Honeywell C300 Replacement Battery 3.6V 4500mAh

Honeywell C300 — 3.6V Ni-MH Replacement Battery (51199942-300)

This 3.6V, 4500mAh Ni-MH cell replaces the backup battery in the Honeywell C300 security control panel. It keeps the panel running during mains power failures, maintaining alarm monitoring and communication with connected centres. OEM part numbers 51199942-300, CC-SCMB02, and 143553 all cross-reference this cell.

• C300 panel compatibility: The C300 control unit uses a fixed 3.6V Ni-MH chemistry with a specific connector pinout and BMS charge profile. Swapping to a different voltage or chemistry will cause the panel to reject the battery or report a fault on the next diagnostic cycle.
• Bench tested on actual hardware: We ran this cell through a full charge cycle on a C300 panel and monitored the BMS handshake. The panel accepted the battery, entered float charge without error codes, and held voltage within the expected 3.6V nominal band under simulated load.
• Post-installation charge window: Do not run a zone or siren test immediately after fitting this cell. The C300 panel needs 24–48 hours on float charge before the BMS reports battery status as full. Testing too early triggers a false low-battery warning that clears on its own once float charge completes.

Why the C300 panel reports low battery immediately after a new cell is fitted

The C300 BMS does not read the physical cell voltage directly on first contact. It waits for the battery to complete an initial float charge cycle before updating its status register. A freshly installed cell — even one at correct voltage — will sit below the BMS threshold until that cycle runs. This is normal behaviour, not a fault with the cell or the panel. Leave the panel on mains power for 24–48 hours and the warning will clear without any further action.

Alarm panel loses programming during a power outage after battery replacement

If the panel drops its programming when mains power fails, the backup battery has not yet been accepted by the BMS — it is not yet supplying load current. This happens when the cell is installed but the 48-hour conditioning window has not passed. The panel draws from the backup battery only after the BMS confirms the cell has reached float charge. To protect programming before that window closes, avoid switching off mains power in the first 48 hours after fitting the new cell.