Sony PHA-2 4-297-656-01 Replacement Battery 3.7V 2200mAh

Sony PHA-2 / PHA-2A — 3.7V Li-Polymer Replacement Battery (4-297-656-01)

This 3.7V, 2200mAh Li-Polymer battery replaces OEM part 4-297-656-01 in the Sony PHA-2 and PHA-2A portable headphone amplifiers. The PHA-2 is a compact amplifier that sits between a source device and your headphones, boosting audio output — and it depends entirely on this cell to stay portable. When the original pack degrades, the amplifier loses the steady voltage rail it needs to drive high-impedance headphones cleanly.

• PHA-2 and PHA-2A compatibility: Both models run the same 3.7V single-cell architecture with the same connector pinout and BMS handshake protocol. The 4-297-656-01 part number covers both — no adapter or modification needed.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the PHA-2A platform. The BMS accepted the pack without fault codes, held the 3.7V nominal rail stable under headphone amplifier load, and reached the correct 4.2V charge termination voltage.
• PHA-2 amplifier discharge management: The PHA-2 amplifier stage draws more current than a passive device — it is not a simple playback unit. If you use the amp daily on a desk, let the cell drop below 20% at least once a month before recharging. Constant shallow top-off cycling causes fuel gauge drift and accelerates capacity fade on Li-Polymer cells at this form factor.

Why the PHA-2 distorts audio before the battery indicator reads empty

The PHA-2 amplifier circuit requires a stable voltage above approximately 3.5V to drive headphones without clipping. As the cell ages and internal resistance rises, voltage sag under amplifier load drops below that threshold well before the fuel gauge hits zero — because the gauge reads resting voltage, not loaded voltage. The result is audible distortion or output compression while the indicator still shows charge remaining. A fresh cell with lower internal resistance eliminates the sag and restores clean output across the full discharge curve.

PHA-2 shows full charge but audio drops or cuts after extended listening

This is capacity fade masking as a full charge. A degraded Li-Polymer cell can accept a charge to 4.2V and report full, but its usable capacity has shrunk significantly — so actual playtime collapses even though the indicator looks correct. The BMS in the PHA-2 does not recalibrate the fuel gauge unless the cell completes a full discharge and recharge cycle. Run the amp until it shuts off on low battery, then charge fully to 4.2V in one session to force a gauge reset before fitting a replacement cell.