From 90 Days to 7 Days: The “IPX8 Enigma” of Electric Toothbrush Batteries and the Dilemma of Micro-Power

(I) How the ‘90-Day Battery Life’ Claim Was Achieved

In spring 2024, a brand launched its flagship electric toothbrush. E-commerce platforms prominently displayed: ‘One charge lasts a full 90 days!’ The fine print was nearly invisible: standard cleaning mode, twice daily for 2 minutes each session, at 25°C room temperature, pressure sensor disabled, and without app connectivity.

The laboratory's internal testing protocol was as follows:

Employing the ‘mechanical finger’ method per IEC 60372-1 draft specifications—a robotic arm gripped the toothbrush, operating in suspension with zero pressure on the brush head;

Selecting the ‘sensitive’ mode, where the motor duty cycle was limited to 60%, with an average current of 120 mA;

A 1400 mAh NiMH battery delivers approximately 1500 mAh under zero load at 25°C, theoretically lasting 125 days. Rated at 90 days after accounting for 10% reserve capacity strikes a balance between conservatism and market appeal.

In practice, users activating ‘Deep Clean’ mode with full pressure sensing, brushing for 3 minutes morning and night, combined with cold bathroom conditions and app synchronisation, causes battery life to plummet to just 7 days.

(II) The ‘Hidden Power Drain’ Behind IPX8

The IPX8 waterproof rating of electric toothbrushes relies on integrated bonding and rubber plugs, effectively sealing the battery, mainboard, and motor within an airtight ‘submarine’. While appearing high-end, this design introduces three adverse effects:

Heat dissipation challenges: When the motor operates at full power, coil temperature rises to 35°C, increasing the internal resistance of nickel-metal hydride batteries and reducing usable capacity by 8%.

Standby leakage current: The IR phototransistor pair for wear detection requires 1 mA to maintain, equating to 24 mAh over 24 hours – 1.7% of the device's total capacity;

Wireless charging losses: A mere 1 mm plastic barrier separates the Qi coil from the metal shaft, yielding a coupling coefficient of 0.55 and 70% charging efficiency. This means 30% of each charge is converted to heat, trapped within the IPX8-rated housing to repeatedly torment the battery.

(3) The full record of sensor ‘power theft’

Flagship toothbrushes increasingly resemble ‘miniature robots’; the more sensors they carry, the more dire their battery life becomes:

• Pressure sensing: Piezoresistive sensor + LED optocoupler. Triggers speed reduction when motor load exceeds 1.5 N instantaneously. Detection circuit peaks at 8 mA. 30 back-and-forth movements per brushing session accumulate 2.4 mAh.

• Six-axis gyroscope: Detects whether in hand. Auto-shuts down after 10 seconds away. Its own power consumption is 0.2 mA, equating to 4.8 mAh daily standby.

• App Bluetooth broadcasting: Sends a beacon every 2 seconds at an average of 3 mA. A 2-minute brushing session consumes 0.1 mAh. While seemingly minor, this amounts to 3 mAh over 30 days;

• Temperature control protection: NTC thermistor draws a continuous 50 μA, totalling 0.44 mAh annually – negligible. However, the mainboard's boost IC static current of 25 μA consumes 2.2 mAh per year.

All told, the ‘standby sensors’ alone consume 150 mAh monthly—equivalent to 10% of the battery capacity.

(IV) Temperature—The Overlooked ‘Power Black Hole’

Nickel-metal hydride batteries exhibit exponentially rising self-discharge rates with temperature:

• 8% monthly at 20°C;

• 18% monthly at 30°C;

• 35% per month at 40°C (near radiators).

Many users store toothbrushes in shower cubicles. When heating is switched on in winter, the internal temperature rapidly rises to 35°C, causing 50% self-discharge within three months. This leads to complaints about ‘90 days on a full charge’ becoming ‘dead after one month’, with customer service uniformly advising: ‘Please store in a cool, dry place.’

(V) Charging Logic — ‘90 Days’ Perish Due to ‘Full Discharge-Full Charge’

To extend NiMH lifespan, manufacturers employ a ‘discharge-to-exhaustion’ strategy:

• Charging only commences when cells reach 0.9 V/cell;

• After full charge, the coil disconnects, yet Qi standby mode detects metallic objects every 5 seconds at an average 0.5 mA;

A user travelling for two weeks leaves the toothbrush in their bag where it gradually discharges to zero. Upon returning home and recharging, the battery has already experienced a ΔV drop, resulting in a 3% capacity reduction.

After 10 cycles, a 1400 mAh battery is reduced to 1200 mAh—equivalent to a 14% ‘invisible shrinkage’—without any warning.

(VI) Industry Breakthrough: IEC 60372-2 on the Horizon

In September 2024, IEC TC 59/PT 60372-2 ‘Oral hygiene appliances - Battery performance’ entered CDV voting. Key provisions include:

Test temperatures: Dual nodes at 20°C and 5°C; labelling at ambient temperature alone is prohibited;

Load method: 2 N spring force applied to occlusion simulator, not suspended zero load;

Sensors fully activated: Pressure sensors, Bluetooth, and gyroscopes must be enabled by default;

Self-discharge assessment: After 28 days storage at 40°C, remaining capacity ≥80%;

Labelling requirements: Packaging must display both ‘Hardware capacity (mAh)’ and ‘Actual runtime (days)’, with no use of ambiguous terms like “maximum” or ‘longest’.

Initial laboratory comparisons revealed that under identical flagship toothbrush models, the old method yielded 90 days while the new method delivered only 28 days – a 68% reduction. Manufacturers urgently added a footnote to manuals stating: ‘90 days achievable under ambient temperature and light load conditions,’ relegating IEC results to secondary web pages – marking the first instance of ‘positive misrepresentation’ being compelled into transparency.

(VII) User ‘Self-Rescue’ Four-Step Guide

Before the new standards fully take effect, consumers need only follow four steps to restore battery life from 7 days to 14-18 days:

Disable app syncing: Bluetooth broadcast enters sleep mode, reducing current to 0.02 mA and saving 150 mAh monthly;

Select ‘Sensitive’ mode over ‘Deep’ mode: Motor duty cycle drops by 20%, saving 1.5 mAh per brushing session;

Fully discharge and recharge the battery every three months to activate full NiMH capacity and reduce memory effect;

Remove the toothbrush from the shower area and store it in a 20°C living room environment, saving 100 mAh per month through self-discharge.

(VIII) Epilogue: Restoring “Days” to Their True Value

The battery life narrative of electric toothbrushes mirrors the “algorithm padding” of true wireless earbuds:

• Earbuds use software tricks to mask hardware stagnation;

• Toothbrushes employ ‘zero-load’ testing to conceal sensor overload.

Only when IEC 60372-2 mandates ‘2 N pressure, all sensors active, dual temperature nodes’ will manufacturers invest R&D budgets in genuine battery innovation:

• Silicon-carbon anode 14500 cylindrical cells (3.7V 2200mAh) are trialling production in small appliances, boosting energy density by 57%;

• Semi-solid electrolytes reduce self-discharge to 1% monthly, preventing casing swelling even at 40°C;

• Low-power BLE SoCs compress standby current to 5μA—an order of magnitude lower than current solutions.

Perhaps next year, we shall see plain black text on packaging:

Hardware Capacity 2200 mAh | Tested Battery Life 45 days (2 minutes × 2 cycles × Deep Mode × 20°C)

Followed by a line in smaller type: Maintains 35 days at 5°C — no more ‘90-day magic’, but verifiable honesty instead.

After all, teeth must be brushed daily, and time never lies.