About this replacement
Sony WF-XB700 Charging Case — 3.7V Li-Polymer Replacement Battery (621733)
This 3.7V, 220mAh Li-Polymer cell replaces the internal battery in the Sony WF-XB700 and WF-XB700N charging cases, as well as the WF-SB700 and WF-SB700N variants. It restores the case's ability to hold charge and cycle power back into the earbuds between wall charges. At 31.44 x 17.40 x 6.30mm, it matches the original cell footprint exactly.
- WF-XB700 and WF-SB700 case compatibility: These case variants share the same internal cavity dimensions, connector orientation, and 3.7V charge management circuit. One cell covers all four models without modification.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge passes on a WF-XB700 case. The BMS accepted the new cell without error, regulated charge cutoff correctly at 4.2V, and passed current to both earbud bays as expected.
- First-cycle case conditioning: After fitting this cell, run the case to full charge before loading the earbuds. The case controller recalibrates its charge state estimate against the new cell on that first unloaded cycle — skipping it can cause the LED indicator to misread remaining capacity.
Why the WF-XB700 case LED shows full but earbuds stop charging early
The case controller stores a charge state estimate in firmware. When the original cell degrades, that estimate drifts away from actual capacity. A replacement cell starts at a different resting voltage than the old cell — typically storage voltage around 3.6–3.8V — so the controller's cached state no longer matches reality. The LED reads from the cached estimate, not a live cell measurement, until recalibration occurs. One full unloaded charge cycle resets the estimate and brings the indicator back in line with actual cell capacity.
Case not charging the earbuds at all after battery replacement
If the case shows no activity after fitting the new cell, the most common cause is a BMS that arrived in deep-sleep lockout — a protective state triggered when a cell sits at or below 3.0V during storage or shipping. The case controller will not draw current from a cell in this state. Connect the case to a USB-C power source and hold it on charge for at least 15–20 minutes without interruption. This trickle-charges the cell past the 3.0V wake threshold, releases the BMS lockout, and allows normal charge negotiation to begin. Once the LED activates, remove from charge and reinsert to confirm the earbuds are receiving current.