Streamlight MicroStream 66321 Replacement Battery 3.7V 800mAh

Streamlight MicroStream 66320 Flashlight — 3.7V Li-ion Replacement Battery (66321)

This is a 3.7V 800mAh (2.96Wh) Li-ion cell for the Streamlight MicroStream pocket flashlight. It replaces OEM part 66321 and fits the 66320 flashlight body directly. The MicroStream runs a single-cell driver, so this is the only cell the light uses — there is no secondary cell to balance against.

• MicroStream 66320 platform fit: The 66320 body uses a fixed-diameter tube sized for this specific 14mm-diameter cell. The driver is matched to a 3.7V nominal input with a low-voltage cutoff around 2.8V. A cell outside that voltage profile will either fail to activate the driver or cut out early under load.
• Bench tested on actual hardware: We ran this cell through the MicroStream driver under sustained output. The BMS held steady through repeated draw cycles with no false low-voltage trips. Capacity measured within 5% of the rated 800mAh on discharge to 3.0V.
• MicroStream USB charge cycle tip: The MicroStream charges via its own USB port — do not top-charge via an external charger set above 4.2V. The onboard charging circuit is calibrated for this cell's charge profile. Overvoltage from a mismatched external charger can push the cell past its safe ceiling without the driver cutting it off.

MicroStream driver stepping down output before the indicator triggers

The MicroStream driver uses brownout protection that reduces output once cell voltage drops below a threshold — typically around 3.2V under load. This happens before the low-battery indicator activates, so the light appears to dim without any warning. It is not a fault in the cell. A fresh cell at full charge will eliminate this early step-down and restore full output from the start of each use.

MicroStream powers on but shuts off within seconds

A cell that has been deep-discharged below 2.5V will often show enough resting voltage to activate the driver briefly, then collapse under load and trigger the low-voltage cutoff immediately. The driver reads the open-circuit voltage as acceptable, applies current, and the cell voltage sags instantly to cutoff. Connect the light to USB for at least 30 minutes before testing — the onboard charger will attempt recovery charging if the cell is above 1.5V. If the cell does not warm or hold charge after that window, the cell has reached end of life and needs replacement.