Putting the phone down on a pad and finding it charged is convenient, especially on a bedside table or a desk. Wireless charging, though, works in a specific way, has efficiency limits and a delicate relationship with heat. Knowing them lets you use it without shortening your battery's life.
The principle: electromagnetic induction
The charging pad contains a coil carrying alternating current, which generates a changing magnetic field. The phone contains a second coil: the field induces a current in it, which is rectified and used to charge the battery. It is the same principle as a transformer, only with the two halves separated by a few millimetres of plastic and glass.
The most widespread standard is Qi, defined by the Wireless Power Consortium. A Qi-certified phone and pad, even from different brands, can talk to each other: the phone states how much power it wants, the pad adjusts and stops delivering power if it detects foreign objects.
The alignment problem
Energy transfer only works well if the two coils are properly aligned. If the phone sits a couple of centimetres off centre:
- efficiency drops;
- the pad has to work harder and heats up;
- in some cases charging stops and restarts continuously.
Anyone who has found their phone flat in the morning because it slid sideways knows the problem. Some pads use several overlapping coils to widen the usable area, but the point stands: alignment matters.
Qi2: magnetic alignment
Qi2 adds a magnetic profile to the standard, based on the MagSafe system introduced by Apple, which made it available to the consortium. A ring of magnets around the coil automatically centres the phone on the charger. The practical benefits are direct:
- always-correct alignment, so fewer losses and less heat;
- the charger can double as a stand, upright or in the car;
- cross-brand compatibility for magnetic accessories too.
A phone without built-in magnets can use magnetic attachment through cases with a magnetic ring, but full Qi2 certification depends on the device.
Efficiency: how much energy is lost
Wireless charging is less efficient than charging by cable: part of the energy is dissipated as heat in the coils and conversion electronics. How much depends on alignment, distance, pad quality and temperature. For the user there are two consequences:
| Aspect | Cable | Wireless |
|---|---|---|
| speed at the same rated power | higher | lower |
| heat produced | moderate | higher |
| convenience | plug in the cable | put the phone down |
| use while charging | easy | awkward, unless on a stand |
The cost of the lost energy for a single phone is modest. The real price to watch is heat.
Heat and the battery
Heat is the main enemy of lithium-ion batteries. Wireless charging produces more of it, and produces it right against the phone. Modern systems reduce power as temperature rises, but a few habits help a lot.
- Thick cases or cases with metal parts: they increase the distance between the coils and trap heat. Wireless-compatible cases are thin or have aligned magnets.
- Objects between phone and pad: cards, keys or coins can get dangerously hot. Certified pads detect foreign objects, but it is better not to rely on it.
- Overnight charging: a moderate-power pad runs cooler and has all night anyway. Turning on optimised charging, which holds around 80% until shortly before you wake, is particularly useful.
- Pad placement: away from radiators, direct sunlight and soft surfaces.
Choosing a pad
A few practical criteria:
- Qi or Qi2 certification, which you can check on the box and in the consortium's database;
- a suitable power adapter: many pads only deliver their rated power with a USB-C adapter of a certain wattage, often sold separately;
- ventilation: some higher-power pads have a small fan, helpful for heat but worth considering for noise in a bedroom;
- shape: upright lets you use the phone while it charges, flat is more stable on a bedside table.
In short
- Qi wireless charging relies on induction between two coils, one in the pad and one in the phone.
- Coil alignment determines efficiency and heat; Qi2 solves it with magnets.
- Wireless is less efficient than cable and produces more heat right against the phone.
- Thin cases, nothing in between and optimised charging protect the battery.
- A certified pad with a suitable adapter actually delivers its rated power.
