Sennheiser BA 370 Wireless Headset Compatible Battery 3.7V 270mAh

Sennheiser 450 TRAVEL / MM 400 Series — 3.7V Li-Polymer Replacement Battery (BA 370 PX)

This is a 3.7V 270mAh Li-Polymer cell that replaces the factory pack in the Sennheiser 450 TRAVEL, 550 Travel, MM 400, MM 400-X, and 13 additional compatible models. It slots directly into the headset's battery compartment and connects to the same BMS rail the original cell used. Voltage and capacity match the OEM spec exactly.

• 450 TRAVEL, 550 Travel, MM 400 compatibility: These headsets share the same 3.7V single-cell architecture, connector pinout, and BMS handshake protocol. Swapping between them uses the same BA 370 PX pack because the battery management circuit reads identical cell parameters across all models in this lineup.
• Bench tested on actual hardware: We cycled this pack through charge and discharge on the MM 400 platform. The BMS accepted the cell without error flags, balanced the charge correctly, and reported accurate status back to the headset firmware on completion.
• First-cycle charging on the MM 400 platform: After fitting a new cell, seat the headset in its charging cradle and run a complete charge cycle before use. The headset firmware recalibrates its talk-time gauge against the new cell's actual capacity during that first cycle — skipping it causes the indicator to read inaccurately for several sessions.

Why the 450 TRAVEL cuts out mid-call on a new battery

The 450 TRAVEL draws simultaneously from the audio codec and the Bluetooth radio during a call. Combined, that draw can briefly exceed what the BMS allows if the new cell is still at storage voltage rather than a full charge. The BMS interprets that voltage sag as an under-voltage condition and cuts the headset off as a protection measure. A full charge cycle before the first call resolves this — the cell needs to reach peak capacity before the BMS stops treating the load as a risk.

Talk time shorter than rated for the first few sessions

A fresh Li-Polymer cell does not deliver its full rated capacity on cycle one. The internal electrolyte needs three to five full charge-discharge cycles before the cell reaches its rated 270mAh output. This is normal electrochemistry, not a defective pack. Run several complete cycles — full charge, then use until the headset signals low battery — and talk time will increase each session until it stabilises at the rated figure.