About this replacement
Logitech Pure-Fi Anywhere Speaker 2nd Generation MM50 — 3.7V Li-ion Replacement Battery (NTA2335)
This 3.7V, 2200mAh Li-ion cell replaces the NTA2335 battery in the Logitech Pure-Fi Anywhere Speaker 2nd Generation (MM50). It fits the original battery bay with the same connector orientation and matches the BMS handshake the speaker's charge circuit expects. Capacity is 8.14Wh — identical to the factory spec.
- MM50 platform fit: The Pure-Fi Anywhere 2nd Gen uses a single-cell Li-ion pack with a specific protection circuit profile. This replacement matches that cell voltage, connector pinout, and charge termination behaviour — the speaker's internal charger sees no difference from the original pack.
- Bench tested on actual hardware: We cycled this cell through the MM50's charge circuit and monitored BMS cutoff at both the low-voltage floor and charge termination threshold. The protection circuit tripped correctly at both ends with no false cutoffs under normal audio load.
- Fuel gauge reset after swap: After fitting this battery, run the speaker down until it shuts off automatically, then charge it uninterrupted to 100% before use. The MM50's fuel gauge calibrates on that first full cycle — skipping it leaves the indicator inaccurate from the start.
Audio distorting before the battery indicator reaches empty on the MM50
At low charge, the amplifier inside the MM50 draws a current spike to maintain output level — especially at mid-to-high volume. If the cell voltage sags below roughly 3.2V under that load, the amplifier clips before the battery indicator has caught up to actual cell state. You hear distortion while the LED still shows charge remaining. This is a cell-voltage issue, not a speaker fault. Fitting a fresh cell with healthy internal resistance eliminates the sag, and the distortion goes with it.
MM50 showing full charge but audio drops after an hour of playback
This is the classic symptom of capacity fade from shallow cycling — the speaker was regularly topped off before dropping below 50%, so the cell never completed a proper discharge-recharge cycle. Over time the usable capacity shrinks while the fuel gauge still reads optimistically. The indicator shows "full" based on voltage at rest, but the cell can't sustain output under real audio load. After fitting this replacement, complete one full discharge-to-shutdown and uninterrupted charge cycle to anchor the gauge to actual capacity.