Battery Voltage Sag

Formula

Description

Voltage sag is the drop in battery terminal voltage under load, caused by current flowing through the internal resistance. Combining the C-rate concept with the internal resistance model: since I = Crate × Q, the voltage drop is Crate × Q × Rint. This is critical for determining whether a battery can maintain the minimum operating voltage of the load at the required current. High-drain applications (power tools, drones, EVs) require low-impedance cells to minimize sag. Voltage sag is worst at low SoC and low temperature when internal resistance is highest.

Variables

• V_oc — Open-circuit voltage at current SoC (V)
• C_rate — Discharge C-rate (dimensionless)
• Q — Battery capacity (Ah)
• R_int — Internal resistance (Ω)

Practical Notes

Example: A 3.7 V, 3 Ah cell with 50 mΩ internal resistance at 3C: Vsag = 3.7 − 3 × 3 × 0.05 = 3.7 − 0.45 = 3.25 V. If the load cutoff is 3.0 V, this leaves only 0.25 V margin. At 5C (15 A), the sag would be 0.75 V, dropping to 2.95 V — below cutoff. This calculation is essential for selecting cells that can support the required discharge rates.