Astell & Kern AK120 II Replacement Battery 3.8V 4100mAh

Astell & Kern AK120 II — 3.8V Li-Polymer Replacement Battery (PLM634786)

This is a 3.8V, 4100mAh Li-Polymer replacement battery for the Astell & Kern AK120 II portable high-resolution audio player. It fits the AK120 II directly and matches the original cell's voltage, capacity, and physical dimensions (86.50 × 47.80 × 7.00mm). Install this when the original cell no longer holds adequate charge or fails to power the device at all.

• AK120 II fit: The AK120 II uses a single Li-Polymer pouch cell on a dedicated BMS board that monitors cell voltage and communicates charge state to the firmware. This replacement matches the original cell's voltage rail and connector layout so the BMS handshake completes without throwing a fault.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the AK120 II platform. The BMS accepted the cell immediately, charge current ramped normally through CC and CV phases, and the protection circuit tripped correctly at the low-voltage cutoff threshold.
• Post-swap initialisation: After fitting a new cell, the AK120 II's fuel gauge IC needs to resynchronise its voltage-to-percentage table. Run the player down to automatic shutdown, then charge uninterrupted to 100% before resuming normal use. Skipping this step leaves the percentage readout unreliable for several cycles.

Battery percentage jumping erratically after cell swap

The AK120 II uses a coulomb-counting fuel gauge that tracks charge state relative to learned cell characteristics. A new cell has no history in that gauge, so the estimated percentage can jump — showing 80%, then 65%, then 72% within minutes. This is not a fault in the replacement cell. One full discharge-to-shutdown followed by a complete uninterrupted charge teaches the gauge the new cell's actual capacity curve. After two or three of those full cycles the readout stabilises.

AK120 II not waking after extended storage

A Li-Polymer cell stored for several months can drop below 3.0V per cell, which triggers the BMS deep-discharge protection and prevents normal power-on. Pressing the power button produces nothing because the BMS is blocking current draw until it confirms the cell voltage is safe. Connect the AK120 II to its charger and leave it for at least 30 minutes without attempting to power on — the charger delivers a slow trickle current that brings the cell back above the BMS re-enable threshold. Once the cell reaches approximately 3.2V, the BMS releases and the device charges normally.