Braun Oral-B Sonic 3721 Replacement Battery 2.4V 1100mAh

Braun Oral-B Sonic 3721 / 4717 / DLX S18.535.3 — 2.4V Ni-MH Replacement Battery (RS-MH 3941)

This is a 2.4V, 1100mAh Ni-MH replacement battery for the Braun Oral-B Sonic 3721, 4717, and DLX S18.535.3 electric toothbrushes. It replaces OEM part numbers RS-MH 3941, 2N-600AE, S-RWT1688, Cd 9S-RWT05, and 1103425149. The cell slots into the toothbrush handle and restores the sonic vibration motor to full operating speed.

• Sonic 3721, 4717, and DLX S18.535.3 compatibility: These three models share the same internal handle diameter, motor voltage rail, and cell mounting points. All three accept the same 56.80 × 17.10 × 17.10mm Ni-MH cell with no connector modification required.
• Bench tested on actual hardware: We cycled this cell through the inductive charging base and confirmed the charging circuit accepted the cell without error. Motor draw at startup and at sustained brushing load stayed within the Ni-MH cell's rated discharge range throughout testing.
• Charging dock management for Ni-MH cells: Remove the toothbrush from the charging base once it reaches full charge and only return it when noticeably below full. Continuous trickle charging from an inductive dock is the primary cause of early capacity fade in Ni-MH toothbrush cells — it is not a storage issue, it is a daily usage habit.

Battery fade from continuous dock charging on Oral-B Sonic handles

Ni-MH cells degrade faster under sustained trickle charge than under normal discharge cycles. The Oral-B Sonic charging base applies a low-level inductive charge whenever the handle is seated — it does not cut off cleanly at full capacity. Over weeks of continuous docking, this trickle current raises the cell's internal temperature slightly and accelerates electrolyte breakdown inside the Ni-MH cell. The result is a battery that tests at 2.4V open circuit but sags below 2.0V under brush motor load far sooner than a properly cycled cell would.

Brush motor running slower than expected immediately after installation

A new Ni-MH cell often delivers noticeably less vibration speed on the first one or two cycles — this is not a faulty cell. Ni-MH chemistry requires two to three full charge-discharge cycles before the cell reaches its rated 1100mAh capacity, as the nickel electrode plates need conditioning to achieve full ion exchange. Running the toothbrush until it slows noticeably, then charging fully on the base, is the correct conditioning step. By the third cycle, motor speed should be consistent and match the original battery's output at full charge.