About this replacement
Astell&Kern AK120 — 3.8V Li-Polymer Replacement Battery (NCP605056)
This is a 3.8V 2600mAh (9.88Wh) lithium-polymer battery for the Astell&Kern AK120 portable high-resolution audio player. It replaces the original NCP605056 cell when the AK120 no longer holds a charge or fails to power on. Dimensions are 56.15 × 49.60 × 6.00mm — confirm these against your existing cell before installing.
- AK120 compatibility: The AK120 uses a flat lithium-polymer pouch cell on a dedicated power rail that feeds both the DAC and the amplifier stage. This cell matches the original connector pinout, voltage profile, and physical footprint so the BMS handshake completes without fault codes.
- Bench tested on actual hardware: We ran the cell through a full charge-discharge cycle on the AK120 platform. The BMS accepted the cell without triggering a protection cutoff, and the charge IC stepped through pre-charge, CC, and CV phases in the correct sequence.
- Post-swap initialisation tip: After fitting this cell, let the AK120 charge for a full uninterrupted cycle before playing audio. The AK120's power management IC recalibrates its voltage-to-percentage curve on the first full charge — interrupting it early causes the gauge to read inaccurately for subsequent cycles.
Battery percentage jumping after a cell swap on the AK120
The AK120 uses a voltage-threshold method to estimate remaining charge — it has no dedicated fuel-gauge IC tracking coulombs. When a new cell goes in, the power management IC has no charge history to reference. Until it completes one full charge-discharge cycle, the percentage reading can jump 10–20% in either direction. Run one complete cycle from 0% shutdown to a full CV-phase charge, and the gauge will stabilise at the correct thresholds.
Playback cutting out before the AK120 shows empty
The AK120's audio amplifier draws a surge current at the end of a discharge cycle when cell voltage drops below roughly 3.5V — the amplifier stage demands more current than a depleted cell can deliver cleanly. This causes the BMS to trip a low-voltage cutoff and halt playback even though the percentage indicator still shows a non-zero figure. It is not a firmware fault. A fresh cell with lower internal resistance will sustain the amplifier load further into the discharge curve and push that cutoff point closer to an actual 0% reading.