🔋 Voltage Sag
The temporary drop in a battery\x27s voltage while it is under heavy load, which recovers when you let off the throttle.
Sag worsens as a pack ages, gets cold, or as its internal resistance climbs. Heavy sag is usually what trips the ESC\x27s low-voltage cutoff part-way through a run.
What it actually does
Voltage sag is the dip in pack voltage that happens the moment you demand a lot of current. Every pack sags; the question is how much. It is what you feel as a car that leaps off the line when fresh and goes soft as the run goes on, and it is the honest measure of a pack's quality.
In practice
Sag comes from the pack's internal resistance, so it gets worse as a pack ages, as it gets cold, and as it empties. A pack that sags badly will trip the ESC's low-voltage cutoff early even though it still has capacity left, which feels like the pack "dying" mid-run.
Worth knowing
If a pack that used to be fine now cuts out early, measure its internal resistance rather than assuming the ESC is faulty. Rising IR is how packs tell you they are done.