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E-bike battery voltage, amp-hours and watt-hours explained

Three numbers describe every e-bike battery: nominal voltage, capacity in amp-hours, and energy in watt-hours. Understanding how they multiply together is the key to comparing packs honestly.

Voltage is set by the cell count

A lithium-ion e-bike battery is built from many small cells wired in series and parallel. Each cell has a nominal voltage of about 3.6 to 3.7 volts. Wiring ten cells in series gives a 36 volt nominal pack, and thirteen in series gives a 48 volt pack. That is why almost every mainstream e-bike is either 36V or 48V: the number is a direct product of how many cells sit in series.

The nominal figure is an average. A fully charged 36V pack actually reads about 42 volts and a 48V pack reads about 54.6 volts, falling steadily as you ride. Chargers are labelled by that full-charge voltage, so a charger marked 42V is a 36V charger and a 54.6V charger is a 48V unit.

Amp-hours measure charge, not energy

Amp-hours, written Ah, describe how much charge the pack holds. A 14Ah battery can in theory deliver 14 amps for one hour, or 7 amps for two hours. On its own, amp-hours tell you nothing about range, because a 14Ah pack at 36V holds far less energy than a 14Ah pack at 48V.

This is the trap in comparing two batteries by amp-hours alone. To compare fairly you have to bring voltage into it, which is what watt-hours do.

Watt-hours is the number that matters

Watt-hours, written Wh, is voltage multiplied by amp-hours. A 36V pack rated at 14Ah stores about 504 watt-hours, because 36 times 14 is 504. A 48V pack at 14Ah stores about 672 watt-hours. Watt-hours is the true measure of stored energy and the honest way to compare any two packs regardless of their voltage.

Most range estimates work backwards from watt-hours. A rough rule used across the industry is that a mid-drive e-bike uses somewhere between 7 and 20 watt-hours per mile depending on assist level, rider weight, terrain and wind. A 500Wh pack therefore covers a wide range of real distances, which is why manufacturers quote a span rather than a single figure.

How to read a pack label

A label reading 36V 14Ah 504Wh is fully self-consistent: the three numbers multiply correctly. If a listing gives only two of the three, you can always calculate the third. Watt-hours divided by voltage gives amp-hours; voltage times amp-hours gives watt-hours.

Be wary of a pack that advertises a huge amp-hour figure without stating voltage, and of any pack whose stated watt-hours does not match its voltage times its amp-hours. That mismatch usually signals an optimistic or incorrect spec.

Related

Sources: Bosch eBike Systems battery documentation · Shimano STEPS technical specifications