Honeywell Lynx 7.2V Replacement Battery 300-03866

Honeywell Lynx Series — 7.2V Ni-MH Replacement Battery (300-03866)

This is a 7.2V 3700mAh Ni-MH backup battery for the Honeywell Lynx alarm panel series. It fits the Lynx, Lynx 5100, Lynx 5200, and Lynx 5210 control panels, among others. When mains power drops, this cell keeps the panel armed and communicating until power returns.

• Lynx panel family compatibility: The Lynx, 5100, 5200, and 5210 all share the same 7.2V battery bay, connector pinout, and charge management circuit. One cell covers the full range without any wiring modification.
• Bench tested on actual hardware: We cycled this cell through the Lynx charge controller and confirmed the BMS accepted the cell without fault, held float charge correctly, and cleared the low-battery condition within 48 hours of initial installation.
• Post-installation panel settling: Do not run a zone diagnostic immediately after swapping the battery. Allow the new cell 24–48 hours on float charge first. The Lynx charge controller needs that window to accept the cell; triggering a test before it completes will cause the panel to flag a false low-battery fault.

Alarm panel showing low battery hours after installing a new cell

The Lynx charge controller does not immediately recognise a fresh Ni-MH cell as fully charged. It runs the cell through its own charge acceptance check, which takes 24–48 hours to complete. Until that cycle finishes, the panel will continue to report low battery — even if the cell voltage reads 7.2V or above on a meter. Leave the panel on mains power and do not clear the fault manually; it will self-clear once the charge controller is satisfied. If the fault persists past 48 hours, confirm the battery connector is fully seated and the terminal voltage under load reads above 6.8V.

Alarm losing programming during a mains outage after battery replacement

If the panel loses its zone programming when mains drops, the backup battery has not yet been accepted by the charge circuit. A newly installed Ni-MH cell needs a full 48-hour conditioning period on mains power before it can sustain the panel during an outage. Installing a new battery and then immediately cutting mains power — or having an outage in that window — will result in the panel dropping to an uncharged cell and losing volatile memory. After replacement, keep the panel on mains for at least 48 hours before testing backup operation. Verify the battery voltage under a brief load reads no lower than 7.0V before treating the conditioning cycle as complete.