About this replacement
LEXIN LX-B4FM / FT4 Pro — 3.7V Li-Polymer Replacement Battery (AHB802540)
This is a 3.7V 800mAh Li-Polymer cell that replaces the original AHB802540 pack in the LEXIN LX-B4FM and FT4 Pro motorcycle Bluetooth headsets. It fits directly into both models, which share the same battery bay dimensions and BMS handshake protocol. Capacity is 800mAh (2.96Wh), matching the factory specification.
- LX-B4FM and FT4 Pro compatibility: Both models run the same 3.7V single-cell architecture with an identical connector and BMS communication line. One replacement cell covers both units without any wiring changes.
- Bench tested on actual hardware: We cycled this pack through charge and discharge on the LX-B4FM platform. The BMS accepted the cell without fault codes, and protection thresholds tripped correctly at low-voltage cutoff.
- First-cycle charging on the headset: After fitting this battery, seat the headset in its charging cradle and let it complete one full, uninterrupted charge before use. The LX-B4FM's firmware logs the new cell during that first cycle — skip it and the talk-time indicator will read inaccurately for the first several sessions.
Why the LX-B4FM cuts out mid-call on a fresh battery
The LX-B4FM combines Bluetooth audio and intercom radio draw simultaneously during a call. That combined current spike can briefly pull cell voltage below the BMS cutoff threshold — especially on a new cell that hasn't completed its first full conditioning cycle. The BMS reads the sag as an undervoltage event and shuts the unit down to protect the cell. This typically resolves after three to five full charge-discharge cycles, once the cell's internal resistance settles to its operating baseline.
Headset shows full charge but cuts off after a few minutes of use
A replacement cell shipped at storage voltage — roughly 3.6–3.7V — can fool the headset's charge indicator into displaying a full bar. The cell isn't actually fully charged; it's just resting above the empty threshold. Seat the headset in the cradle, wait for the indicator to cycle through to a confirmed full charge, then disconnect. If the issue persists after two full cycles, check that the cradle contacts are clean and making solid connection with the headset pins.