Voltage Sag

Where it comes from

Every pack has internal resistance: in the cells, in the welded joints, in the nickel strip, in the BMS, and in the connectors. Push current through resistance and you lose voltage across it.

The relationship is direct — the drop is the current multiplied by the total internal resistance. Draw 30A through a pack with 50 milliohms of internal resistance and you lose 1.5V before the electricity reaches the controller.

The energy does not vanish; it becomes heat inside the pack, which is why hard-working packs run warm.

What it looks like to a rider

Sag is the most commonly misdiagnosed battery behaviour in the field.

A rider starts a climb, the display drops two bars, and at the top they stop and the bars come back. Nothing is faulty. Under the extra current the pack sagged, and any fuel gauge inferring state of charge from voltage read the sag as lost capacity.

The more serious version happens at low charge. A pack that is genuinely near empty is already at a low resting voltage; a burst of current sags it past the BMS cutoff and the pack shuts off. The rider reports “it died suddenly with 20% showing”.

What increases and reduces it

Sag grows with current draw, with cold — internal resistance rises sharply at low temperature, which is why winter range disappoints — and with age, since resistance climbs as cells wear.

It shrinks with a higher parallel count, because the current divides across more cells, and with low-resistance cells, good welds, and adequate wire and connector sizing.

Sizing around it

Tell us your peak current, not just your continuous draw, and mention cold-weather markets explicitly. Sizing a pack on continuous current alone produces one that sags into cutoff on the first hard start of a winter morning — a fault that will be reported as a defect and is actually a specification error.