Load testing reveals how much the pack voltage falls when current flows. Measure an open-circuit voltage, apply a known stable load, record the loaded voltage quickly, then calculate R ≈ (Vopen − Vload) ÷ (Iload − Irest). This is an approximate DC resistance, not a laboratory impedance measurement.
Why results vary
Temperature, state of charge, current ramp, cable resistance, connector resistance and meter timing all change the result. Compare packs at the same state of charge and temperature. A weak cell group can cause disproportionate sag even when the pack’s average voltage looks normal.
Stop if a pack becomes hot, unstable, swollen or emits an unusual smell. Never short a battery to “test” it.