How far will a battery pack take you?

Short answer: range comes from watt-hours, not from cell count or amp-hours on their own. A 13S5P on 18650 is about 819 Wh and roughly 41–58 km on medium assist for a 75 kg rider; the same configuration on 21700 is about 1170 Wh and around 43% further. Assist level changes the figure more than any hardware choice.

Watt-hours is the number to compare

Two packs of different voltages are not comparable by amp-hours. A 48 V 17.5 Ah pack and a 52 V 16.1 Ah pack are both around 840 Wh, and that is what the motor consumes. Multiply voltage by amp-hours and you have the only figure that travels between builds.

What each configuration works out to

For a 75 kg rider on an average 25 kg e-bike, mixed terrain, no headwind, cells at 3.5 Ah (18650) or 5.0 Ah (21700). Figures are ranges because the real answer depends on how you ride, not just what you built.

What moves the figure

Assist level is the biggest single variable and it is entirely under your control — the same pack can give twice the distance on eco as it does on full assist. Rider and load weight is second: every 10 kg is roughly 6% more consumption. Terrain and wind come third, and a sustained headwind on an exposed road can cost as much as a hill.

Getting more range without a bigger case

The cheapest gain is not a bigger housing, it is the same configuration on 21700. A 13S5P is 65 cells whichever cell you use, so it fits the same moulds — but on 21700 it is about 1170 Wh against 819 Wh, roughly 43% more energy in the same envelope. If your frame limits the housing size, that is where the range has to come from.

Beyond that, the housing becomes the constraint and you are choosing between formats. The configuration index lists every buildable combination with the housings that take it, and the pack calculator will work out the watt-hours for any series and parallel count you are considering.

What to check before you commit to a range figure

Three things undermine a range estimate more often than the arithmetic does. A pack built to the absolute ceiling of a housing has no room for the BMS, so a plan that fits on paper may not close — the usual reason is the BMS. Cells that are not what the listing claimed will under-deliver on capacity. And a pack run near its continuous current limit sags, which costs range at exactly the moment you need it.