Action HDMax Extreme 3.7V Replacement Battery 1050mAh Li-ion

Action HDMax Extreme — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1050mAh (3.89Wh) lithium-ion battery for the Action HDMax Extreme action camera. It fits directly into the HDMax Extreme body and powers both video recording and playback. No OEM part number is published for this cell.

• HDMax Extreme fit: The HDMax Extreme uses a compact 40.20 x 40.90 x 6.50mm cell with a 3.7V nominal rail. This replacement matches that footprint and voltage exactly, so the camera's internal BMS receives the correct charge parameters without modification.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the bench. The BMS accepted the cell, held charge termination at the correct upper threshold, and showed no fault flags across multiple cycles.
• First-cycle initialisation: On first install, run one complete charge cycle through the camera body or OEM charger before heavy shooting. Some action camera BMS systems need that initial cycle to calibrate the battery-remaining indicator accurately against the new cell's discharge curve.

HDMax Extreme showing dead battery indicator on a partially charged replacement cell

The HDMax Extreme maps its battery indicator to a specific voltage-threshold curve stored in the camera firmware. A new replacement cell may sit at a resting voltage that the firmware reads as critically low even when the cell is not discharged. This happens because the open-circuit voltage of a fresh lithium-ion cell can settle between threshold bands after shipping. One full charge cycle from inside the camera body resets the indicator's reference point and resolves the false-empty reading.

Battery percentage jumping erratically during recording on the HDMax Extreme

Erratic percentage readouts during video capture usually point to a mismatch between the cell's discharge curve and the camera's voltage-to-percentage mapping table. The HDMax Extreme draws current in bursts — sensor, processor, and storage writes all spike simultaneously — which causes instantaneous voltage sag the BMS misreads as a step-drop in capacity. Completing two full charge-discharge cycles typically allows the firmware to recalibrate. If the jumping persists beyond three cycles, check that the cell is seated flush and the contacts are clean — even a 0.1V drop from a dirty contact will skew the reading.