Electric Cars Are Ever So Slightly Lighter With Dead Batteries

By Chris Mills on at

Drive your car until the tank’s empty, and it will probably be 45 kilos lighter (minus the six extra Red Bulls consumed). But drive an electric car until it’s out of juice, and on a much smaller scale, it will also have shed weight.

Whether or not batteries get lighter as they lose energy is a question that has vexed Yahoo Answers since amateur physicists first found the internet. So to settle the question, YouTuber Tom Scott tracked down half a dozen renowned professors (and a Tesla Model S). Their answer? A dead battery is lighter than a charged one, but on a microscopic scale.

As Scott explains, the mass lost is equivalent to the battery’s energy, divided by the speed of light squared (we know this thanks to e=mc^2). Anything divided by the speed of light squared is going to be an unfathomably tiny number — in this case, less than the weight of the rubber burned off the tires.