Nightstick XPR-5580 Replacement Battery 3.7V 6800mAh

Nightstick XPR-5580 / XPR-5581RX / XPR-5542GMX — 3.7V Li-ion Replacement Battery (5500-BATT)

This is a 3.7V 6800mAh lithium-ion replacement battery for the Nightstick XPR-5580, XPR-5581RX, and XPR-5542GMX tactical flashlights. It uses OEM part number 5500-BATT and fits the cylindrical single-cell format these lights require. Capacity is 25.16Wh — matched to the product data, not estimated.

• XPR-5580 / XPR-5581RX / XPR-5542GMX platform fit: All three models share the same cell format, voltage rail, and driver interface. The 5500-BATT cell slots into each without modification — same connector, same BMS voltage thresholds, same physical dimensions at 139.30 × 24.00 × 24.00mm.
• Bench tested on actual hardware: We ran this cell through full charge-discharge cycles on the XPR-5580 driver board. The BMS held cutoff at the correct low-voltage threshold and did not trip prematurely during turbo-mode draw spikes.
• Turbo mode draw and cell stress: Turbo mode on the XPR-5580 pulls significantly higher current than standard or low modes. Keep the light body clear of obstructions during extended turbo use — heat buildup at the head transfers back to the cell and accelerates capacity fade over time.

Why the XPR-5580 steps down output before the battery indicator reads low

The XPR-5580 driver uses brownout protection that reduces lumen output before the cell actually reaches low-voltage cutoff. This is intentional — the driver detects voltage sag under load, not resting voltage, so it steps down to protect the cell under high-current draw. The battery indicator reads the resting voltage, which still looks acceptable. If you see output step down on turbo or high mode, switch to a lower mode — the cell still has usable charge, just not enough headroom to sustain peak draw.

Flashlight cycling through modes or flickering near end of charge

Mode-cycling or rapid flicker near the end of a charge cycle is the driver repeatedly tripping and recovering as cell voltage dips below the brownout threshold under load, then recovers at rest. This is not a faulty battery or driver — it signals the cell is nearly depleted. Stop using the light and recharge immediately. A healthy replacement cell should show a resting voltage of 3.6–3.7V off the charger before first use — verify with a multimeter if the light exhibits this behaviour from day one.