How Should Charge and Discharge Cut-Off Voltages Be Understood?

Every lithium cell is designed around an upper and a lower voltage limit. Charging past the upper limit or discharging below the lower limit degrades the cell, so cut-off voltages are the boundaries of safe use.

The upper (charge) cut-off

Charging normally proceeds in constant-current then constant-voltage (CC-CV) mode. Once the cell reaches the charge cut-off, typically 4.2 V for standard cells and higher for LiHv cells, the charger holds that voltage until the current tapers off. Pushing beyond this point risks overcharging, lithium plating and gas generation.

The lower (discharge) cut-off

On discharge, the protection board disconnects when the cell falls to its discharge cut-off, commonly 2.5 to 3.0 V. This reserves a small margin and prevents deep over-discharge. Running below it repeatedly shortens cycle life.

Matching the system

The charger termination voltage, the protection board over-charge and over-discharge thresholds, and the cell specification must all agree. Deviating from the specified window to squeeze extra runtime is the most common cause of premature aging or swelling in custom packs, and the damage accumulates cycle after cycle.