Sena SRL2 Replacement Battery 3.7V 700mAh Li-Polymer

Sena SRL2 — 3.7V Li-Polymer Replacement Battery (XK701953)

This 3.7V, 700mAh lithium-polymer cell replaces the original battery in the Sena SRL2 motorcycle Bluetooth headset. It restores wireless audio, intercom, and communication functions when the factory cell no longer holds a charge. OEM part number XK701953 confirms the match.

• SRL2 fit: The SRL2 runs a single-cell Li-Polymer pack at 3.7V nominal. That voltage rail drives both the Bluetooth radio and audio amplifier through one shared BMS. Any cell outside that spec will either fail the handshake or undervolt the radio during transmission.
• Bench tested on actual hardware: We cycled this cell through full charge-discharge runs while monitoring BMS cutoff thresholds. The protection circuit tripped correctly at both the high-voltage and low-voltage limits, and the cell delivered rated capacity within the first two cycles.
• First-cycle protocol for the SRL2: Seat the headset in the SRL2 clamp mount and let it charge uninterrupted until the LED indicator goes solid. The SRL2's firmware logs the new cell during that first complete cycle — skip it and the talk-time estimate will read incorrectly for several sessions.

Why the SRL2 cuts out mid-call on a new battery

The SRL2 draws simultaneously from the Bluetooth radio and the audio amplifier during an active intercom call. On a cell that hasn't completed a full conditioning cycle, that combined draw creates a brief voltage sag below the BMS cutoff threshold — typically around 3.0V. The protection circuit reads this as a dead cell and shuts the unit down, even though resting voltage is fine. Run two to three full charge-discharge cycles and the cell's internal resistance drops enough to handle the combined load without sagging.

SRL2 showing full charge but dropping off after a few minutes of use

This happens when the replacement cell arrives at storage voltage — around 3.6–3.8V — and the charger circuit terminates early because it misreads that as a near-full pack. The headset then starts from a partial state of charge rather than a true 100%. Seat the unit in the mount, confirm the charging LED is active, and allow a full uninterrupted charge to 4.2V before first use. That resets the charge curve and the firmware's capacity register.