Does my electric vehicle weigh more after I have charged it?
Alex McDowell
London, UK
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According to Albert Einstein’s theory of special relativity, energy has mass. Through his equation E = mc2, we can deduce the mass of energy put into batteries.
The battery of a large Tesla holds 100 kilowatt hours, or 3.6 × 108 joules, hence the extra mass from charging it would be 4 × 10-9 kilograms. This is small compared with the mass of the car, which is typically more than 2000kg for a large Tesla.
Eric Kvaalen
Les Essarts-le-Roi, France
If you add 100kWh to your battery, or 360 megajoules, according to special relativity (E = mc2), you increase its mass by 4 micrograms – around the mass of a small grain of sand.
When charging a lithium-ion battery, for example, one electrode loses mass and the other one gains it as lithium ions move from one to the other, but the net change in the mass of the battery will be 4 micrograms.
David Muir
Edinburgh, UK
When a rechargeable battery is being charged, most people understand that a direct electric current is flowing into the battery at the negative electrode.
It is less appreciated that the same current is flowing out of the battery from the positive electrode. There are no more electrons in the battery after charging than there were before. The thing that has increased is the amount of chemical energy that can transform to electrical energy when the battery is discharged.
Spencer Weart
Hastings-on-Hudson, New York, US
Your electric car will weigh less after charging.
No doubt some readers will calculate the gain through E = mc2, but this will be far smaller than the loss from molecules knocked loose when you insert and remove the charging nozzle.
Additionally, a tiny warming in components of the electrical system will probably evaporate some water and other molecules from car surfaces into the air.
Adrian Bowyer
Foxham, Wiltshire, UK
Yes, the mass of the car will increase. A bit of this will be because the stored energy has mass. But most will be due to the dust the car gathered while parked.
Incidentally, all the electricity that humanity generates in a year, around 9 × 1019 joules, has a mass of about 1 tonne – less than half of the total mass of an electric Renault Zoe.
Nick Leach
Watford, Hertfordshire, UK
It is estimated that a person infected with covid-19 contains 1 to 100 micrograms of virus, so the mass gained by charging an electric car, 4 micrograms, is about the same as the mass of covid-19 virus in an infected person. I feel there is a new unit of measurement in here somewhere…
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