About this replacement
Sena SF2 / SF4 — 3.7V Li-Polymer Replacement Battery (AHB802540)
This is a 3.7V, 800mAh Li-Polymer cell built to the AHB802540 specification. It fits the Sena SF2 and SF4 Bluetooth motorcycle helmet headsets. Swap it when the original cell no longer holds a charge or the headset shuts down mid-ride.
- SF2 and SF4 compatibility: Both models share the same battery bay, connector pinout, and BMS handshake protocol. One cell covers both headsets without any hardware modification.
- Bench tested on actual hardware: We cycled this cell through the SF2 charging circuit and monitored BMS communication. The protection circuit engaged correctly at low-voltage cutoff and the base station reported a full charge state after a complete cycle.
- First charge after installation: Place the headset in the base station and let it run a complete charge cycle before pairing or taking a call. The SF2 base needs to complete this cycle to calibrate its talk-time indicator — skipping it causes the display to misreport remaining charge from the first use.
Why the SF2 cuts out mid-call even with a new battery installed
The SF2 draws current from two loads simultaneously — the audio amplifier and the Bluetooth radio. Under combined load, a cell sitting at storage voltage can sag below the BMS cutoff threshold briefly, triggering a shutdown. This is not a faulty cell. Run two or three full charge-discharge cycles through normal use and the resting voltage stabilises above the cutoff point. If drop-outs continue after three cycles, check the connector seating — the SF2 bay is shallow and a partially seated cell creates resistance that worsens voltage sag.
Base station shows charging error after fitting a replacement cell
The SF2 base station performs a BMS handshake at the start of every charge session. A new cell shipped at storage voltage (typically 3.6–3.7V) can cause the base to flag an error before the handshake completes. Remove the headset from the base, wait ten seconds, and reseat it firmly. If the error persists, connect the base to power first, then place the headset — this sequence lets the base initialise its charge controller before attempting communication with the new cell.