Why does a 15W wireless charger
often deliver only 8–11W to the battery?

Understanding where wireless charging efficiency is gained—or lost—and how engineers optimize every stage from adapter to battery.

ⓘ

Wireless charging efficiency is the ratio of energy stored in the battery to the energy drawn from the charger input. It is typically measured end-to-end, from the USB-C input connector to the battery terminal.

≈80%
System Efficiency

40%
Heat = Largest Loss

Qi2
Better Alignment

>80%
Engineering Target

Figure 1 — Energy flows from adapter through TX coil, air gap, RX coil, and charging IC to the battery. Overall system efficiency is typically ≈80%.

Engineering Insight: The air gap between TX and RX coils is typically the single largest variable. In many commercial designs, magnetic coupling coefficient k drops with the cube of distance — doubling the gap can reduce efficiency by 8–12%.

Figure 2 — Four primary loss categories. Heat generation accounts for ~40% of total loss, followed by alignment and spacing at ~25%.

Engineering Insight: Heat and alignment together may account for ~65% of total loss. Addressing these two factors — through coil design and magnetic alignment — often delivers the highest return on engineering effort.

Engineering Test Data

18W
Input Power

14.5W
Battery Power

80.6%
Efficiency

41.8°C
Surface Temperature

Reachinno RK-TX01 transmitter + RK-RX03 receiver, Qi EPP profile, 25°C ambient, 30-minute test duration.

Figure 3 — Start with your measured efficiency. If below 75%, check alignment and gap. Above 80% is typically considered good for commercial designs.

Engineering Insight: In many designs, a 3mm lateral offset can reduce efficiency from ~82% to ~75%. Adding magnetic alignment (Qi2-style) often recovers 5–8% of system efficiency with minimal BOM cost.

Figure 4 — Five thermal zones from cool (15–25°C) to critical (75°C+). Efficiency typically drops rapidly above the 45°C threshold.

Engineering Insight: Three strategies dominate effective thermal management: spread heat with PCB copper pours, sink heat through thermal pads to the chassis, and manage heat via temperature-aware firmware power folding — gradually reducing TX power rather than abruptly shutting down.

Figure 5 — Eight prioritized optimization steps. Items 1–4 typically achieve ~78% efficiency; all eight steps target premium-grade performance above 80%.

Engineering Insight: Coil design and magnetic alignment are typically the highest-impact optimizations. Litz wire construction and Qi2-compatible magnet rings often deliver measurable efficiency gains in the first design iteration.

From 65% to 82% — That’s the Reachinno Difference

Our engineering team has tested hundreds of coil configurations, thermal layouts, and firmware profiles. Whether you’re designing a Qi2 charger, a multi-device pad, or an automotive wireless dock, the Efficiency Chain™ framework gives you a systematic approach to maximizing performance.

Talk to Our Engineering Team →

Wireless Charging Efficiency Chain™ — White Paper

Complete test data, coil design guidelines, thermal simulation results. 12 pages.

Download PDF →

Talk to Engineering Team →