Oral-B 9000 Replacement Battery 3.7V 800mAh Li-ion

Oral-B 9000 / Genius X / Smart 6000 — 3.7V Li-ion Replacement Battery (4235 010 18141)

This is a 3.7V, 800mAh Li-ion cell that replaces the internal battery in the Oral-B 9000, 7000, Genius X, and Smart 6000 electric toothbrushes. It fits the oscillating brush drive mechanism and restores full motor voltage when the original cell has degraded. Dimensions are 51.50 × 15.10 × 14.20mm — confirm these against your existing cell before fitting.

• Cross-series fitment: The 9000, 7000, Genius X, and Smart 6000 share the same internal cell form factor, connector footprint, and nominal voltage rail. That's why one part number covers all of them — the brush motor and BMS circuitry draw from the same 3.7V supply across all these handles.
• Bench tested on actual hardware: We cycled this cell through the inductive charging base and confirmed the BMS accepted charge without fault. Motor drive voltage held steady at load, and the protection circuit tripped correctly at low-voltage cutoff without needing a manual reset.
• Dock charging discipline: Remove the toothbrush from the charging base once full and only return it when the cell is below roughly 30%. Continuous trickle charging through an inductive base accelerates capacity fade on this Li-ion cell faster than partial-charge cycling does — more so than on most other small Li-ion devices because the base never fully disconnects.

Why the brush motor slows down before the low-battery indicator triggers

As this Li-ion cell ages, internal resistance rises. Under the brief high-current pulse of the oscillating motor, voltage sags at the cell terminals even when resting voltage looks acceptable. The toothbrush firmware reads resting voltage to decide when to show a low-battery warning — not terminal voltage under load. So the motor loses torque and speed noticeably before the indicator responds. A new cell with lower internal resistance eliminates this lag and restores consistent motor output from the first stroke to the last.

Toothbrush not charging on the base after a battery swap

Inductive charging on these handles is sensitive to coil alignment — the internal receive coil must sit within a few millimetres of the base transmitter coil. If the handle isn't fully seated or the replacement cell sits fractionally higher than the original, coupling efficiency drops and the unit may show no charge activity at all. Check that the brush head is removed during initial seating and press the handle firmly onto the base. If charging still doesn't start, verify the replacement cell's solder joints haven't lifted the PCB, then reseat and try again — most cases resolve at that step.