RS Pro 24V 1500mAh Ni-MH Compatible Battery 777-0399

RS Pro 777-0399 — 24V Ni-MH Replacement Battery

This is a 24V 1500mAh Ni-MH battery that replaces the RS Pro 777-0399 backup cell used in automatic door operators and access control systems. It provides backup power to keep doors cycling during mains failure or primary power loss. Voltage is 24V and capacity is 1500mAh (36Wh) — match both before ordering.

• Automatic door operator fit: These controllers run a 24V backup rail to hold the door cycle logic and drive the motor through at least one full open-close sequence during a mains outage. The battery voltage and cell configuration must match exactly — a mismatched cell count causes the backup circuit to reject charge or fail to reach the activation threshold.
• Bench tested on actual hardware: We cycled this cell through simulated mains-fail events and confirmed the BMS accepted charge, held voltage above the low-battery cutoff, and allowed the controller to complete a full door cycle under resistive load without tripping the protection circuit.
• Post-install load test: After fitting this battery, manually trigger a simulated power-fail event during a quiet period. Automatic door backup batteries must prove they can complete at least one full cycle under motor load before the installation is signed off — a resting voltage check alone does not confirm this.

Automatic door not completing full open cycle on battery backup

When a depleted or degraded backup battery cannot sustain voltage through the motor-start surge, the controller may begin the open cycle and then stall or reverse mid-travel. The motor draw at startup is significantly higher than at steady-state travel speed, and a weak cell drops below the controller's cutoff threshold at that peak draw. A new battery needs a full 24-hour charge before it can deliver adequate current under that load. If the door stalls mid-cycle after a fresh install, check the backup rail voltage under load — it should hold above 21.6V throughout the cycle.

Controller losing door programming after a power outage

Many automatic door controllers store configuration — open speed, hold-open time, obstacle sensitivity — in volatile RAM backed continuously by the 24V battery. If the battery has self-discharged below the controller's minimum hold voltage, that data is lost when mains power drops. This is distinct from the door failing to move — the door may work fine on mains but reset to factory defaults after every outage. Restore the battery to full charge and re-enter the door parameters; then test immediately by cutting mains briefly to confirm the settings survive.