Why your charger gets hot.

Where the heat actually comes from

A charger rectifies 120V AC from the wall, chops it at high frequency through a transformer, and rectifies it again into the 5V, 9V, 15V or 20V rail the device asked for. Each stage loses energy as resistive and switching loss. Modern silicon designs land around 87–90% efficiency; GaN designs reach 93–95%. The difference between input and output is heat, with nowhere to go but the case.

Load: a 100W charger feeding a phone at 20W dissipates under 2W and feels cool. Airflow: the same brick behind a couch, under a pillow or buried in a bag cannot convect, so surface temperature climbs until it throttles.

What "normal" is

There is no single number to look up — safety certification (IEC/UL 62368-1, the standard behind the UL and ETL marks) sets accessible-surface limits that vary by material and expected contact time, which is why a metal-shelled brick is allowed to run cooler than a plastic one. The practical test is the five-second rule: if you can hold the charger in a closed hand for five seconds without wanting to let go, it is inside its design envelope.

Also normal: the charger being hotter than the phone, the brick being warmest on the face nearest the wall, and a multi-port unit running warmer with three devices attached than with one — more total watts converted means more total heat from the same shell.

Thermal throttling, on both ends

Chargers above roughly 45W carry an NTC thermistor next to the transformer. When it reads too high, the controller steps output down — a 65W unit may hold 45W instead. You never see a warning, just a slower charge — one of the causes covered in fixing slow charging.

The device throttles too, and more aggressively. Apple specifies a 0–35°C operating range for iPhone and iPad; above it, iOS pauses charging and can display a "charging on hold" message until the phone cools. Android phones do the same thing silently. A phone charging in direct sun, in a thick case, or while running navigation and GPS will slow down no matter what brick is attached; the fix is removing the case or moving the phone, not more watts.

Four signs it is time to replace it

1. Any smell. Hot electronics, burnt plastic or fish-like odours from flame-retardant compound mean thermal damage has already happened. Unplug and bin it. 2. Deformation or discolouration — a bulged seam, a glossy melted patch, browning around the vents. 3. Buzzing or crackling under load, usually a failing capacitor or arcing. 4. Hot with nothing plugged in. A charger with no device attached should draw a fraction of a watt and stay at room temperature; heat at idle means the primary side is failing.

Counterfeits run far hotter than they should, because the transformer, isolation and heat-spreading are exactly where the cost was removed. If the brick was cheap and unbranded, read how counterfeit listings work and what the UL and ETL marks certify. A charger that is warm overnight on the nightstand is fine — see overnight charging.

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