Sony LinkBuds S Charging Case Compatible Battery 3.85V 450mAh

Sony LinkBuds S / WF-LS900N Charging Case — 3.85V Li-Polymer Replacement Battery (ACE731834)

This is a 3.85V, 450mAh Li-Polymer battery for the Sony LinkBuds S and WF-LS900N charging cases. It replaces OEM part ACE731834 when the original cell no longer holds enough charge to keep your earbuds topped up through the day. Dimensions are 34.00 × 16.80 × 7.00mm — measure your original before fitting.

• LinkBuds S and WF-LS900N case compatibility: Both cases share the same internal battery bay, connector pinout, and BMS handshake voltage threshold. One cell covers both SKUs without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a LinkBuds S case. The BMS accepted the cell without fault flags, and the case passed charge current to the earbuds at the expected rate from the first cycle.
• Earbud case charge cycle tip: These cases sit on a desk charging constantly without ever fully discharging. That shallow-cycle pattern causes fuel gauge drift over months. Let the case drain fully until the earbuds stop receiving charge, then do a full recharge — do this once a month to keep the gauge accurate.

Why the LinkBuds S case shows full charge but stops topping up the earbuds early

The case relies on a fuel gauge IC that estimates remaining capacity based on charge cycles and voltage curves. When the cell is repeatedly charged from 40–50% without a full drain, the IC loses calibration and reports a higher state of charge than the cell can actually deliver. The earbuds stop receiving current not because the case failed, but because the cell voltage drops below the threshold the BMS needs to sustain output — even though the LED still shows green. A full discharge-then-charge cycle resets the gauge reading to match actual cell capacity.

Case not waking up on USB-C after sitting unused for several weeks

A deeply discharged Li-Polymer cell can drop below 2.5V after extended storage, which is under the minimum acceptance voltage for most USB-C PD controllers. The case appears completely dead — no LED, no charging activity. Plug it into a low-current 5V USB-A source rather than a fast charger, and leave it for 30–60 minutes to allow the BMS to trickle the cell back above 3.0V before normal charging resumes. If the cell does not recover above 3.0V after that window, the original cell is failed and replacement is the correct next step.