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THREE NUMBERS

Charge, energy and power

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Capacity (Ah)

How much charge the cell moves: 1 Ah is 1 amp for 1 hour

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Energy (Wh)

Capacity × voltage: what actually does work

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Power (W)

How fast the energy can come out

A 3 Ah, 3.6 V cell stores about 11 Wh.

C-RATE

Current relative to capacity

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0.1C A gentle 10-hour discharge, used to measure true capacity
1C Empties the cell in about one hour
2C About 30 minutes: a fast charge
10C+ Minutes or less: power tools, drones and racing
C-rate lets you compare a tiny coin cell and a car cell on the same scale.

PER KILOGRAM, PER LITRE

Energy density

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Specific energy (Wh/kg)

Energy per mass: matters for cars, drones and planes

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Energy density (Wh/L)

Energy per volume: matters for phones and watches

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Cell vs pack

Packs add cases, cooling and wiring, so they score lower than their cells

Lithium-ion cells reach roughly 150–300 Wh/kg; lead-acid manages about 30–40.

NAME THAT NUMBER

Which number answers the question?

Range, run time, punch and charging speed each depend on a different number on the spec sheet.

Ah is charge, Wh is energy, W is power, C-rate is how hard you push.

SCALE

From earbud to electric car

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Earbud About 0.2 Wh
Phone About 15–20 Wh
Laptop About 50–100 Wh
E-bike About 400–700 Wh
Electric car About 50–100 kWh: thousands of phones' worth
The same chemistry, scaled over about six orders of magnitude.

UNIT 3 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen the difference between charge, energy and power, and how C-rate and energy density compare batteries.