Hot Weather Performance: How Motors Thermally Derate Above 40°C

In Gulf summers and heatwave Europe alike, we get the same support ticket: "the chair got slow in the afternoon and recovered by evening." That is thermal derating doing its job. Motors, controllers and batteries all lose capacity as temperatures climb — here is what happens, with numbers.

Why motors derate

A motor's rated power assumes it can shed heat; that ability depends on the temperature difference between windings and ambient. Class B insulation (130 degrees C rating) with a 40 degrees C ambient allows a winding rise of about 80-90 degrees C. Raise ambient to 50 degrees C and the allowed rise shrinks — the motor must run at reduced current or cook its insulation. In practice, a wheelchair gear-motor rated 300W continuous at 25 degrees C delivers roughly 85% of that at 40 degrees C and 70-75% at 50 degrees C. Controllers enforce this with thermal sensors that cap current before damage — experienced as a speed ceiling that appears after 20-40 minutes of hard hill work in the sun.

The battery joins in

Lithium cells are happiest at 15-35 degrees C. Internal resistance roughly doubles from 25 to 60 degrees C, so voltage sag under load deepens — the controller sees "low battery" earlier and cuts peak current. Combined with motor derating, a hot-afternoon chair can feel 20-25% less powerful even with a full pack. It recovers fully when temperatures drop; nothing is broken.

What we specify for hot climates

• Motor temperature sensors wired to the controller (not just thermal fuses) so derating is graceful, not a shutdown.
• Controllers rated to 60 degrees C ambient with conformal-coated boards for humid heat.
• Sun-shade seat fabrics and lighter frame colors — a black seat frame in direct sun can run 15-20 degrees C above ambient.
• Battery placement away from motor housings; on our heavy-duty series the pack sits under-seat mid-frame, out of both motor radiance and direct sun.

Practical habits for hot regions

Charge in the coolest hours (the BMS will refuse charging if the pack exceeds ~45 degrees C anyway — another "broken charger" mystery solved). Park in shade; a chair that starts at 35 degrees C instead of 50 keeps full performance for the whole outing. Plan steep ramps for mornings. And if a dealer reports recurring afternoon power loss on a fleet, check that the motor thermal sensor is actually connected — a disconnected sensor makes controllers fall back to conservative default limits, permanently slow.

The verification question

For buyers in hot markets, one question cuts through: "At what ambient temperature is the rated output valid, and what percentage does the controller allow at 45 degrees C?" The honest answer includes two numbers and a curve. Spec sheets that only say "operating range -10 to 50 degrees C" tell you the chair survives the heat — not what it can do in it.