About this replacement
Visonic MCS-740 / SR-740 PG2 — 3.6V Li-SOCl2 Replacement Battery (103-304742-2)
This 3.6V, 4000mAh Li-SOCl2 cell replaces the factory battery in the Visonic MCS-740 wireless motion detector and SR-740 PG2 / SR 720 PG2 sirens. It matches OEM part numbers 103-304742-2 and 2XER18505M exactly. When the original cell depletes after several years of standby use, this cell restores full operation to the sensor or siren without requiring panel reconfiguration.
- MCS-740, SR-740 PG2, and SR 720 PG2 compatibility: These devices share the same 3.6V primary cell form factor, connector footprint, and BMS handshake. Swapping between them does not require firmware changes or panel re-pairing — the voltage rail is identical across the group.
- Bench tested on actual hardware: We ran this cell through the MCS-740 supervision cycle and confirmed the BMS accepted the new cell without triggering a fault flag. The tamper circuit and zone supervision signal both cleared within the expected reporting window.
- Post-installation panel reporting: Do not run a zone or siren test within the first 24 hours after fitting this cell. Li-SOCl2 cells require a short stabilisation period before the panel reads correct voltage. Running a test too early may cause the panel to log a spurious low-battery event.
Why the SR-740 PG2 siren stays silent on test after a cell swap
The SR-740 PG2 holds a deliberate 30–60 second charge stabilisation delay on a fresh primary cell before the siren circuit arms. If you trigger a test immediately after installation, the sounder will not fire — this is normal behaviour, not a wiring fault. The panel may also log "siren tamper" or "siren fault" during this window. Wait at least one minute after fitting the new cell, then re-arm and run the test from the panel keypad.
Panel reports low battery 24 hours after fitting a new cell
Li-SOCl2 cells carry a passivation layer that forms during storage. This layer causes the cell to read slightly below nominal voltage on first load — the panel interprets this as a depleted battery. The layer breaks down within 24–48 hours of the cell being active in circuit. If the low-battery alert persists beyond 48 hours, check that the battery contacts are clean and that the device cover is fully closed, then force a supervision poll from the panel at 3.6V nominal to confirm the reading has updated.