About this replacement
Bose SoundSport Free Charging Case — 3.7V Li-ion Replacement Battery (NTA3522)
This is a 3.7V, 700mAh Li-ion cell for the Bose SoundSport Free charging case. It fits case model 423729 and replaces the original NTA3522 cell. When the case battery degrades, the case stops topping up the earbuds between uses — this cell restores that function.
- SoundSport Free case compatibility: The case uses a single-cell 3.7V Li-ion pack with a small BMS to manage earbud charging cycles. Voltage, connector type, and physical dimensions must match exactly — this cell measures 34.60 × 17.80 × 16.50mm, matching the original cavity.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on the SoundSport Free case and confirmed the BMS accepted the new cell without error, triggering normal earbud charging behaviour on the first cycle.
- Case-specific charge tip: After installing, run the case to full charge via USB before placing the earbuds in — the case BMS needs one complete charge cycle to calibrate its fuel gauge and report accurate earbud charge levels.
Why the SoundSport Free case stops charging earbuds even when plugged in
The case charges the earbuds from its internal cell, not directly from USB. If the cell voltage drops below roughly 3.0V, the BMS cuts output to protect the cell — so even with USB power connected, the earbuds receive nothing. A degraded or deeply discharged cell triggers this cutoff repeatedly. Replacing the cell resolves the problem at the source; USB pass-through charging is not a feature this case supports.
Case LED flashing but earbuds not charging after new cell installed
A flashing LED with no earbud charge transfer usually means the replacement cell arrived at storage voltage — typically around 3.6V — and the BMS has not yet completed its initialisation cycle. Place the case on USB charge for a full cycle before inserting the earbuds. Once the cell reaches 4.1–4.2V and the BMS logs the new pack, earbud charging resumes normally. Do not test earbud charging until the case LED shows a solid full-charge indicator.