Philips BT9290 Replacement Battery 2.4V 700mAh

Philips BT9290 / HC3410 Series — 2.4V Ni-MH Replacement Battery (4238 118 20585)

This is a 2.4V, 700mAh Ni-MH replacement battery for the Philips BT9290 cordless shaver and compatible grooming devices in the HC3410, HC3426, and HC5446 series. It replaces OEM part number 4238 118 20585. Use this when the original cell no longer holds enough charge to complete a full shave cycle.

• BT9290 and HC-series compatibility: These models share the same 2.4V two-cell Ni-MH configuration, connector pitch, and physical form factor — 44.50 x 21.00 x 10.70mm. The shaver's charge management circuit expects that specific voltage window, so swapping in any other chemistry will trigger a protection fault.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge runs on the BT9290 platform. The charge circuit accepted the battery without fault codes, and the cell held voltage within the expected Ni-MH discharge curve under motor load.
• Waterproof model port care: Dry the charging port completely before connecting to the charger after rinsing. Moisture bridging the charging contacts causes a protection trip that the shaver reports as a dead battery — not a charging error. A 30-minute air dry resolves it in most cases.

Capacity fade from daily partial charging on the BT9290

Ni-MH cells develop a voltage depression effect when repeatedly charged before fully depleting — this is the memory effect specific to this chemistry. On a shaver used daily and docked after every use, the cell learns a shorter voltage window and the indicator drifts early. Running the battery fully down to motor cutoff once every three to four weeks resets the effective capacity the charge circuit sees. This is not a fault with the replacement cell — it is a characteristic of Ni-MH under shallow cycling.

Shaver motor slows down before the low-battery indicator activates

This happens because the shaver's indicator circuit reads a threshold voltage, but the motor draws enough current to sag the cell voltage below the motor's effective operating point before that threshold triggers. The indicator is measuring resting voltage, not load voltage. Under the motor's pull, an aged or partially degraded cell drops faster than the display reflects. If this replacement cell shows the same symptom after a full break-in cycle, check that the dock contacts are clean — oxidised contacts increase resistance and amplify voltage sag at the cell terminals. Clean contacts with a dry cloth and retest at 2.4V nominal.