Nightstick 5566 Replacement Battery 3.7V 2600mAh

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

This is a 3.7V Li-ion replacement battery for the Nightstick 5566, 5568, XPP-5566, and XPR-5568 tactical flashlights. It carries 2600mAh (9.62Wh) and uses OEM part number 5568-BATT. Swap it in when the original cell no longer holds a charge or output has dropped off noticeably.

• 5566 and 5568 platform fit: The 5566, XPP-5566, 5568, and XPR-5568 all share the same battery bay geometry, connector pinout, and 3.7V cell voltage — so one cell covers the full model group without modification.
• Bench tested on actual hardware: We cycled this cell through multiple charge and discharge runs on the 5566 platform. The BMS accepted the charge curve without fault flags, and the protection circuit tripped cleanly at the low-voltage cutoff threshold.
• Turbo-mode draw on single-cell lights: The 5566 runs a single Li-ion cell, so turbo mode pulls the full current demand from one source. Avoid leaving the light in turbo for extended continuous runs — the driver will step down output as cell voltage sags under sustained high-current draw, which is normal protection behaviour, not a fault.

Flashlight driver stepping down output before the battery indicator shows low

The 5566's driver monitors cell voltage directly, not a separate fuel gauge. Under high-current draw in turbo mode, internal resistance in an aging cell causes the terminal voltage to sag below the driver's brownout threshold even while the indicator still shows partial charge. The driver interprets this as a low-voltage condition and steps output down to protect the cell. Replacing the cell restores the voltage headroom the driver needs to sustain full output.

New battery not charging after fitting into the 5566

If the charger shows no activity after fitting a new cell, the cell may have arrived at a storage voltage below the charger's detection threshold — typically under 2.5V. Some chargers for this platform require the cell to present above that threshold before initiating a charge cycle. Try a compatible Li-ion charger with a recovery or trickle-charge mode to bring the cell up to around 3.0V, at which point the standard charge cycle should begin normally.