Sena SMH-5 Replacement Battery 3.7V 350mAh Li-Polymer

Sena SMH-5 — 3.7V Li-Polymer Replacement Battery (ICP40/25/40P)

This is a 3.7V 350mAh lithium-polymer cell for the Sena SMH-5 Bluetooth motorcycle intercom headset. It replaces OEM part ICP40/25/40P and fits directly into the SMH-5 housing. Voltage and physical dimensions match the original: 41.30 × 24.90 × 3.98mm.

• SMH-5 platform fit: The SMH-5 runs a single-cell 3.7V Li-Po pack with a compact flat form factor to fit inside the clamp-mount unit. This cell matches the OEM footprint and connector orientation so the BMS handshake completes correctly on first charge.
• Bench tested on actual hardware: We cycled this cell through the SMH-5 charge circuit and confirmed the BMS accepted the new pack without error, reached a full 4.2V charge termination, and discharged cleanly across the combined Bluetooth radio and audio draw.
• First-charge protocol for the SMH-5: Seat the headset in its mount cradle and run a full charge cycle before pairing or taking calls. The SMH-5 firmware calculates talk-time estimates from the first logged charge cycle — skipping this step causes the battery indicator to read inaccurately for several sessions.

Why the SMH-5 cuts out mid-call on a degraded cell

The SMH-5 draws current from two loads at once — the Bluetooth radio and the audio amplifier. Under that combined draw, a worn cell sees a sharper voltage sag than either load alone would cause. When cell voltage dips below the BMS cutoff threshold, the unit shuts off even if the indicator still showed charge. A fresh 350mAh cell at full 4.2V holds enough headroom to handle the combined draw without triggering the cutoff.

Talk time shorter than expected on the first few charges

A new Li-Po cell does not reach its rated capacity on the first cycle. Usable capacity typically increases over three to five full charge-discharge cycles as the electrolyte fully wets the electrode surfaces. If talk time feels short after the first charge, that is expected behaviour — not a fault. Run three complete cycles from full charge to automatic shutoff and capacity will stabilise near the rated 350mAh.