One number for three levels
A specification sheet that offers three heat levels and one runtime figure is telling you about the level the factory chose to measure, and nothing else. The figure usually comes from the lowest setting, run in a room at a comfortable temperature, on a fully charged pack, until the control board cuts out. Change any one of those four conditions and the number moves. Yet the sheet presents the result as a property of the product rather than a property of the test.
For a buyer, that matters at the point of comparison. A warmer advertised with a long single figure may simply have a lower bottom setting, or a looser cutoff rule, or a measurement taken without a hand wrapped around it. None of that is deception by itself; it is missing context, and missing context is what turns a purchase order into a dispute eight weeks later.
What a runtime figure has to be paired with
The first pairing is the heat level, named as the supplier names it, not as a rank. Two suppliers can both call something "medium" while one is driving a single element and the other is driving two faces at a reduced share.
The second is the starting charge. A runtime measured from a full pack is the best case; a runtime measured from a pack at the shipping state of charge is a different and often more honest figure for a user who opens the box and switches on immediately. The record should say which one it is, and whether the pack was rested after charging or tested straight off the charger.
The third is the ambient temperature, because the warmer is losing heat to the room the whole time. A cold room raises the duty cycle the control board applies, which pulls the pack down faster. The fourth is the load condition: free air, an insulating sleeve, or a hand. The fifth is the cutoff criterion, which deserves its own paragraph below because it is where most of the ambiguity hides.
Reading a per-level table
What each row should contain
| Row | What belongs in it | Why it changes the answer |
|---|---|---|
| Heat level | The supplier's own level name plus the switching state (one face or two) | Two faces at one setting divide the same budget differently |
| Starting charge | Full, rested, or shipping state | Sets the best case versus the realistic case |
| Ambient | The room temperature held during the run | Colder ambient raises duty cycle and shortens runtime |
| Load | Free air, sleeve, or simulated grip | A hand removes heat and changes the control loop |
| Cutoff rule | Low-voltage cut, thermal derate, or user-perceived decline | Each produces a different end point on the same pack |
| Sample identity | Golden sample or production batch, with the date | Separates a design record from a batch claim |
Ask for the table, not the headline. A supplier who can fill six rows has run the test and kept the record; a supplier who can only repeat one number has not, or has decided not to share it.
Test conditions that quietly move the result
Ambient is the largest single lever. A warmer in a cold room fights a bigger temperature difference, so the element spends more time at full duty, and the pack drains faster than the same unit in a mild room. That is physics, not a defect, but it means a runtime figure without an ambient value cannot be compared with a competitor's figure.
Load is the second lever. A unit running in free air holds its setpoint with less energy than one with a hand wrapped around it, because the hand keeps removing heat that the element then has to replace. A sleeve has the opposite effect and lengthens the run. Third is intermittency: a user who switches the warmer off between uses gets a different total than a continuous run, and neither figure is wrong if the method is stated.
The control board's own logic is the fourth. A firmware that derates early to protect the pack will report a shorter runtime than one that runs to the low-voltage cut, even on identical hardware. Ask whether the runtime record ends at a protective derate or at the low-voltage cutoff, because the two produce very different user experiences at the end of the run.
Pack behaviour at the end of a run
The last portion of any runtime is the least comfortable and the least documented. As the pack voltage falls, the control board either reduces the duty cycle fed to the heating element, which the user experiences as a gradual cooling, or maintains duty until the protection trips, which the user experiences as an abrupt stop. Both are legitimate, and the choice belongs in the firmware note.
Cell temperature belongs in the same discussion. On a dual-sided unit with two elements and a cell between them inside the battery compartment, a long run at a high level pushes the pack thermistor up, and a sensible BMS derates before the cell reaches its limit. Where the derating is instead a hardware cut, it is the OTP protector that ends the run, and the runtime record will show a step rather than a slope. A supplier reporting a long runtime should be able to say whether the run stayed inside the pack's thermal envelope.
Longest runtime ≠ usable warmth
The boundary to state plainly: the longest runtime is not a measure of useful heat, and a short runtime is not automatically a worse product. A unit that holds a modest, stable surface temperature throughout can be more useful than one that starts warm, derates early and spends its last third lukewarm.
The second boundary is that a runtime figure is not a battery capacity claim. Capacity lives in the pack specification, and converting a runtime into a capacity expectation requires the element power, the duty cycle and the losses, none of which a runtime figure contains. If a supplier uses runtime to imply capacity, ask for the pack datasheet instead.
Five questions for the supplier
- For each heat level, what runtime was recorded, and what was the switching state of the elements? Level name plus element state is the minimum for a comparable row.
- What was the starting charge, and was the pack rested after charging? A rested full pack and a freshly charged one can differ noticeably.
- What ambient temperature was held during the run, and was the unit in free air, in a sleeve or in a simulated grip? All three belong in the record.
- What ended the run: low-voltage cutoff, thermal derate, or the point where the surface stopped being useful? The end point defines the number.
- Was the run continuous or intermittent, and which sample and batch does it represent? Continuous and intermittent runs are different tests, and a golden sample is not a batch guarantee.
Next step
Request the six-row table for every level, plus the firmware note that explains the cutoff behaviour, and keep both with the sample approval file. When the numbers arrive in that shape, the comparison with a second supplier becomes mechanical rather than interpretive. Related reading: USB-C input and full charge time for the other end of the cycle, mAh vs Wh capacity confusion for the pack figures, and the golden sample signoff form.
Written for the Emberly heating-element programme. Specifications are illustrative; confirm every figure against a supplier test record.