
Lateral offset — coupling degrades faster than the air gap alone predicts, and beyond a critical offset angle (θ ≈ 54.7°, where 3cos²θ − 1 = 0) the dipole term crosses zero and the induced voltage would flip sign — the same "null" alignment installers are trained to avoid.
Quality factor Q — higher-Q resonant coils (better copper, lower losses) push more of a given coupling k into delivered power, per the U = k√(Q₁Q₂) figure of merit.
Delivered power is η × 3.7 kW, the SAE J2954 WPT1 reference power level used for light-duty EV charging.

Energy flux (particle flow) Transmitter / receiver coils

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