Voltage sag

When you demand power (acceleration, hill climb), battery voltage temporarily drops. When you stop demanding, it recovers. This isn’t the battery losing charge - it’s the battery’s internal resistance causing a temporary voltage dip under load.

Why it matters

Most EUC apps estimate battery percentage from voltage. Voltage sag makes that estimate wildly inaccurate during hard riding. You might see 40% at cruise, 25% when climbing, and 35% when you stop - all within seconds.

The safety implication

Safety margin and alarms are often voltage-based too. Voltage sag means your actual margin under load is thinner than what the display shows between efforts. The beeps that fire at 30% battery on flat ground might fire at 45% on a steep hill.

555 take

Voltage sag is why battery percentage is a lie under load. The number bouncing around isn’t your battery breaking - it’s physics. But it means you need more conservatism at lower SoC than the display suggests. The range article and battery article show how sag turns into real-world distance and safety margin.