Altec Lansing Mini H2O 3 Compatible Battery 3.7V 400mAh LZ502535

Altec Lansing Mini H2O 3 / IMW258 — 3.7V Li-Polymer Replacement Battery (LZ502535)

This 3.7V 400mAh (1.48Wh) Li-Polymer cell is the direct replacement for the LZ502535 pack inside the Altec Lansing Mini H2O 3 and Mini H2O 2 portable Bluetooth speakers. It fits the IMW258 and IMW258BLK variants as well as three additional IMW258-series SKUs. The slim 36.82 × 24.80 × 5.50mm profile matches the original cell footprint in the waterproof housing.

• IMW258-series compatibility: All listed models share the same 3.7V single-cell power rail, the same connector pinout, and the same BMS handshake threshold. Swapping between these SKUs does not require firmware or hardware changes — the protection circuit reads identically across the IMW258 family.
• Bench tested on actual hardware: We cycled this cell through the Mini H2O 3 at sustained mid-volume output and monitored BMS cutoff voltage. The protection circuit tripped cleanly at the low-voltage threshold with no false cutoffs under combined amplifier and Bluetooth radio draw.
• Fuel gauge reset after installation: On the Mini H2O 3, the onboard fuel gauge can misread state-of-charge after a cell swap. Run the speaker down until it shuts off automatically, then charge uninterrupted to 100% — this recalibrates the gauge to the new cell's actual capacity.

Bluetooth dropping at high volume on the Mini H2O 3

At maximum volume, the amplifier and Bluetooth radio draw current simultaneously, causing a momentary voltage sag on the 3.7V cell. If the cell is aged, internal resistance rises enough that sag crosses the BMS undervoltage threshold — the radio drops to recover the rail. A fresh 400mAh cell with low internal resistance handles the combined current spike without the sag crossing that threshold. After fitting the new cell, test at full volume for 30 seconds — sustained audio without dropout confirms the BMS is not tripping.

Speaker shows full charge but audio cuts before the indicator reaches empty

This is amplifier clipping under voltage sag, not a charging fault. As the cell depletes past roughly 3.5V, the amplifier rail can no longer sustain clean output at high gain — the speaker either clips audibly or the BMS cuts power early to protect the cell. A fuel gauge drifted from shallow cycling reports more charge remaining than actually exists, so the cutoff appears premature. Replace the cell and complete one full discharge-to-shutdown cycle before recharging to sync the gauge with real capacity.