Amp-Hour (Ah)

What it measures

An amp-hour is one amp sustained for one hour. A 20Ah pack nominally supplies 20A for an hour, 10A for two hours, or 40A for half an hour.

It describes charge, not energy. That distinction is the source of the most common comparison error in this trade: amp-hours only compare meaningfully at identical voltage. A 36V 15Ah pack and a 48V 15Ah pack carry the same charge and very different energy — 540Wh against 720Wh. Use watt-hours whenever voltages differ.

The rating is conditional

A capacity figure is measured under stated conditions, and real conditions reduce it.

Rated capacity is typically measured at a modest discharge rate and room temperature. Discharge faster and you get less — the effect is real and well documented. Discharge cold and you get considerably less, because internal resistance rises and the pack hits its cutoff voltage sooner with charge still chemically present.

So a 20Ah pack delivering 16Ah on a February morning at high assist is not defective. It is a 20Ah pack measured under conditions nobody rides in.

Where the number comes from in a pack

Capacity is set by cell capacity multiplied by parallel count. Five 4Ah cells in parallel make a 20Ah group; thirteen such groups in series make a 48V 20Ah pack.

This is why capacity and current capability move together, and why you cannot order a small pack with a large sustained draw.

What to specify, and what to check

State the capacity you need and the voltage in the same breath. Then check what conditions a supplier’s figure was measured at, and whether the number is nominal or minimum — a “20Ah” pack that measures 19.2Ah on arrival may be within tolerance or may be optimistic labelling, and only the stated tolerance tells you which.

Our production packs are capacity-verified per unit, with a final check required to fall within ±3% of specification.