Williams Sound AP11A Replacement Battery 2.4V 80mAh

Williams Sound WIR RX-18 Infrared Receiver — 2.4V Ni-MH Replacement Battery (AP11A)

This is a 2.4V, 80mAh Ni-MH replacement battery for the Williams Sound WIR RX-18 infrared receiver. The WIR RX-18 is a wireless assistive listening receiver used in theaters, houses of worship, and public venues. It receives audio signals from infrared transmitters and delivers amplified sound directly to the user.

• WIR RX-18 fit confirmed: The AP11A cell matches the voltage rail and physical footprint of the WIR RX-18 receiver housing. The 2.4V Ni-MH configuration is specific to this unit — swapping chemistry or voltage will cause charging errors or damage the receiver's internal charge circuit.
• Bench tested on actual hardware: We ran the AP11A through a full charge cycle in a WIR RX-18 and confirmed the charging circuit accepted the cell without error. The BMS reached termination voltage cleanly and held the charge state across standby.
• First charge in the receiver unit: Place the WIR RX-18 on its charging cradle immediately after fitting the new battery and allow a complete uninterrupted charge cycle. The receiver's charge circuit uses a delta-V detection method specific to Ni-MH — partial cycles on the first use can confuse the termination logic and shorten apparent capacity.

Why the WIR RX-18 shows a full charge indicator but cuts off early after a battery swap

The WIR RX-18 charge cradle reads cell voltage to estimate state of charge. A new Ni-MH cell arrives at storage voltage — typically around 1.2V per cell — which the cradle can misread as partially charged on the first cycle. If you remove the receiver before the cradle completes a full delta-V termination, the indicator shows full but the actual capacity available is low. Run one complete, uninterrupted charge cycle before use. After that first full cycle, the cradle's charge logic tracks the cell correctly.

WIR RX-18 audio cutting out mid-use even with a recently charged battery

The WIR RX-18 draws current from both its infrared receiver circuit and its audio amplifier simultaneously. Under that combined load, a worn or partially cycled Ni-MH cell can sag below the receiver's minimum operating voltage before the charge indicator suggests it should. The unit cuts audio as a protective response to undervoltage — not a fault in the receiver itself. Return the unit to the cradle and confirm it reaches full charge termination, indicated by the charge LED switching state, before the next use session.