What is the Difference Between Battery Capacity and Energy Density?

Battery capacity, measured in mAh or Ah, describes how much electric charge a cell can store and deliver during a discharge. Energy density, measured in Wh/kg (gravimetric) or Wh/L (volumetric), describes how much energy is packed into a unit of weight or volume. The two are related but never interchangeable.

From mAh to Wh

Capacity alone tells little about runtime. Energy (Wh) equals capacity multiplied by nominal voltage: a 3.7 V, 2000 mAh cell holds about 7.4 Wh. Two cells rated at the same mAh can therefore deliver very different usable energy if their voltage differs, which is why a high-voltage chemistry often wins on runtime even at the same mAh.

Why Energy Density Drives Design

In wearables, earbuds and slim devices, Wh/L decides how long a product runs inside a fixed space; Wh/kg matters more for drones, handheld tools and portable instruments. Comparing cells only by mAh frequently misleads buyers into choosing a heavier, bulkier pack.

How to Compare Properly

Always compare cells of the same chemistry and voltage, and request both Wh and Wh/L when quoting a custom pack. Minimum order quantity and tooling for a shaped cell are subject to project evaluation rather than a fixed table.