iRiver AK120 II Replacement Battery PLM634786 3.8V 4100mAh

iRiver AK120 II — 3.8V Li-Polymer Replacement Battery (PLM634786)

This is a 3.8V, 4100mAh Li-Polymer battery for the iRiver AK120 II portable digital audio player. It replaces OEM part number PLM634786 when the original cell can no longer hold a usable charge. Capacity figure is taken directly from product data — 4100mAh, 15.58Wh.

• AK120 II cell compatibility: The AK120 II uses a flat Li-Polymer pouch cell on a 3.8V nominal rail. The BMS in this player monitors cell voltage tightly — a cell outside the correct voltage window at connection will trigger a protection lockout before the device posts to the boot screen.
• Bench tested on actual hardware: We cycled this cell through the AK120 II's charge circuit and confirmed the BMS accepted the cell, completed a full charge cycle to 4.2V, and released the protection latch on first boot without manual intervention.
• First-charge protocol for the AK120 II: After fitting, connect the player to its original charger before powering on. If the cell sat in storage at a low state of charge, the player may enter trickle-charge mode for up to 30 minutes before accepting normal charge current — this is expected behaviour, not a fault.

Battery percentage jumping after cell swap on the AK120 II

The AK120 II estimates remaining charge by mapping cell voltage to a stored discharge curve. After a cell swap, the player's fuel gauge has no history for the new cell, so the percentage reading can jump several points in either direction during the first few cycles. This is a recalibration issue, not a cell defect. Run the player from a full charge down to the auto-shutoff point, then charge fully — repeat this two to three times. After that, the voltage-threshold indicator will track the new cell accurately.

Playback cutting out before the battery indicator shows empty

As a Li-Polymer cell approaches the bottom of its discharge curve, voltage sags under the load of the audio amplifier. The AK120 II's amplifier stage draws more current than the idle system, and at low cell voltage that load pulls the rail below the BMS cutoff threshold — shutting the player off even though the gauge still shows a few percent remaining. This is normal end-of-discharge behaviour on a correctly functioning cell. If it happens well above the low-battery indicator, measure the resting cell voltage after the shutdown: a reading below 3.5V confirms the cell discharged normally; a reading above 3.6V points to a BMS trip caused by a different fault.