Bose QuietComfort 35 Replacement Battery 3.7V 400mAh

Bose QuietComfort 35 — 3.7V Li-Polymer Replacement Battery (AHB110520CPS)

This is a 3.7V, 400mAh lithium-polymer replacement battery for the Bose QuietComfort 35 wireless headphones. It fits the QC35 and QC35 II (model 419811) and replaces OEM part AHB110520CPS. Swap it in when the original cell has degraded and the headphones can no longer hold a usable charge.

• QC35 and QC35 II compatibility: Both headphone generations share the same battery footprint, connector pinout, and BMS handshake protocol. The cell dimensions — 47 × 10 × 10mm — match the housing cavity exactly, and the protection circuit communicates correctly with the QC35 charge management IC.
• Bench tested on actual hardware: We ran this cell in a QC35 unit through charge, discharge, and ANC-active listening cycles. The BMS accepted the pack without error codes, charge termination triggered cleanly at full voltage, and the protection circuit tripped correctly under simulated over-discharge conditions.
• First-cycle charge on QC35: Place the headphones on the charging cable and run a full charge cycle before use. The QC35 firmware recalibrates its fuel gauge against the new cell's actual capacity on that first cycle — skipping it causes the battery indicator to read inaccurately from the start.

Why the QC35 cuts out mid-song on a new battery

The QC35 draws current from two loads simultaneously — the audio amplifier and the active noise cancellation circuit. Under combined draw, a cell at storage voltage (typically around 3.6–3.7V shipped) can sag below the BMS undervoltage threshold momentarily, triggering a shutdown. This is not a faulty cell. Running one full charge-discharge cycle brings the cell to operational voltage and the sag disappears on subsequent use.

Battery indicator stuck at the same level after fitting a new cell

The QC35 estimates remaining charge by tracking coulombs from a reference point set during the last full charge cycle. A brand-new cell has no reference point stored, so the indicator reads stale data from the old cell and displays it as current. Charge the headphones to 100% and discharge them fully once — the firmware resets the coulomb counter against the new cell's actual 400mAh capacity. After that cycle, the indicator tracks accurately.