About this replacement
Sony MDR-DS6500 / MDR-RF985R Series — 3.7V Li-Polymer Replacement Battery (1-756-920-21)
This 3.7V 550mAh Li-Polymer cell replaces the original battery in the Sony MDR-DS6500, MDR-RF865R, MDR-RF985R, MDR-XB650BT, and related wireless headset models. It matches the OEM dimensions at 52.35 × 34.10 × 3.20mm and fits the existing battery compartment without modification. Voltage, connector, and BMS signalling match the original specification.
- MDR-DS6500 and RF-series compatibility: These models share a common battery platform — same 3.7V rail, same physical form factor, and the same BMS handshake protocol used by the base station to track charge state. A cell that passes one model's BMS check passes all of them.
- Bench tested on actual hardware: We cycled this cell through the Sony base station charging circuit and confirmed the BMS handshake completed without error flags. Charge acceptance was consistent from the first cycle, and the protection circuit triggered correctly at both low-voltage cutoff and overcharge thresholds.
- Base station first-cycle logging: Place the headset in the base station immediately after fitting this cell and allow a complete charge cycle to finish before use. The Sony base station logs the new cell during this first cycle — skipping it means the talk-time indicator will be inaccurate until the log completes.
Why the MDR-DS6500 cuts out mid-transmission with a new battery
The MDR-DS6500 draws from two loads simultaneously — the audio amplifier and the DECT radio transmitter. Under that combined draw, a cell that hasn't completed its first conditioning cycle can sag below the BMS cutoff threshold, causing the headset to drop out briefly even though the charge indicator looks fine. This isn't a fault with the cell — it's the protection circuit doing its job on an unconditioned pack. Running two to three full charge-discharge cycles through the base station brings the cell to its rated capacity and eliminates the sag. After conditioning, mid-transmission dropout from voltage sag stops.
Base station shows full charge but headset cuts off after a few minutes
A new Li-Polymer cell ships at storage voltage — typically around 3.6–3.7V — not full charge. The base station's charge indicator can read this as "charged" before the cell has actually completed a full charge cycle. The result is a headset that appears ready but shuts off quickly under load. Fix this by placing the headset in the base, letting the charge LED cycle through to its completion state, and leaving it seated for another 30 minutes past that point. After one genuine full charge, the base station's reading and actual cell state will align.