Fischer NLK1U003SC Replacement Battery 4.8V 600mAh Ni-MH

Fischer NLK1U003SC Series — 4.8V Ni-MH Replacement Battery (NLAKKU4808)

This is a 4.8V, 600mAh Ni-MH rechargeable battery for Fischer emergency lighting units. It fits the NLK1U003SC, NLK2U003SC, NLK3U003SC, and NLS1D003SC series fittings. When the original cell degrades, the fitting either fails its duration test or stops activating on mains failure — this cell restores both functions.

• NLK and NLS series compatibility: These fittings share the same 4.8V charge rail, connector footprint, and trickle-charge controller logic. The BMS handshake expects a Ni-MH cell in the 4.8V nominal range — substituting a different chemistry causes the charge controller to misread cell state and flag a permanent fault LED.
• Bench tested on actual hardware: We cycled this cell through a full mains-failure simulation on an NLK1U003SC fitting. The charge controller accepted the cell within two minutes, the fault LED cleared, and the cell held load voltage above the fitting's cutoff threshold throughout the full test duration.
• First-cycle activation on emergency fittings: Within 24 hours of installation, hold the test button for the full rated duration. This forces one complete discharge-to-cutoff cycle, which allows the charge controller to calibrate its float charge register to the new cell. Without this step, the controller may undercharge the cell before the next scheduled compliance test.

Charge indicator staying red after new battery installation

The NLK series charge controller uses open-circuit voltage to determine whether to enter float charge mode. A cell that has sat in storage for several months will present a lower resting voltage than the controller's float acceptance threshold — typically around 5.4V for a fully charged 4.8V Ni-MH pack. When the cell voltage is too low, the controller locks in a fault state and holds the red LED. Leave the fitting connected to mains for 12–16 hours without triggering the test button. The controller runs a slow trickle pre-charge below the float threshold, and the LED should transition to green once the cell crosses approximately 5.2V.

Emergency light dims and cuts out partway through a duration test

This happens when the cell has not completed a full conditioning cycle after installation. The fitting activates and draws load current, but the cell's available capacity is below the 600mAh rated figure because it has not been fully charged from its storage state. Voltage sags under load, and the fitting's low-voltage cutoff trips before the rated duration is reached. Run the fitting on mains charge for a full 24 hours after installation, then perform the manual test — this gives the charge controller enough time to bring the cell to a true full-charge state before you verify duration.