Nine battery prototypes, one that lasted the week

The first Kin prototype died before dinner. That is the honest start of the battery story. A device worn by a child, a dog or a parent who will not remember to charge it every night cannot be a nightly ritual. It has to last the week.

What we threw away

We tried denser cells, then a bigger case, then a version that reported every few seconds whether anyone was looking. That last one felt "live" in a demo and useless by Thursday. Live should mean live while you are looking — not a permanent radio.

• Position polls every two seconds only in follow mode, and follow mode stops when the phone backgrounds.
• Everyday reporting is arrivals and departures against a place, not a trail.
• Routes are discarded at midnight. History is events, not a map of the day.

A tracker that needs charging every night is a tracker that is flat when it matters.

— Hardware notes

Where we landed

Kin One is designed for up to a month between charges in ordinary use, and a few days when someone is being followed. The number we print on the box is the conservative one. The number we chased in the lab was "still on at the end of the week."