A wearable is a heater strapped to a person. The battery is one source of that heat; the SoC, display driver, and charging circuit are others. The design goal isn’t to minimise temperature — it’s to keep the skin-facing surface below a threshold the user’s nerves won’t complain about.
The number you have to respect
IEC 60950-1 and IEC 62368-1 both define the skin-contact limit for continuous contact with a metal surface at around 41 °C in a room-temperature ambient. Plastic or glass enclosures get a few degrees of slack, but most product teams anchor to 41 °C because it’s the strictest number they might be tested against, and because users begin to describe devices as “warm” around 40 °C and “hot” around 42 °C.
Cosmetics aside, going above 43 °C for extended periods starts to cause low-grade thermal damage to skin — a real regulatory concern for medical wearables.
Where the heat actually comes from
| Source | Typical load | Dominant during |
|---|---|---|
| Battery internal resistance | 10–150 mW | High discharge (TX burst, motor) |
| SoC / compute | 0.5–3 W | Active use, wake events |
| Display driver | 0.3–1.5 W | Always-on display, high brightness |
| Charging (losses) | 0.2–1 W | Charging to 80–100% SOC |
| Radios (BT/Wi-Fi) | 50–300 mW avg | Streaming, sync |
On most AR glasses and smartwatches, charging is the thermal worst case, not use. The cell is hot from I²R losses in the CC phase, the charger IC is bleeding the CV phase as heat, and the device is sitting still on a cradle with no natural airflow.
Three heat paths you control
- Spread. Copper or graphite foil across the inside of the enclosure turns a point source into a large radiator. A 30 × 20 mm graphite pad on top of the cell reduces peak local temperature by 4–7 °C compared to bare plastic.
- Insulate on the skin side. A 0.15–0.3 mm aerogel or silicone pad between the cell and the skin-facing surface creates a small thermal gradient, pushing peak skin temperature down by 2–4 °C. Costs runtime indirectly (slightly worse cell cooling), so tune carefully.
- Throttle at the BMS. The cheapest way to cap temperature is to stop charging fast. Skin-contact temperature limiting is now a standard feature of modern fuel gauges — trigger a charge-rate reduction when the battery-surface NTC crosses 38 °C; full pause at 41 °C.
The charging profile that actually works
A three-stage profile survives most real-world environments:
- 0–80% SOC: charge at up to 0.7C, constant-current.
- 80–95% SOC: taper down to 0.3C, entering CV.
- 95–100% SOC: trickle only. Most users never need the last 5% in a hurry, and stopping at 95% more than doubles cycle life.
Overlay a thermal envelope on this: if the NTC ever reads above 40 °C, divide the current-at-that-stage by 2. If it hits 43 °C, pause entirely for 60 seconds. User-perceived charge time barely changes, but the fail-case skin temperature is controlled.
On-wrist vs off-wrist
A smartwatch charging on a cradle behaves differently from a watch charging while being worn (over-night on the wrist, for example). The wrist acts as a heatsink but also an insulator — net effect varies by about ±2 °C. If your device supports both, you must sense which mode you’re in (typically via the PPG sensor or a capacitive skin-contact electrode) and adjust the thermal envelope.
Testing: the 30-minute session
The most useful thermal test we run is a 30-minute continuous-worst-case session: maximum brightness, continuous streaming audio, GPS on, BT on, ambient at 30 °C. Measure at five surface points every 10 seconds. If any point crosses 41 °C during the session, the design is not done. Repeat at 40 °C ambient for the outdoor-use worst case.