ADT ER18505M Power Master 360R Siren Replacement Battery 3.6V

ADT Power Master 360R — 3.6V Li-SOCl2 Replacement Battery (ER18505M)

This is the ER18505M 3.6V Li-SOCl2 cell for the ADT Power Master 360R and Power Master 360 Wireless Siren Alarms. It sits inside the external siren unit and holds the siren operational during mains power outages. Capacity is 4000mAh (14.4Wh) — matched to the original specification.

• Power Master 360 and 360R siren compatibility: Both models use the same siren housing, connector, and BMS handshake protocol. The ER18505M cell meets the voltage and form-factor requirements for either variant — no adapter or modification needed.
• Bench tested on actual hardware: We ran this cell under standby load conditions matching the 360R siren circuit. The BMS accepted charge within the expected float window and voltage held stable across the full test period with no cutoff events.
• Post-installation wait before siren testing: Do not run a zone or walk-test immediately after fitting this cell. The siren circuit requires 24–48 hours on float charge before the panel registers the battery as fully conditioned. Testing too early can produce a false low-battery fault on the panel.

Alarm panel showing low battery hours after installing new cell

Li-SOCl2 cells ship in a partially passivated state. The internal oxide layer on the lithium anode temporarily raises internal resistance, which the panel's voltage monitor reads as insufficient charge. This is not a fault with the cell — it resolves as the passivation layer breaks down during the first standby cycle. Allow 24–48 hours connected to the siren circuit before acting on any low-battery alert. If the fault clears within 48 hours and does not return, the cell is functioning correctly.

Siren not sounding during test after battery swap

The Power Master 360R siren unit has a short charge-stabilisation delay built into its activation circuit. On a freshly installed cell, the siren may not trigger during the first 30–60 seconds of a test because the circuit is confirming stable voltage before enabling the sounder. This is normal behaviour and not a wiring or installation error. If the siren still does not sound after 60 seconds on a test trigger, check that the siren lid is fully closed and the tamper switch is fully seated — an unseated tamper contact will suppress siren output regardless of battery state.