The Voltage Sag Calculator determines how much your eBike battery voltage drops when current flows under load. This voltage drop is caused by internal resistance inside the battery cells and wiring. If voltage sag becomes too high, the controller may trigger low-voltage cutoff, which suddenly turns the motor off even when you still have usable charge remaining.
Voltage sag happens when current passes through a battery with internal resistance, causing an internal voltage drop equal to I × R. The more current drawn, the more voltage drops. This effect becomes very noticeable during acceleration, hill climbing, heavy load, or when battery health is degrading.
As lithium-ion cells age, their internal resistance increases. Higher IR means more voltage sag under load, more heat, lower efficiency, and reduced overall power delivery from the battery. Measuring internal resistance is one of the best ways to estimate battery aging and performance loss.
When your battery voltage sags, the motor temporarily receives lower voltage, resulting in reduced power output, limited speed, and potential motor cut-offs. You may feel sudden loss of power especially on steep hills, at high throttle, or while carrying heavy loads.