Sony WF-1000XM5 Charging Case Replacement Battery 3.8V 500mAh

Sony WF-1000XM5 Charging Case — 3.8V Li-Polymer Replacement Battery (SP772020)

This 3.8V, 500mAh Li-Polymer cell replaces the internal battery inside the Sony WF-1000XM5 charging case. It powers the case's ability to store charge and refill the earbuds between wall-power top-ups. When the original cell degrades, the case loses capacity to complete full earbud charge cycles — this battery restores that function.

• WF-1000XM5 case compatibility: The case uses a small Li-Polymer cell with a specific footprint — 30.00 x 20.80 x 7.40mm — and a low-current charge circuit matched to 3.8V nominal. Swapping to a cell outside that voltage window triggers the case's protection circuit and stops charging entirely.
• Bench tested on actual hardware: We cycled this cell through the case's charge management circuit and confirmed the BMS completed handshake without fault codes. Charge flow to the earbud contacts resumed normally after the first full cycle from the wall.
• First charge after installation: Place the case on the charging mat or USB-C cable immediately after fitting the new cell. The case's charge controller needs a full charge cycle to log the new cell's capacity before the LED indicator reports accurately.

Why the WF-1000XM5 case LED shows full but stops charging the earbuds early

The case measures its own state of charge through a fuel gauge IC tied to the cell's voltage curve. A degraded cell sits at a resting voltage that looks full to the gauge but collapses under the load of pushing current into both earbuds simultaneously. The result is a false full reading followed by an abrupt cutoff. Replacing the cell resets the voltage reference the IC uses, and the LED behaviour returns to normal after one complete charge-discharge cycle.

Case not charging the earbuds at all after battery replacement

If the earbud contacts receive no current after fitting the new cell, the most likely cause is that the BMS has not yet completed its initialisation. This happens when the replacement cell ships at storage voltage — typically around 3.6V — which sits below the threshold the case controller expects before enabling the output path. Connect the case to USB-C power and allow it to charge uninterrupted to 4.2V. Once the cell reaches that voltage, the output circuit re-enables and earbud charging resumes.