What the Display Is Guessing and What It Is Measuring

There is a short answer about e-bike displays and a useful one, and they are not the same. What follows is the useful one.

Measured, inferred and invented

A display shows a mixture of directly measured values, calculated estimates and extrapolations, and it presents all of them with equal confidence. Speed and distance are genuinely measured, from a magnet and sensor counting wheel revolutions against a stored wheel circumference.

Battery state is usually inferred from pack voltage, which sags under load and recovers at rest, making the figure move around. Range remaining is an extrapolation from recent consumption, which assumes the next stretch resembles the last one. Knowing which category a number falls into tells you how much weight to put on it when planning a ride.

Why the battery gauge lies in a predictable way

Lithium cells hold a fairly flat voltage across the middle of their discharge, then drop away quickly near the end. A gauge reading voltage therefore falls slowly through the middle of a ride and then plummets over the last portion. Under heavy load the voltage sags, so the gauge can drop several bars on a climb and recover most of them at the top.

Cold makes the same thing happen more dramatically, because cold cells hold a lower voltage under the same load. The practical response is to treat the gauge as a rough indicator and to plan by distance and terrain instead.

Range estimates and the hill they cannot see

A range figure is calculated from how much energy you have used per kilometre recently, projected forward onto the remaining charge. That works well on a consistent route and badly on one where the character changes, which describes most real journeys.

With a loaded rack, ride ten flat kilometres and the estimate becomes optimistic, then collapses the moment you start climbing. Riding into a headwind produces the same effect, and turning around at the halfway point can transform the figure in either direction. The estimate is most useful as a trend rather than a value, and a figure that is falling faster than your distance is a genuine warning.

Speed, wheel size and the settings nobody checks

Speed is calculated by multiplying wheel revolutions by a stored circumference, so an incorrect setting produces a proportionally incorrect speed. Fitting different tyres changes the rolling circumference slightly, and nobody updates the setting afterwards. That matters more on an assisted machine, because the assistance cut-off is enforced against the speed the system believes it is doing.

A wheel circumference set too small makes the system think you are slower than you are, which is why deliberately altering it is treated seriously.

Tampering with speed settings changes the machine's legal classification in many places, and the rules on that vary by country.

Assistance levels are not percentages of anything fixed

Assistance settings scale the motor's contribution relative to your input or to a preset level, depending on the sensor type fitted. The same numbered level therefore delivers different amounts of power on different systems, and the numbers are not comparable between machines. Riding a level lower than habit usually costs very little speed on flat ground and saves a noticeable amount of energy.

The largest consumption is at low speeds under high load, which is why stop-start city riding drains a pack faster than a steady run. Watching consumption change as you move between levels on a familiar route teaches you more than the manual will.

Range and charging figures are quoted under conditions nobody commutes in.

Using the display as a diagnostic

A trip that suddenly consumes far more energy than usual over a familiar route points at a mechanical drag, a soft tyre or a headwind. Error codes vary between manufacturers and are worth photographing rather than clearing, since they narrow a diagnosis considerably. A display that flickers or resets while riding usually indicates a connection problem rather than a fault in the display itself.

Sudden capacity readings far below normal are more often a management or sensing fault than genuine overnight cell failure. Recording the ordinary numbers for your regular route gives you a baseline, and the baseline is what makes an abnormal reading visible.

The takeaway

Plan by distance and terrain, and use the display to notice when something has changed rather than to predict the future.

Most of riding well is being boring and predictable to everyone else on the road.