Sony WF-1000XM4 Charging Case Compatible Battery 3.85V 500mAh

Sony WF-1000XM4 Charging Case — 3.85V Li-Polymer Replacement Battery (LP702428)

This is a 3.85V, 500mAh Li-Polymer replacement cell for the Sony WF-1000XM4 charging case. It fits the case unit directly — not the earbuds themselves. When the case battery degrades, the case loses its ability to top up the earbuds away from a wall outlet; this cell restores that function.

• WF-1000XM4 case compatibility: The case uses a compact Li-Polymer pouch cell running a 3.85V nominal rail. The LP702428 part number matches the OEM cell dimension at 29.80 × 23.10 × 7.00mm. The BMS inside the case reads cell state-of-charge to manage earbud charging passes — a mismatched cell will cause the case to report incorrect charge levels or refuse to pass charge to the buds.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench. The BMS accepted the new cell without fault codes, and the case resumed initiating earbud charging normally. Cell voltage settled at 3.85V under no-load conditions after a full charge cycle.
• First charge after installation: After fitting the new cell, place the case on a USB-C charger and run it to 100% before placing the earbuds inside. The WF-1000XM4 case calibrates its earbud charge-pass count against the case cell's full capacity — skipping this step can cause the case indicator to misread available passes.

Why the WF-1000XM4 case stops charging earbuds before the LED shows empty

The case BMS applies a low-voltage cutoff to protect the cell — typically around 3.0V. When an aged cell's internal resistance rises, voltage drops sharply under the load of pushing charge into the earbuds, tripping the cutoff even if the indicator still shows one or two bars. A new cell with lower internal resistance holds the voltage rail steady through that load. After fitting this replacement, the case should complete full earbud charging passes without early cutoff.

Case LED flashing but earbuds not charging after battery swap

A flashing LED after a cell swap usually means the BMS has not yet completed its initialisation cycle with the new cell. The case needs to see a full charge from near-empty to 100% before it trusts the cell's state-of-charge data. Connect the case to USB-C, let it charge uninterrupted to full, then seat the earbuds. If the fault persists, check the cell connector seating — the LP702428 uses a small JST-style contact that can sit slightly proud if not pressed fully home.