Sony HMP-A1 Replacement Battery 3.7V 2200mAh Li-ion

Sony HMP-A1 — 3.7V Li-ion Replacement Battery

This 3.7V, 2200mAh Li-ion cell replaces the original internal battery in the Sony HMP-A1 portable media player. It fits the HMP-A1 directly and restores the device to normal playback operation. Capacity figure is taken from product data: 2200mAh (8.14Wh).

• HMP-A1 fit: The HMP-A1 uses a single-cell 3.7V Li-ion pack with a compact form factor — 65.80 x 18.57 x 18.57mm. This cell matches that footprint and voltage rail, so the onboard charge controller recognises it without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the HMP-A1 platform. The BMS accepted the cell cleanly, entered CC/CV charge mode at the correct thresholds, and terminated charge without error flags.
• Post-install charge cycle: After fitting a new cell, connect the HMP-A1 to its charger before powering on. Cells shipped in storage state sometimes sit below the device's wake threshold — a 30-minute trickle charge pulls them above the minimum voltage the firmware needs to boot.

Battery percentage jumping after cell swap on the HMP-A1

The HMP-A1 estimates remaining charge by reading cell voltage against a fixed lookup table stored in firmware. When a new cell goes in, the firmware's internal reference points no longer match the new cell's actual voltage curve. That mismatch causes the indicator to jump — sometimes from 80% to 20% mid-session. Run two or three full charge-to-discharge cycles and the voltage curve re-aligns with the firmware table. After that, the percentage display stabilises.

Playback cutting out before the battery indicator shows empty

The audio amplifier circuit in the HMP-A1 draws a short current spike every time it drives the output stage. Near the end of a Li-ion cell's discharge curve — typically below 3.5V — internal cell resistance climbs enough that those spikes cause momentary voltage sag. The BMS reads that sag as an undervoltage event and cuts power to protect the cell, even though the indicator still shows charge remaining. This is not a fault with the cell. Charge the device back to 4.2V and the issue won't recur until the cell approaches that same low-voltage window again.