Nightstick 9844-BATT Flashlight Replacement Battery 3.7V 3400mAh

Nightstick NSR-9844XL — 3.7V Li-ion Replacement Battery (9844-BATT)

This is a 3.7V, 3400mAh Li-ion replacement battery with OEM part number 9844-BATT. It fits the Nightstick NSR-9844XL rechargeable tactical flashlight and several related models in the NSP-9842XL and USB-578XL lines. Capacity figure is taken directly from product data — 12.58Wh at 3.7V nominal.

• Cross-model fit — NSR-9844XL, NSP-9842XL, USB-578XL series: These models share the same single-cell 18650-format Li-ion platform, voltage rail, and driver circuit. The BMS handshake and connector pinout are consistent across the group, so one cell covers the full compatibility list.
• Bench tested on actual hardware: We cycled this battery through charge and discharge on the NSR-9844XL driver. The BMS held cutoff correctly at low-voltage threshold and accepted a full charge without thermal event or protection trip. Cell voltage at full charge measured 4.19V — within spec.
• Turbo mode draw management: The NSR-9844XL turbo mode pulls significantly higher current than standard output modes. Avoid leaving the light in turbo while stationary — sustained high-current draw without airflow over the head raises cell temperature and accelerates capacity loss over repeated cycles.

Flashlight driver stepping down output before the indicator shows low

The NSR-9844XL driver monitors cell voltage continuously. When voltage sags under high-current draw — even briefly — the driver steps output down to protect the cell before the low-battery indicator triggers. This is brownout protection, not a fault. The indicator lags because it reads resting voltage, not the voltage under load. If the light dims suddenly during turbo use but recovers on a lower mode, the cell is at the bottom of its usable voltage curve — typically below 3.5V under load.

Battery showing full charge but delivering noticeably less output than when new

Li-ion cells lose capacity gradually with each charge cycle — this is normal chemistry, not a defective cell. A cell that reads 4.2V fully charged can still have shed 15–20% of its original 3400mAh after 300–500 cycles. The flashlight driver does not compensate for reduced capacity, so output duration drops even though the charge indicator reads full. If charge-to-dim time has shortened significantly, replace the battery and confirm the new cell reaches 4.19–4.20V at end of charge.