At the heart of every wireless charger lies a deceptively simple component: a coil of copper wire. This coil, and its counterpart inside your phone, enable the seemingly magical transfer of energy through thin air. Understanding how these coils work illuminates why alignment matters so much for wireless charging efficiency, and why magnetic standards like Qi2 and MagSafe represent such a significant advancement.
This guide takes you inside the technology, explaining the physics in accessible terms while providing practical knowledge you can apply to get better charging performance from your devices.
The Science of Electromagnetic Induction
Wireless charging relies on electromagnetic induction, a phenomenon discovered by Michael Faraday in 1831. The principle is elegantly simple: a changing magnetic field can induce an electrical current in a nearby conductor.
How It Works
In a wireless charger, this process happens in two stages:
- Creating the magnetic field: When electricity flows through the transmitter coil in your charger, it creates an electromagnetic field around the coil. This field oscillates (switches direction) rapidly—typically 100-200 thousand times per second in Qi chargers.
- Inducing current: When the receiver coil in your phone enters this oscillating magnetic field, the changing field induces an alternating current in the receiver coil. This induced current is then converted to direct current and used to charge your battery.
The efficiency of this energy transfer depends critically on how well the two coils align. Perfect alignment means maximum energy transfer; poor alignment means significant energy is lost as heat rather than reaching your battery.
Engineers measure alignment efficiency using the "coupling factor" (k), ranging from 0 (no coupling) to 1 (perfect coupling). Well-aligned Qi2/MagSafe coils achieve coupling factors around 0.5-0.6, while misaligned standard Qi charging might drop to 0.3 or lower—meaning nearly half the energy is wasted.
Anatomy of Charging Coils
Understanding what's inside your charger and phone helps explain why different chargers perform differently.
Transmitter Coil (In the Charger)
The transmitter coil consists of several key elements:
- Copper wire winding: Multiple turns of insulated copper wire form the coil. More turns generally allow for stronger magnetic fields.
- Ferrite backing: A layer of ferrite material behind the coil helps direct the magnetic field upward (toward your phone) rather than letting it dissipate in all directions.
- Shielding: Metal shielding prevents the magnetic field from interfering with the charger's electronics and other nearby devices.
- Driver electronics: Sophisticated circuits control the frequency and power level of the current flowing through the coil.
Receiver Coil (In Your Phone)
Phone receiver coils are similar but optimised for space efficiency:
- Flat coil design: Phone coils are extremely thin (less than 1mm) to fit within the tight confines of modern smartphones.
- Receiver electronics: Circuits convert the AC current from the coil to DC for battery charging and communicate with the charger about power needs.
- Heat management: Materials around the coil help dissipate heat generated during charging.
- Position: In most phones, the coil is located in the upper-centre of the back panel.
Why Alignment Matters So Much
The relationship between coil alignment and charging efficiency isn't linear—small misalignments can have outsized effects on performance.
The Alignment Sweet Spot
Maximum energy transfer occurs when the centres of both coils are perfectly aligned and the coils are as close together as possible. In this ideal position:
- Most of the magnetic field from the transmitter passes through the receiver coil
- Minimal energy is lost to the surrounding environment
- Heat generation is minimised
- The charger can deliver its rated power output
Effects of Misalignment
As the coils move out of alignment:
- Reduced power transfer: Less of the magnetic field reaches the receiver, reducing charging speed
- Increased heat: Energy that doesn't reach the receiver dissipates as heat in both devices
- Charging may stop: Beyond a certain misalignment threshold, the charger may not detect the phone at all
- Battery stress: Inconsistent power delivery can stress the battery management system
Studies show that even 10-15mm of horizontal misalignment can reduce charging efficiency by 20-40%. This is why magnetic alignment in Qi2/MagSafe chargers represents such a practical improvement—it eliminates this variable entirely.
How Magnetic Alignment Solves the Problem
MagSafe and Qi2 chargers use a ring of magnets around the charging coil to solve alignment challenges permanently.
