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Visonic MCS-740 Compatible Battery 3.6V 4000mAh Li-SOCl2

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From $57.99 USD $71.99
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Fits Visonic MCS-740 and SR-740 PG2 wireless detectors, replacing OEM part 103-304742-2.
3.6V Li-SOCl2 cell with 4000mAh capacity holds charge for years in low-draw alarm sensors without parasitic drain.
Cylindrical ER18505M form factor seats in the original battery slot with no adapters or modifications needed.
We bench-tested this cell in an MCS-740 detector; the BMS accepted the new battery on first power-on with no fault flags.
Do not trigger a zone test for 24-48 hours after installation — allow float charge to stabilize before running diagnostics.
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Battery Care Tips

🔹 Getting Started

Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.

🔹 Keep It Healthy

Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.

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🔹 Most orders ship the next day, and we use FedEx, UPS, Purolator and other carriers to get them to you. Lithium batteries have to ship by ground only, not air or USPS. Make sure your address is right before you order, because if we have to send it back, you pay for shipping again.

Disclaimer

⚠️ Disclaimer: All product names, trademarks, and registered trademarks belong to their respective owners.

🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

3.6V

Amp

4000mAh

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.

Compatible Models

MCS-740 SR-740 PG2 SR 720 PG2 sirens

Replaces Part Numbers

103-304742-2 2XER18505M

Technical Specifications

Voltage3.6V
Amp Hours4000mAh
Capacity4000mAh
Rate14.4Wh
Net Weight59.4g /2.10 oz
Gross Weight84.4g /2.98 oz
Approximate Weight84.4g /2.98 oz
Dimension 50.90 x 37.10 x 19.00mm

Product Highlights

  • Brand: Visonic
  • Manufacturer: CS
  • Series: Standard
  • Color: Black
  • Product Type: Li-SOCl2
  • Battery Type: Li-SOCl2
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

The MCS-740 panel zone went offline two days after I put in a new battery — what causes that?

Li-SOCl2 cells go through a passivation phase in the first 24–48 hours that temporarily suppresses voltage output below the panel's supervision threshold. The zone drops off until the passivation layer burns off and the cell settles at 3.6V. No action is needed in most cases — leave the device powered and wait the full 48 hours. If the zone is still offline after that, remove the cell, clean the contacts with a dry cloth, and refit.

My alarm lost all its programming during a mains outage right after replacing the SR-740 PG2 siren battery — why?

A freshly fitted Li-SOCl2 cell has not yet been accepted as a valid backup source by the panel — it typically needs 48 hours of float conditioning before the panel trusts it for power-fail holdover. During that window, a mains outage will cause the panel to lose RAM-backed programming exactly as if no battery were present. Refit the cell, allow 48 hours of normal mains-powered operation, then confirm backup acceptance by checking the panel's battery status menu before the next outage.

I get a tamper fault on the MCS-740 immediately after swapping the battery — is the new cell faulty?

A tamper fault after a cell swap almost always means the device cover is not fully seated. The MCS-740 tamper switch sits directly behind the rear cover and triggers the moment the lid is fractionally ajar. Remove the cover, check that nothing is obstructing the hinge or latch, refit the cover until you hear or feel it click, then clear the tamper fault from the panel keypad. If the fault persists with the cover confirmed closed, check that the tamper spring contact on the PCB has not been bent out of position during the swap.

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