Key points
- A runtime claim needs three definitions fixed in writing: minutes per day, load condition, and what counts as empty.
- A 3.7 V / 500 mAh cell holds 1.85 Wh nominal and roughly 1.3 Wh usable; free-run draw roughly doubles the apparent runtime against a loaded one.
- The same product can honestly be claimed at ~56 days free-run or ~21 days loaded. Only one of those describes brushing.
- Ageing and standby drain are why a claim that holds on unit one fails at month 18.
“Up to 30 days on a single charge” is one of the most common specifications in this category and one of the least reproducible. This note works through the arithmetic, then sets out how to write a claim that survives a marketplace test or a buyer's own lab.
The claim is only meaningful with three definitions
- Runtime definition. Minutes of brushing per day. The category convention is two minutes, twice a day — four minutes. A claim built on one minute a day is arithmetically true and commercially useless.
- Load definition. A motor at free run draws a fraction of what it draws with the bristles pressed against a wet tooth surface. Free-run figures are the single biggest source of inflated claims.
- End-of-life definition. Cut-off voltage, or the point at which the user perceives the brush as weak. These are different points, often several days apart.
The arithmetic, worked
Take a typical handle with one 3.7 V, 500 mAh cylindrical cell. Nominal energy is 3.7 × 0.5 = 1.85 Wh. You cannot use all of it: the firmware cuts off above the cell's true empty point to protect cycle life, and there are conversion losses in the driver, so usable energy is typically around 70 % — roughly 1.3 Wh.
| Metric | Free-run basis | Loaded basis |
|---|---|---|
| Motor draw | 0.35 W | 0.9 W |
| Daily energy at 4 min/day | 0.023 Wh | 0.060 Wh |
| Days from 1.3 Wh usable | ~56 | ~21 |
| Typical published claim | “up to 60 days” | — |
| Same product, honest claim | — | “up to 21 days” |
The gap between the two columns is the whole story. Neither number is a lie in isolation; only one of them describes brushing.
What else moves the number
- Mode and intensity. A high-intensity mode can draw twice the current of a gentle one. If the claim is made in gentle mode, say so on the pack.
- Standby and self-discharge. An indicator LED or a radio that never sleeps can drain a cell in weeks on its own. Most generous “30-day” claims quietly assume the handle is off, not on standby.
- Temperature. Capacity falls in the cold. A brush specced in Guangdong and claimed in Helsinki will underperform the headline number in winter.
- Ageing. A cell at 300 cycles has lost a meaningful share of its original capacity, so a claim measured on a fresh cell will not hold at month 18 — which is exactly when the two-star reviews arrive.
How to specify a claim that holds
- Fix the three definitions above in the specification, in writing, before tooling.
- Test on a full run of finished units, not on a bench cell — at least five units, in the temperature band of the destination market, in the mode you intend to claim.
- Test again after an ageing cycle. A claim that only holds on unit one is a warranty problem.
- Publish the basis. “Up to 21 days — two minutes twice daily, standard mode, 25 °C, new cell” is more persuasive to a professional buyer than an unqualified larger number, because it is checkable.
- If the retail pack needs a headline, use a range plus the basis: “Up to 3 weeks (2 × 2 min/day)”.
What we do by default
We quote battery life on a loaded basis with the three definitions attached, and we keep the ageing and duration records for the batch so the number can be re-verified later. If your marketing team wants a larger figure, the route is a bigger cell — and that changes the transport watt-hour classification, which is worth checking before the artwork is signed off.
Written from the factory side for professional buyers. Figures are indicative and dated; regulatory summaries are not legal advice.