The Magnet Ring
Both your phone and the charger contain a ring of magnets positioned precisely around their respective coils. When you bring the phone near the charger:
- The magnets attract and guide the phone into position
- The ring design ensures the phone can only settle in one position—perfectly aligned
- The magnetic force holds the phone in place, preventing drift from vibrations
Consistent Results
With magnetic alignment:
- Every charging session achieves optimal efficiency
- No need to check positioning or adjust after placement
- Stands can hold phones vertically without slipping
- Accessories like wallets and battery packs align correctly
Coil Configurations in Different Chargers
Not all wireless chargers use the same coil configuration. Understanding these differences helps explain varying performance.
Single Coil Chargers
The simplest and most common design. A single transmitter coil must align with the phone's receiver coil. This design works well with magnetic alignment but requires careful positioning on non-magnetic chargers.
Multi-Coil Chargers
Some chargers include multiple overlapping coils to create a larger "sweet spot." The charger detects where the phone is placed and activates the appropriate coil(s). This design offers more forgiving placement on non-magnetic chargers but adds cost and complexity.
Free-Position Chargers
Advanced chargers use arrays of small coils that can power different areas of the charging surface. These allow you to place your phone anywhere on the pad. While convenient, these are typically more expensive and may not achieve the same peak efficiency as well-aligned single-coil systems.
Factors Affecting Coil Performance
Beyond alignment, several factors influence how well the coils transfer energy:
Distance Between Coils
The magnetic field strength decreases rapidly with distance (following an inverse square relationship). This is why:
- Thin phone cases work better than thick ones
- Wireless charging doesn't work well with very thick cases (>3-4mm)
- Magnetic chargers that pull the phone closer charge more efficiently
Coil Quality
Better chargers use higher-quality coils with:
- More turns of finer wire for stronger fields
- Better ferrite materials for improved field direction
- Superior shielding to prevent interference
- More sophisticated driver electronics for cleaner power delivery
Operating Frequency
Qi chargers operate at frequencies between 110-205 kHz. Higher frequencies can transfer more power but generate more heat. The charging system negotiates the optimal frequency based on the devices' capabilities and current conditions.
If you're using a non-magnetic charger and notice slower-than-expected charging or excessive heat, try repositioning your phone. Move it slightly in different directions until you find the position with fastest charging and least heat—that's where your coils are best aligned.
Practical Applications of Coil Knowledge
Understanding coil behaviour helps you optimise your wireless charging experience:
Choosing Cases
For best results, choose cases that:
- Are made of non-metallic materials (metal blocks magnetic fields)
- Are relatively thin (under 3mm is ideal)
- For MagSafe/Qi2 phones, have integrated magnets that align with the phone's magnet ring
- Don't have metal plates (like those for magnetic car mounts) over the charging coil area
Charger Placement
Position your charger where:
- It won't be disturbed (vibrations can shift alignment on non-magnetic chargers)
- Metal objects aren't nearby (they can interfere with the magnetic field)
- It has good ventilation (heat affects coil efficiency)
- It's convenient for your workflow (you're more likely to use it consistently)
Troubleshooting with Coil Knowledge
When charging isn't working properly:
- Check for objects between phone and charger (even credit cards can interfere)
- Verify alignment, especially on non-magnetic chargers
- Remove cases temporarily to test if case thickness is the issue
- Check for debris on charging surfaces that might create distance
- Ensure the charger's coil area isn't damaged or warped
The Future of Charging Coil Technology
Research continues to improve wireless charging coils. Emerging developments include:
- Higher efficiency coils: New materials and designs reduce energy loss
- Multi-directional charging: Coils that work regardless of device orientation
- Longer-range charging: Experimental systems can charge devices several centimetres from the pad
- Higher power delivery: Coils capable of supporting 50W+ wireless charging
Understanding the fundamental principles of coil operation and alignment gives you the foundation to make the most of current technology and appreciate future advances as they arrive. Whether you're troubleshooting slow charging or shopping for a new charger, this knowledge helps you make informed decisions.