Nightstick 5566 Replacement Battery 3.7V 3400mAh

Nightstick XPP-5566 / XPR-5568 Series — 3.7V Li-ion Replacement Battery (5568-BATT)

This is a 3.7V, 3400mAh Li-ion replacement for the original 5568-BATT cell used in Nightstick tactical flashlights. It fits the 5566, 5568, XPP-5566, and XPR-5568 models. Swap it in when the original cell no longer holds a full charge and the light starts stepping down output earlier than it should.

• 5566 and 5568 platform fit: These four models share the same battery bay dimensions, connector, and 3.7V single-cell driver circuit. One cell format covers the entire platform — no adapter or wiring change needed.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the XPP-5566 driver. The BMS accepted the charge handshake cleanly, held full capacity through turbo-mode draw, and tripped the protection circuit at the correct low-voltage threshold without drop-out errors.
• Single-cell replacement note: This is a single-cell light. When the cell is replaced, condition it with two or three full charge cycles before relying on it for duty use — the driver's fuel gauge calibrates itself against actual cell capacity over those first cycles, so the output step-down timing becomes accurate.

Why the XPP-5566 steps down to a lower mode before the indicator shows low

The 5566 driver uses brownout protection — it monitors cell voltage in real time, not just state of charge. Under turbo-mode draw, internal resistance in an aged or depleted cell causes voltage to sag below the driver's threshold even when the indicator still shows partial charge. The driver steps output down to protect the cell and maintain stable light output. This is normal behaviour on a fatigued cell. A fresh cell at full capacity holds voltage above the sag threshold longer, so the step-down happens much later in the discharge cycle.

Flashlight cycling through modes or flickering at low charge

If the light starts switching modes on its own or flickering near end of charge, the driver is hitting its brownout threshold repeatedly — voltage dips below the cutoff, the driver resets, voltage recovers slightly, and the cycle repeats. This is not a fault in the driver or the switch. It means cell voltage is at or below 3.0V under load. Switch to a lower output mode immediately or charge the cell — do not leave it cycling, as repeated BMS trips at the low-voltage boundary accelerate cell degradation. Charge before cell voltage drops below 3.0V under no-load measurement.