Streamlight Stylus Pro COB 66143 3.7V Replacement Battery

Streamlight Stylus Pro COB / Stylus Pro USB — 3.7V Li-ion Replacement Battery (66143)

This 3.7V 640mAh Li-ion cell replaces the factory battery in the Streamlight Stylus Pro COB and Stylus Pro USB flashlights. Both lights run a single-cell configuration, so this is a direct swap once the original cell degrades. Capacity figure is 640mAh (2.37Wh) — use that number when comparing against third-party cells.

• Stylus Pro COB and USB compatibility: Both models share the same single-cell bay, voltage rail, and driver board. The COB uses a chip-on-board emitter array; the USB variant adds an onboard charging circuit. Neither requires BMS handshake authentication, so the cell seats and powers up without firmware intervention.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the Stylus Pro COB driver. The BMS held cutoff at 2.75V under load and accepted a full charge termination at 4.2V without deviation across three consecutive cycles.
• USB model charging port caution: On the Stylus Pro USB, charge the battery inside the light via the built-in micro-USB port only. Charging the bare cell externally on a generic charger bypasses the light's onboard protection circuit and can push voltage past the cell's rated ceiling.

Stylus Pro COB dimming before the battery reads empty

The COB driver uses brownout protection that steps output down before the cell actually hits cutoff. This is intentional — the driver reduces current draw to keep voltage above 3.0V and protect the cell from a hard drop. Users often see noticeable dimming at around 30–40% of remaining capacity. Stepping down to a lower output mode at that point lets the driver stabilise and extends usable light before the cell fully depletes.

Replacement cell not reaching full brightness after first charge

A new Li-ion cell sometimes shows reduced output on the first one or two cycles because the electrodes haven't reached full ionic mobility yet. This isn't a faulty cell — it's normal formation behaviour. Run two full charge-discharge cycles and output should normalise. If brightness is still noticeably low after the second cycle, check resting cell voltage with a multimeter: a healthy fully-charged cell should read between 4.15V and 4.20V.