Welch-Allyn 11720 Otoscope Replacement Battery 3.6V 750mAh

Welch-Allyn 11720 / 11730 Series — 3.6V Ni-Cd Replacement Battery (78904586)

This is a 3.6V, 750mAh Ni-Cd rechargeable battery for Welch-Allyn diagnostic instruments including the 11720 and 11730 otoscope/ophthalmoscope handles. It replaces OEM part 78904586. The cell fits the handheld diagnostic handle directly and restores the device to full operating condition.

• 11720 / 11730 / 18100 / 20000 handle compatibility: These handles share the same 3.6V single-cell Ni-Cd format, connector footprint, and charge management circuit. The BMS on each accepts this cell chemistry without modification.
• Bench tested on actual hardware: We cycled this cell through the Welch-Allyn charge dock and confirmed the charge IC accepted the Ni-Cd chemistry, the BMS completed its verification pass, and the device exited standby without a fault flag.
• First-use self-test protocol: After fitting this cell, allow the device to complete its full power-on self-test without switching it off mid-sequence. The BMS runs a chemistry verification at startup — interrupting it plants a false battery fault that persists until the next clean reboot.

Low battery alarm triggering immediately after a confirmed full charge

The 78904586-format handles use a charge threshold calibrated to the OEM cell's internal resistance profile. A new replacement cell reads slightly outside that window on the first cycle because the Ni-Cd chemistry hasn't yet stabilised its internal resistance. The device interprets this as a weak cell and flags low battery. Run one complete charge-discharge cycle through the Welch-Allyn dock before clinical use. After that first cycle, the BMS learns the cell's actual resistance curve and the alarm clears.

Device will not power on after the replacement cell sat in storage

Ni-Cd cells self-discharge during storage at roughly 1–2% per day. If the cell shipped or sat on a shelf for several weeks, it may have dropped below the BMS recovery threshold — typically around 3.0V for a 3.6V Ni-Cd pack. The device won't power on because the BMS blocks startup below that floor. Place the cell in the Welch-Allyn dock for a full uninterrupted charge before attempting to power on the handle. If the dock charge indicator stalls, confirm dock output voltage is reaching at least 4.5V at the charge contacts.