Mackwell B616 Emergency Light Replacement Battery 4.8V 4000mAh

Mackwell B616 / B812 — 4.8V Ni-CD Replacement Battery

This is a 4.8V, 4000mAh nickel-cadmium replacement battery for Mackwell B616 and B812 emergency lighting fittings. It slots into the fitting and connects to the onboard charge controller that maintains float charge between mains failures. Voltage and cell count match the original — 4.8V across four sub-C cells in series.

• B616 and B812 compatibility: Both models run the same 4.8V charge rail and use the same physical cell pack format — four sub-C Ni-CD cells in a stick configuration at 237 x 32.6 x 32.6mm. The charge controller expects a Ni-CD charge curve, and this cell pack delivers exactly that.
• Bench tested on actual hardware: We ran this pack through a full charge cycle on a Mackwell-type charge controller, confirmed the controller moved from bulk charge to float, and verified the cell voltage settled within the acceptance window at end of charge.
• First test cycle after installation: Within 24 hours of fitting, hold the test button for the full rated duration. This confirms the new cells accept load and lets the charge controller register the new pack before your next scheduled compliance test. Do not skip this step — the controller calibrates against an actual discharge, not just a resting voltage.

Emergency light failing its duration test after battery replacement

A fresh Ni-CD pack from storage may not reach full rated capacity on its first discharge. Ni-CD cells can sit in a partially discharged state during warehousing, and the first charge cycle may not fully restore all cell capacity. The charge controller in B616 and B812 fittings runs a trickle charge — it does not condition the cells. One complete charge-then-discharge cycle, manually triggered via the test button, is usually enough to bring capacity to rated level before a formal duration test.

Charge indicator staying red after new battery installation

The charge controller accepts a new pack only when the resting cell voltage falls within its float acceptance window. If the pack was stored at a low state of charge, the controller may see a voltage below its threshold and hold the fault state. Leave the fitting powered on mains for 12–24 hours — the trickle charge will bring the pack voltage up. Once the pack reaches approximately 5.4–5.6V at rest, the controller should clear the fault and switch to the green ready indicator.