The Battery Internal Resistance Calculator lets you quickly estimate the internal resistance of a lithium eBike battery using measured no-load and load voltages. Internal resistance (IR) affects performance, peak current capability, efficiency and heat production when the battery is under load. Higher IR causes more voltage sag, meaning less available power under acceleration.
Internal resistance refers to the inherent electrical resistance inside a battery pack. Every lithium cell has internal impedance made of ionic resistance, contact resistance, separator resistance and connection losses. As a battery ages, resistance increases due to chemical degradation and electrode wear.
Higher internal resistance causes voltage to drop more when the battery delivers current. This voltage drop reduces peak power and acceleration, especially at high loads. As a battery ages or overheats, IR increases, which is why older batteries suffer weaker performance even when fully charged.
Lithium internal resistance increases with cell aging, temperature cycling, calendar aging and deep discharge stress. Cells stored at high temperature develop higher internal impedance. Daily high current usage also increases IR faster than shallow discharge operation.
A healthy modern eBike battery pack may show internal resistance between 40–120 mΩ depending on quality and chemistry. Values above 200 mΩ typically indicate noticeable performance loss.