iRiver SlimX iMP-350 Compatible Battery 1.2V 1200mAh

iRiver SlimX iMP-350 / iMP-400 / iMP-550 — 1.2V Ni-MH Replacement Battery

This 1.2V, 1200mAh Ni-MH cell replaces the original rechargeable battery in the iRiver SlimX iMP-350, iMP-400, iMP-550, MP-350, and compatible models. It restores power to the portable CD/MP3 player when the original cell has degraded and can no longer hold a usable charge. Capacity matches the OEM spec at 1200mAh (1.44Wh).

• SlimX series compatibility: The iMP-350, iMP-400, and iMP-550 share the same slim-form battery bay, voltage rail, and charge-termination circuit. All three models use a single-cell Ni-MH configuration with delta-V cutoff charging, so one cell fits each variant without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the SlimX platform and confirmed the onboard charge controller reached delta-V cutoff correctly without overcharging. The BMS terminated charge cleanly at full capacity on each run.
• First-use charge protocol for SlimX players: If the player has been in storage, connect it to the charger for a full uninterrupted charge cycle before powering it on. The SlimX charge circuit requires voltage above a minimum threshold before it will initialise normal charge current — skipping this step can leave the cell in a partial trickle state.

Playback cutting out before the battery indicator shows empty

The SlimX audio amplifier draws a brief current spike when buffering and decoding high-bitrate tracks. Near the end of a Ni-MH cell's discharge curve, internal resistance rises sharply — enough that those spikes pull cell voltage below the player's minimum operating threshold. The player shuts down to protect the circuit even though the fuel gauge still shows charge remaining. After a full slow charge cycle, this cutoff point resets and the full usable capacity returns.

Battery percentage jumping erratically after cell swap

The iRiver SlimX uses a voltage-threshold indicator, not a coulomb counter, to estimate remaining charge. A new Ni-MH cell has a flatter discharge curve than a worn original, so the player's voltage readings temporarily map to the wrong percentage steps. Run two full charge-discharge cycles and the indicator stabilises. If the display still jumps after two cycles, check that resting voltage sits between 1.2V and 1.3V at full charge — anything outside that range points to a cell fault.