Wireless charging: convenient, and the least efficient way to do it
The short answer
A coil in the pad induces current in a coil in the phone. A meaningful share of the energy becomes heat, in the phone, right beside the battery. It is fine for topping up on a desk and a poor choice for a warm overnight charge under a pillow.
How it works, and why it warms up
The pad runs alternating current through a flat coil, which produces a changing magnetic field. A second coil in the back of the phone sits in that field and has current induced in it. No contacts, no port, no wear.
The inefficiency is inherent. Energy that does not make it into the second coil becomes heat in both, and alignment makes a large difference: a phone sitting slightly off centre wastes considerably more.
That heat lands inside the phone, next to the battery, which is the one place you would rather not put it. Heat is what ages a lithium cell fastest, so wireless charging is convenient and it is not kind.
Qi, Qi2 and MagSafe
The magnets are the real improvement and not a gimmick. Most of the wasted energy in older wireless charging was misalignment, so holding the coils concentrically makes it meaningfully faster and cooler at the same time.
| Standard | What it adds | Typical speed |
|---|---|---|
| Qi | The original open standard, no alignment help | 5W to 15W, often slower in practice |
| MagSafe | Magnets that align the coils, on iPhone | Up to 25W on recent iPhones with the right charger |
| Qi2 | Magnetic alignment brought into the open standard | 15W and up, on a growing number of Android phones |
When it is the wrong choice
When it is the right choice: a desk, an office, a kitchen counter, where the phone goes down and comes up repeatedly through the day and never runs flat. That is what it is genuinely good at.
- Overnight in a warm room, and particularly on a bedside table under a pillow or a book
- In a car in summer, on a vent mount in direct sun
- While gaming or navigating, when the phone is already producing heat of its own
- Through a thick case, a wallet case or anything with a metal plate, which blocks it or makes it worse
- When you are in a hurry, because a cable is faster on essentially every phone
After a back glass repair
The charging coil sits directly under the back glass, so a repair there involves it and an impact that broke the glass can have damaged it.
Tell us if wireless charging stopped at the same time as the glass broke. It changes what we order and it is far cheaper to sort while the phone is already open than as a second job.
We test wireless charging on a pad after any back glass repair before the phone goes back, because it is exactly the sort of thing that gets discovered on the walk home.
Common questions.
Not directly. It is the least efficient method and the wasted energy becomes heat next to the cell, and heat is what ages a battery fastest. Fine on a desk, poor for a warm overnight charge.
Some of the energy never reaches the phone, and alignment makes a big difference. Magnetic standards like MagSafe and Qi2 hold the coils concentric, which is why they are both faster and cooler.
Through most thin cases, yes. Not through anything with a metal plate, and poorly through a thick or wallet case. If charging has become slow or intermittent, take the case off and try again.
The charging coil sits directly under the back glass and can be damaged in the same impact. Say so when you book the back glass repair, because it is much cheaper to sort while the phone is open.
Close. Qi2 brings the magnetic alignment idea into the open standard, so Android phones can have what MagSafe gave iPhones. Speeds and accessory fit differ slightly by device.
Where this leads.
Next in batteries and charging.
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