The original note, as written at the time
A charging current winds a spring in an EV as the car moves. When the battery of the EV runs out, the spring charges the battery using its stored potential energy, increasing the EV’s range.
Ongoing development
A charging current winds a spring in an electric vehicle as it moves. When the battery runs out, the spring charges the battery from its stored potential energy, extending the vehicle’s range.
The idea is a mechanical reserve, and the honest question is whether it stores enough to matter.
A spring stores energy as force times distance, and the energy density of even a good spring is poor compared with a battery — roughly two orders of magnitude worse per kilogram. So a spring that weighs as much as a small battery would hold a fraction of that battery’s energy, and the arithmetic of carrying it looks bad at first sight.
But the case does not depend on energy density. It depends on the fact that the vehicle is shaking constantly. Every bump, every brake, every stop is energy currently dissipated as heat, and a spring that captures even a little of it is recovering something that was being thrown away — which is a different calculation from storage.
The right place to put it is not the drivetrain but the suspension, where the motion already exists. Regenerative damping is the serious version of this idea, and it works.
And there is a safety argument for a small mechanical reserve: a limp-home capability that does not depend on chemistry, temperature or software. A spring works when it is cold, when it is hot and when the computer has crashed.
Store what the road is already giving you. Do not expect it to replace the battery.