Volvo 30659883 Alarm Siren Replacement Battery 2.4V 700mAh

VOLVO FOMOCO Alarm Siren — 2.4V Ni-MH Replacement Battery (30659883)

This is a 2.4V, 700mAh Ni-MH rechargeable battery for the Volvo FOMOCO Alarm Siren. It fits the siren unit used across the S60, V60, S80, and XC60 (2008–2018) platforms. When the original cell depletes, the alarm loses its ability to sound independently of the vehicle — this battery restores that standalone sounder function.

• S60, V60, S80, XC60 siren compatibility: These models share the same FOMOCO siren housing and internal cell specification across the 2008–2018 production run. The 2.4V two-cell Ni-MH configuration matches the original charge circuit in the siren module — no wiring changes needed.
• Bench tested on actual hardware: We cycled this cell through the siren's charge and discharge profile. The BMS accepted charge at the correct rate and the cell held voltage at the activation threshold required to trigger the sounder circuit.
• Post-install cover check: After swapping this battery, close the siren cover completely and verify the tamper switch clicks into place. The Volvo alarm ECU reads an open tamper contact as a fault — some panels flag this as a battery error, which causes unnecessary further diagnosis.

Why the FOMOCO siren goes silent after sitting unused for an extended period

Ni-MH cells self-discharge at a higher rate than lithium chemistry — typically 1–3% of capacity per day at room temperature. In an outdoor siren exposed to seasonal temperature swings, that rate climbs further. A cell left uncharged for several months can drop below the minimum voltage the sounder circuit needs to activate. Reconnecting the siren to the vehicle allows the alarm ECU to trickle-charge the cell back up — allow at least 24 hours before testing.

Siren activates briefly during alarm test then cuts out before completing the cycle

This points to a cell that cannot sustain output current for the full siren activation duration. Ni-MH cells that have experienced repeated deep discharge or prolonged storage lose internal capacity even if they accept a surface charge. The voltage appears acceptable at rest but collapses under the current draw of the sounder. Replace the cell and allow a full charge cycle — rest voltage after charging should measure at or above 2.4V before testing.