Monster SUPERSTAR FXN-40 Replacement Battery 3.7V 5200mAh

Monster SUPERSTAR / S310 — 3.7V Li-ion Replacement Battery (FXN-40)

This is a 3.7V, 5200mAh Li-ion replacement battery for the Monster SUPERSTAR and S310 portable Bluetooth speakers. It slots in to restore wireless audio playback after the original cell degrades. Capacity figures here come from the product data, not estimated specs.

• SUPERSTAR and S310 shared platform: Both models run the same 3.7V single-cell architecture with the FXN-40 footprint. The connector pinout and BMS handshake protocol are identical across the two — one battery covers both without adaptation.
• Bench tested on actual hardware: We ran the FXN-40 through a full charge cycle on the SUPERSTAR platform and confirmed the BMS accepted the cell, completed charge termination correctly, and reported state-of-charge accurately to the speaker's fuel gauge.
• Monthly discharge cycle for the SUPERSTAR: Speakers left on a desk and topped off before they drop below 50% develop fuel gauge drift within a few months. Let the cell discharge below 20% at least once a month before plugging back in — this recalibrates the gauge and slows capacity fade on the Li-ion cell.

Why the SUPERSTAR clips audio before the battery indicator reaches empty

At high playback volume, the amplifier draws a sharp current spike that sags cell voltage below the amp's clean-power threshold — even when the fuel gauge still reads 20–30%. The BMS does not cut out entirely, but the cell can no longer hold voltage steady under the combined amplifier and Bluetooth radio draw. The result is audible clipping or distortion before any low-battery warning appears. Dialling volume back to around 70% keeps instantaneous current draw within the cell's stable output range and eliminates the sag.

USB charging won't start after the speaker sat unused for weeks

Li-ion cells self-discharge slowly in storage. If the SUPERSTAR sat unused long enough, the cell drops below the minimum acceptance voltage for USB-C PD negotiation — the charger and speaker fail to handshake and charging never begins. The fix is to use a 5V USB-A cable rather than USB-C for the first 10–15 minutes, which bypasses PD negotiation and trickle-charges the cell back above the recovery threshold. Once the fuel gauge shows any sign of life, switch back to your normal charger. Target at least 3.0V on the cell before expecting a normal charge session to proceed.