Sony Inzone H9 Replacement Battery 3.85V 500mAh Li-Polymer

Sony Inzone H9 — 3.85V Li-Polymer Replacement Battery (FT702028P)

This is a 3.85V, 500mAh Li-Polymer replacement cell for the Sony Inzone H9 wireless gaming headset. It fits the internal battery slot directly, restoring charge capacity when the original cell has degraded. Also compatible with the WF-1000XM4 Charging Case, WH-CH720N, and WH-XB910N where this cell is used.

• Inzone H9 and WH-series compatibility: These models share the same 3.85V cell format and connector footprint. The BMS in each headset communicates with the cell over the same charge-management lines, so one cell services the full compatibility group without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Inzone H9 platform. The BMS accepted the pack on first insertion, completed a full charge cycle without error flags, and held voltage within the expected 3.85V nominal range under combined noise-cancellation and wireless radio draw.
• First-cycle charge protocol for the Inzone H9: Place the headset in the base station immediately after fitting the new cell and allow a complete charge cycle before use. The Inzone H9 base station logs the new cell during this first cycle — skipping it causes the talk-time indicator to misread remaining capacity for the first several sessions.

Voltage sag under combined noise-cancellation and wireless radio draw in the Inzone H9

The Inzone H9 runs two power-hungry systems simultaneously — the ANC processor and the wireless radio. Under that combined load, a degraded or storage-voltage cell can sag below the BMS cutoff threshold even when the headset indicator still shows charge remaining. A fresh cell at full nominal voltage holds the rail stable. If the headset cuts out mid-session rather than on startup, voltage sag under load is the most likely cause, not a BMS fault.

Base station showing full charge but headset shuts off after a few minutes

This happens when a new or storage-discharged cell sits at low resting voltage — typically below 3.5V — before its first full charge. The base station charges to the threshold it detects and then signals complete, but the cell hasn't reached a true full charge. The fix is straightforward: leave the headset docked for a full uninterrupted charge cycle. After that cycle, resting voltage should read at or near 3.85V and the headset will hold a normal charge.