About this replacement
Oral-B Triumph 4000 / 9000 Series — 1.2V Ni-MH Replacement Battery (3731 / 3738)
This is a 1.2V 2500mAh Ni-MH cell for Oral-B Triumph toothbrushes. It fits the Triumph 4000, 5000, 9000, and 9400 series, along with several other compatible models. Swap it in when the original cell no longer holds enough charge to complete a full brushing session.
- Triumph 4000–9400 platform compatibility: These models share the same internal cell format, voltage rail, and motor driver circuit. The 3731 and 3738 part numbers both reference this same 1.2V Ni-MH specification, so either number confirms a correct fit across the range.
- Bench tested on actual hardware: We cycled this cell through the inductive charging base on a Triumph 9000 unit. The cell accepted a full charge without interruption, and motor speed held steady across the brush stroke cycle from first charge through the third conditioning cycle.
- Dock charging discipline on Ni-MH cells: Remove the toothbrush from the charging base once fully charged and only return it when the charge is noticeably low. Continuous trickle charging from an inductive base accelerates Ni-MH capacity fade faster than partial-charge cycling does. This matters more for Ni-MH chemistry than it would for lithium cells.
Battery fade from continuous dock charging on Oral-B Triumph models
Oral-B Triumph bases deliver a continuous low-level inductive charge when the toothbrush sits on the dock. Ni-MH cells degrade faster under constant trickle current than they do under regular charge-discharge cycles. Over weeks, this compresses the usable capacity — the toothbrush appears charged but the motor slows down well before the brush head indicator signals low. Store the toothbrush off the base between uses once the cell is full to slow this effect.
Toothbrush not charging on the inductive base after battery replacement
The Triumph charging base uses inductive coupling, so the coil alignment inside the handle must line up precisely with the base transmitter coil. If the brush head is not fully seated or the handle is slightly off-centre on the base, the transfer is too weak to charge the cell. After a battery swap, internal components can shift slightly during reassembly — re-seat the brush head firmly and make sure the handle sits flush and centred on the charging ring. If the charge indicator still does not light, check the cell polarity is correct before looking at the base itself.