Battery Loaded Voltage

Formula

Description

A battery under load delivers less voltage than its open-circuit voltage due to the internal resistance voltage drop. Internal resistance increases as the battery ages, discharges, or operates in cold temperatures. For lithium-ion cells, fresh internal resistance is typically 20-80 mΩ, rising to 200+ mΩ near end of life. At high discharge currents, the voltage sag can be significant and may cause the terminal voltage to drop below the minimum operating voltage of the connected circuit even when significant capacity remains.

Variables

• Vload — Terminal voltage under load (V)
• Voc — Open-circuit voltage (V)
• I — Load current (A)
• Rint — Battery internal resistance (Ω)

Practical Notes

Internal resistance can be measured by comparing voltage at two different load currents: Rint = (V1-V2)/(I2-I1). For high-current applications (power tools, EVs), low internal resistance is critical. Cell manufacturers specify AC impedance at 1 kHz and DC resistance separately. Battery management systems monitor internal resistance as a health indicator.