Mitel 6865i Replacement Battery 3.7V 240mAh Li-Polymer

Mitel 6865i / 6863i Series — 3.7V Li-Polymer Replacement Battery (50006764)

This 3.7V 240mAh Li-Polymer cell replaces the internal battery in Mitel DECT wireless headsets. It fits the 6865i, 6863i, 6865, 6867i, and nine additional models in this range. Swap it when your headset no longer holds charge or drops off the base station mid-cycle.

• 6860 series headset compatibility: These models share a common 3.7V single-cell architecture and the same connector footprint. The BMS on each variant communicates charge state to the base station over the same handshake protocol, so one cell covers the full range without firmware conflicts.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a 6865i base station. The BMS completed its handshake correctly, and the base station's charge indicator progressed normally from empty to full without throwing a fault.
• First-cycle base station registration: On first use, seat the headset in the base and let it charge through one complete uninterrupted cycle before making a call. The base station logs the new cell during this cycle — skipping it causes the talk-time estimate to read inaccurately for the first several uses.

Base station not recognising a new pack after swap

When a fresh cell is installed, the BMS starts at storage voltage — typically around 3.6V — which sits outside the range the base station expects for a registered headset. The base may flash an error or refuse to show a charging state. This is not a fault with the cell. Place the headset on the cradle and leave it undisturbed for a full charge cycle; the BMS handshake completes once voltage climbs past the base station's recognition threshold near 3.7V.

Headset cutting out mid-call on a fresh battery

A 240mAh cell in a compact headset housing carries a combined load — audio amplifier plus DECT radio transmitter — both drawing current simultaneously during a call. If the cell hasn't completed its first full conditioning cycle, its internal resistance is temporarily elevated, causing a voltage sag under that combined draw. The BMS interprets the sag as a low-cell condition and cuts power to protect the cell. Complete three to five full charge-discharge cycles and the sag behaviour normalises; mid-call dropouts typically stop after cycle three.