AT&T 401 Cordless Phone Replacement Battery 3.6V 1300mAh

AT&T 401 / 4126 / 24032X — 3.6V Ni-MH Replacement Battery

This is a 3.6V, 1300mAh Ni-MH battery for the AT&T 401, 4126, 24032X, and A36 cordless phone handsets. It slots into the handset battery compartment and restores power to the unit when the original pack can no longer hold a charge. Dimensions are 50.57 x 42.47 x 14.35mm — confirm these against your existing pack before ordering.

• 401 / 4126 / 24032X / A36 compatibility: These AT&T handsets share the same 3.6V battery bay, connector orientation, and contact spacing. One replacement pack covers all four models without modification.
• Bench tested on actual hardware: We cycled this pack through charge and discharge on a cordless phone test rig. The BMS accepted charge from a standard AT&T base cradle without triggering a fault light, and cell voltage held steady across the full discharge curve.
• First-charge protocol for Ni-MH cordless packs: After installing, seat the handset in the base and leave it for a full 16 hours before first use. Ni-MH cells shipped in partial-charge state need a slow initial charge to reach rated capacity — skipping this shortens talk time from the start.

Range dropping after a battery swap on the AT&T 401

DECT handsets draw a sharp current spike every time the RF transmitter fires. If a Ni-MH pack hasn't been fully conditioned, internal resistance is higher than rated, and voltage sags under that transmit load. The handset interprets the voltage drop as a weak signal and reduces transmit power — which cuts range. Running three to five full charge-discharge cycles brings internal resistance down and stabilises the voltage under RF load.

Base station showing no charge or fault light with a new Ni-MH pack

Ni-MH batteries can sit in storage long enough for cell voltage to drop below the threshold the AT&T base cradle uses to detect a valid pack. When the cradle sees voltage below roughly 3.0V, it may refuse to initiate a charge cycle and show a fault or no-charge indicator instead. The fix is to briefly trickle-charge the pack externally with a universal Ni-MH charger until it reaches 3.2V or above, then return it to the base — the cradle will recognise it and resume normal charging.