Qi2 Magnet Ring Specifications: The Precision Engineering Behind 25W Wireless Charging
Aim Magnet Engineering Team
Every Qi2 wireless charger on the market shares one critical component: a precision NdFeB magnet ring. But not all magnet rings are created equal. In this post, we dissect the exact specifications, tolerances, and material choices that separate a certification-ready magnet ring from a commodity part.
The Qi2 Magnet Ring: Dimensional Specification
The WPC Qi2 standard defines a magnet array based on Apple's MagSafe geometry. Here are the critical dimensions:
Ring Geometry
|
Dimension |
Specification |
Tolerance |
|
Outer Diameter (OD) |
56.0 mm |
±0.05 mm |
|
Inner Diameter (ID) |
44.0 mm |
±0.05 mm |
|
Ring Width |
6.0 mm |
— |
|
Thickness |
0.8–1.2 mm |
±0.02 mm |
|
Flatness |
< 0.05 mm warpage |
— |
Magnetic Performance Requirements
The holding force of a Qi2 magnet ring is a function of three variables: F = Br × Vmagnet × 1/dgap, where Br is residual flux density, V is magnet volume, and d is the gap distance.
Recommended Magnet Grades
|
Grade |
Br (kGs) |
Hcj (kOe) |
BHmax (MGOe) |
Use Case |
|
N48 |
13.8–14.2 |
≥11 |
46–49 |
Budget chargers, thin cases |
|
N50 |
14.0–14.5 |
≥11 |
49–52 |
Mainstream Qi2 chargers |
|
N52 |
14.3–14.8 |
≥11 |
51–54 |
Premium chargers, car mounts |
|
N52SH |
14.3–14.8 |
≥20 |
51–54 |
High-temp (car, 25W fast charge) |
N52 provides ~15% more holding force than N48 at the same volume. N52SH is mandatory for 25W Qi2.2.
Why Tolerance Matters: Real-World Comparison
Consider two magnet rings, both labeled "Qi2 compatible":
|
Parameter |
Budget Ring |
Aim Precision Ring |
|
OD tolerance |
±0.10 mm |
±0.02 mm |
|
Thickness tolerance |
±0.05 mm |
±0.02 mm |
|
Holding force at 1mm gap |
~800 gf |
~950 gf (+19%) |
|
WPC cert pass rate |
~60–70% |
> 95% |
Coating: Corrosion Resistance
NdFeB magnets oxidize rapidly without proper coating. For Qi2 applications:
|
Coating |
Thickness |
Salt Spray |
Recommended For |
|
Ni-Cu-Ni |
15–25 μm |
48h |
General indoor use |
|
Epoxy + Ni |
20–30 μm |
96h |
Humid env, power banks |
|
Parylene |
3–5 μm |
200h+ |
Medical, high-end |
Thermal: 15W vs 25W Qi2.2
|
Aspect |
15W Qi2 |
25W Qi2.2 |
|
Magnet temp exposure |
~40–50°C |
~55–70°C |
|
Recommended grade |
N48–N52 |
N50SH–N52SH |
The rule: at 25W, ordinary N-grade magnets can lose 3–5% of magnetic strength within months. SH-grade (Hcj ≥ 20 kOe) is the minimum recommended for Qi2.2.
Checklist: Specifying Qi2 Magnet Rings
- OD 56.0 mm ±0.05, ID 44.0 mm ±0.05
- Flatness < 0.05 mm, Grade ≥ N50 (N52SH for 25W)
- Ni-Cu-Ni coating ≥ 15 μm
- Segmented arc configuration (18+ segments preferred)
- Gauss measurement data per production batch
- WPC certification test reports available
- Supplier provides magnetic field simulation support
About Aim Magnet
We provide certified Qi2 magnet rings in N48 through N52SH grades, with ±0.02 mm precision, full batch traceability, and magnetic field simulation support. ISO 9001:2015 certified.
