Why does wireless charging run hotter than wired?

Power passes through more stages. Wired charging loses energy only in the charger IC and battery, but wireless adds receiver-coil and rectifier losses inside the phone. Here, putting 10 W into the battery generates 1.27 W of heat in the phone when wired and 2.09 W wirelessly. Transmitter-side losses (1.98 W) heat the pad.

What happens if the phone is misaligned?

Coil coupling weakens, so more coil current is needed to deliver the same power, raising losses. Here, when transmitter-to-receiver efficiency falls from 0.90 to 0.75, wall power rises from 13.67 to 16.40 W, phone heat from 2.09 to 3.39 W and temperature rise from 19.9 to 32.3 K.

Why does charging slow down partway through?

Control lowers charging power to keep the battery and phone from getting too hot. Here, holding the rise to 15 K requires cutting battery power from 10 to 7.9 W. The phone's thermal time constant is about 29 minutes, so temperature climbs over the first hour or so and charging slows partway.

Electromagnetic-Thermal Analysis of Wireless Charging

Category: Coupled Analysis – Electromagnetic-Thermal | Revised 2026-10-01

Theory: where losses go and the phone's heat balance

Overview

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What should I pin down first when thinking about wireless-charging heat?

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Where losses arise and which side they accumulate on. Between wall and battery there are several stages: transmitter inverter, transmitter coil and inter-coil coupling, receiver coil and rectifier, and charger IC. Each stage's loss becomes heat on the spot, but heat in the pad and heat in the phone mean different things. Heat in the phone warms the battery, which ages faster when hot, so rising phone temperature forces charging power down. Hence the coupling: electromagnetic analysis gives coil and component losses, which feed a thermal analysis of phone and battery temperature.

Efficiency chain and phone heat

$$ P_{wall} = \frac{P_{bat}}{\eta_{chg}\,\eta_{rx}\,\eta_{cp}\,\eta_{inv}},\qquad Q_{phone} = P_{bat}\!\left(\frac{1}{\eta_{chg}\eta_{rx}} - 1\right) + I_{bat}^2 R_{bat} $$

Here battery power P_bat = 10 W (3.85 V, 2.60 A), charger IC efficiency 0.92, receiver coil and rectifier 0.93, coupling and transmitter coil 0.90, inverter 0.95 and battery internal resistance 0.06 Ω (all illustrative).

Phone temperature

$$ C\,\frac{d\Delta T}{dt} = Q_{phone} - hA\,\Delta T,\qquad \Delta T_\infty = \frac{Q_{phone}}{hA},\qquad \tau = \frac{C}{hA} $$

A is top-face area (150×70 mm), h combined convection and radiation coefficient (10 W/m²K) and C phone heat capacity (200 g × 900 J/kgK). Heat loss through the back into the pad is neglected because the pad warms too.

Coffee Break Trivia

A common standard for drop-and-charge

Sending power by electromagnetic induction dates to the 19th century, but drop-and-charge smartphones arrived after an industry group released the common Qi standard in 2010. Being able to use the same pad across brands drove adoption. Deliverable power was raised step by step, and at the same time limiting heat in the phone became a major design challenge.

Worked examples

Example 1: loss breakdown for 10 W into the battery

Item	Aligned	Misaligned	Wired
Wall power	13.67 W	16.40 W	—
Overall efficiency	73%	61%	—
Charger IC loss	0.87 W	0.87 W	0.87 W
Receiver coil and rectifier loss	0.82 W	0.82 W + 1.30 W	—
Heat inside battery	0.40 W	0.40 W	0.40 W
Phone heat	2.09 W	3.39 W	1.27 W
Pad heat	1.98 W	3.42 W	—
Phone temperature rise	19.9 K	32.3 K	12.1 K

For misalignment, transmitter-to-receiver efficiency drops from 0.90 to 0.75, with half the extra loss assumed to arise in the phone's receiver coil.

Example 2: warm-up speed and allowable power

Item	Value
Time constant	1,714 s (about 29 min)
Time to 90% of final rise	about 66 min
Battery power for a 15 K rise	7.9 W (phone heat 1.57 W)

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The breakdown shows the charger IC losing more than the receiver coil. So analysing the coil alone isn't enough?

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Exactly — heat is set by the overall heat balance. Coil field analysis is essential for losses under misalignment or with foreign objects, but phone temperature depends on heat from coil, rectifier, charger IC and battery together, and how it spreads through the phone and escapes. Clustered heat sources also make local temperatures exceed the 20 K average. Batteries are especially temperature-sensitive: long periods hot and near full charge accelerate ageing. Real designs therefore add heat-spreading sheets between receiver coil and battery or lower power in the later part of charging. Analysis passes the field-derived loss distribution to a thermal model and steps through time with component temperatures and charging control together.

Modelling workflow

Solve fields for coils, nearby metal and ferrite sheets to get losses versus position.
Express rectifier, charger IC and battery losses as functions of power and temperature.
Solve component temperatures over time with thermal models of phone and pad.
Include control that lowers charging power with temperature.
Compare charge time and peak temperature across layout and cooling options.

Coffee Break Trivia

“Charging that slowed with a case on”

Users of one model reported that charging with a thick case took much longer. Analysis showed the case increased coil separation, weakening coupling and raising losses, while also insulating the phone so heat escaped less easily. Together these made temperature hit its limit sooner, cutting charging power. Redesigning the receiver coil to improve coupling at larger separation and enlarging the heat-spreading sheet reduced the extra charging time with a case on.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Coil loss only	Phone temperature underestimated	Include every stage
Aligned case only	Worst-case heat missed	Sweep offset and gap
Steady state only	Mid-charge throttling missed	Solve in time
Over-crediting back-face cooling	Temperature underestimated	Solve pad temperature too
Ignoring cases and placement	Slow charging in real use	Cover realistic conditions

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