How it works
Wireless charging for phones, earbuds and watches relies on electromagnetic induction. A charging pad contains a coil of wire. When alternating current flows through that coil, it produces a changing magnetic field. A matching coil inside the device picks up that changing field, which induces an electric current in it. Circuitry in the device converts this current into the form the battery needs.
The pad and the device have to be close together and reasonably well aligned, because the magnetic field weakens quickly with distance. Nothing is actually “wireless” about the power source: the pad still plugs into the wall or a USB adapter. Only the last few millimetres are cable-free.
The two sides also talk to each other. The device tells the pad how much power it needs, and the pad adjusts, which is how the pad can stop when the battery is full or when something unsuitable is placed on it.
Standards and compatibility
The most common standard for phones and small devices is Qi (pronounced “chee”), maintained by the Wireless Power Consortium. A Qi-certified pad should work with a Qi-compatible device, within the limits of the power levels each supports. A newer version of the standard, Qi2, adds magnets to help align the phone with the pad, and it is supported by some recent devices.
Compatibility can still trip people up:
- Not every phone, watch or earbud case supports wireless charging.
- Some smartwatches use their own charging puck that is not interchangeable with Qi pads.
- A pad may support a lower power level than the device is capable of, so charging works but more slowly.
- Thick or metallic cases, card holders or magnetic rings can interfere. Metal between the coils can block the field or heat up.
Checking the specifications for both the pad and the device is the surest way to know what to expect.
Speed
Wireless charging is often slower than wired charging, although the gap varies a lot between products, and some high-power pads can charge quickly. Wired fast-charging standards typically allow more power through the cable than many wireless pads deliver. If you need a quick top-up before leaving the house, a cable is usually the better choice.
For overnight charging or topping up at a desk, speed matters less, and convenience matters more.
Efficiency and heat
Induction is less efficient than a direct cable connection. Some of the energy is lost as heat in the coils, the circuitry, and the metal parts of the phone, and some of the magnetic field never reaches the receiving coil. Efficiency figures differ between products and conditions, so specific numbers should be treated cautiously, but it is widely accepted that more energy is drawn from the wall for each unit that reaches the battery.
The practical effects are:
- Warmth. The phone and pad can both get warm during charging. This is normal within limits, but heat is not good for batteries over time. Many phones slow charging when they become hot.
- Misalignment makes it worse. If the coils are poorly lined up, more energy is wasted, and temperatures may rise. A slightly shifted phone can charge slower or stop charging.
- Energy use. Over a year, the extra electricity for one household is small, but it adds up when multiplied across many users.
Heat and charging habits both influence the long-term capacity loss that causes battery health to decline. A pad is not necessarily harmful to the battery, but one that runs the phone hot, especially under a thick case or in a warm car, may add to wear.
Advantages
- Less plugging and unplugging. This reduces wear on the charging port, which can fail after many insertions. Dust and lint in the port are also avoided.
- Convenience. Placing the device on a pad is easy, especially for one-handed use or bedside tables.
- Shared use. One pad can serve different compatible devices, and some stands charge several at once.
- Sealed devices. Wireless charging makes it simpler to build water-resistant devices without an open port.
- Built into furniture and cars. Some desks and car consoles have built-in pads, though these tend to be slower and run hot.
Disadvantages
- Slower in many cases, and less efficient.
- Alignment needed. If the device moves, charging can stop, which is a nuisance at night.
- Heat. Warm devices charge more slowly and may age batteries a little faster.
- Not ideal while using the phone. You can use a phone on a stand-style pad, but lying flat makes it awkward to use, and a cable still offers the freedom to hold it.
- Cost. Good, certified pads cost more than a plain cable and adapter.
- Quality varies. Poorly made, uncertified pads may be inefficient or lack safety features such as protection against overheating or against metal objects on the pad.
Safety tips
- Use certified products from reputable sellers, along with a suitable power adapter.
- Remove any metal objects, bank cards or key fobs between the phone and pad. Cards with magnetic stripes or chips are generally recommended to be kept away.
- Keep the pad ventilated, rather than under a pillow or in a bag.
- People with implanted medical devices such as pacemakers should ask their doctor about magnets and charging fields. Manufacturers often advise keeping some distance, and this is a question for a medical professional.
When a pad makes sense
A pad suits a desk or bedside table, where the device sits still for hours and speed is not a concern. A cable is better for rapid charging, for travel, for charging while holding the device, and when the device’s wireless support is limited.
Many people simply use both: a pad for routine charging at home and a cable when in a hurry.