Nightstick XPR-5580 Replacement Battery 3.7V 5200mAh

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

This is the OEM-equivalent 3.7V Li-ion cell for the Nightstick XPR-5580, XPR-5581RX, and XPR-5542GMX tactical flashlights. It matches the original 5500-BATT specification at 5200mAh (19.24Wh). These lights are used by law enforcement and emergency responders, so a weak cell is not a minor inconvenience.

• XPR-5580 series fit: All three listed models share the same battery bay geometry, cell voltage, and driver communication protocol. The flashlight driver reads cell state directly — wrong voltage or a mismatched BMS response will cause the driver to default to a reduced output mode before the battery indicator shows low.
• Bench tested on actual hardware: We cycled this cell through the XPR-5580's full output modes including turbo. The BMS held the 3.7V nominal rail without false low-voltage cutoff during high-current draw spikes, and the protection circuit triggered correctly at the lower discharge threshold.
• Turbo mode storage caution: Storing this light after extended turbo sessions with the cell still warm accelerates electrolyte stress. Let the cell return to ambient temperature before capping and storing the light — turbo draw generates more internal heat than standard or low modes.

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

The XPR-5580 driver monitors cell voltage in real time and applies brownout protection before the fuel gauge threshold triggers. As the cell voltage sags under high-current draw — especially in turbo — the driver steps output down to protect the cell and maintain a stable arc. This is normal circuit behaviour, not a fault. A degraded or partially discharged cell will trigger this step-down earlier and more frequently. Replacing the cell with a fresh 5200mAh unit restores the full output window before brownout kicks in.

Flashlight driver cycling through modes or flickering at end of charge

If the XPR-5580 begins rapidly cycling modes or flickering near the end of a charge, the driver is hitting the brownout threshold repeatedly as the cell voltage oscillates under load. This is distinct from a dead cell — the battery still has charge, but voltage is dropping below the driver's minimum under draw and recovering at rest. Switch to a lower output mode immediately to stop the cycling and draw down the remaining charge safely. Once recharged with a healthy cell, this behaviour stops; if it continues on a fresh charge, check the charger contact pins for corrosion.