AT&T 3B Cordless Phone Replacement Battery 3.6V 2700mAh

AT&T 3B / 3B2 Series — 3.6V Li-MnO2 Replacement Battery

This is a 3.6V Li-MnO2 cell rated at 2700mAh (9.72Wh), built to replace the original battery in the AT&T 3B, 3B2, 405371493, and 6286 cordless telephone handsets. When the original cell degrades, the handset loses talk time and standby capacity. Swapping this cell restores full handset function without replacing the base station or the entire phone system.

• AT&T 3B and 3B2 handset compatibility: These models share the same 3.6V single-cell architecture and physical cell format. The 60.55 × 18.50 × 18.35mm dimensions match the original cavity directly, and the Li-MnO2 chemistry matches the voltage curve the handset charging circuit expects.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the 3B handset platform. The charging circuit accepted the cell without error, and the BMS held the float voltage within spec throughout the test cycle.
• First-charge protocol for Li-MnO2 cordless cells: Li-MnO2 cells ship at partial state of charge. Place the handset in the cradle immediately after installation and allow a full uninterrupted charge cycle before use — interrupting the first charge can skew the handset's charge gauge and cause premature low-battery warnings.

Why the AT&T 3B handset reports low battery immediately after a new cell is installed

Li-MnO2 cells arrive at storage voltage, typically 3.2–3.3V, which sits below the handset's full-charge threshold. The 3B charging circuit reads this incoming voltage and flags it as a depleted cell. This is not a fault with the cell. Place the handset in the cradle and leave it undisturbed through one full charge cycle. The charge gauge recalibrates once the cell reaches its nominal 3.6V float and the low-battery indicator clears automatically.

Handset powers on but drops calls or loses range far sooner than it used to

This symptom points to internal cell resistance in the original battery, not the base station or antenna. As a Li-MnO2 cell ages, internal resistance climbs and the cell voltage sags under the transmit current draw — the handset's RF output drops and range collapses even when the charge gauge still shows partial charge. Replacing the cell eliminates the resistance problem at the source. After installing this cell, confirm the handset charges to 3.6V on the cradle before returning it to service.