threshold

/ˈθrɛʃhəʊld/ noun

The strip of wood or stone at the bottom of a doorway.
The level at which something starts to happen.

26 November 2025 · Berlin · 1 minute

Drivers have a threshold of charge below which they stop enjoying the drive. For most it is around twenty per cent; for some it is forty. Below it, every kilometre is spent watching a number.

Measuring it

The threshold is personal, and it moves with experience.

If $s$ is the charge and $θ$ the threshold, comfort behaves roughly like a step:

(1)

comfort (s) = \frac{1}{1 + e^{- k (s - θ)}}

with $k$ large for a new driver, whose comfort falls off a cliff, and small for an experienced one, who has learned that the last ten per cent is real.

Designing for it

A route planner that knows the threshold can plan stops that keep the charge above it. It costs a few minutes more. It gives back a calmer drive, which is the thing the driver was buying. See range and forecast.