Does “electricty” or whatever it’s called that goes into your electric vehicle battery, have mass or weigh anything? My car has a 100KW battery. Is the weight substantially different when it has 0 charge versus when it is fully charged? It seems like a lot of energy goes into the battery, it must add to the net mass of the vehicle?
In: Technology
A charged battery “weighs” more, yes, because it holds potential energy, and the mass-energy equivalence tells us that mass has energy and energy has apparent mass. The relationship between the two is given by E = mc^2
We can use E = mc^2 to determine how much additional mass an object will have if its energy increases.
In the case of a battery, when it discharges it is said to convert chemical potential energy to electrical energy through the chemical reactions in the galvanic cell. This causes the mass to decrease (the products weigh less than the reactants; the balance is converted into energy). When a battery is being charged, the cell becomes electrolytic and electrical energy is converted to chemical potential, reforming the original reactants – therefore the mass increases.
For a 100kWh capacity battery, we can convert this to the SI unit (joules). 1kWh = 3.6 megajoules so 100kWh would be 260MJ (3.6×10^8 J).
According to E = mc^2, 1kg of mass would be converted into 9×10^16J of energy, so your 100kWh battery would see its mass increase by (3.6×10^8)/(9×10^16) kg, or about 4 microgrammes
The battery in the electric car doesn’t store electricity. It uses electricity to drive a chemical reaction in a battery. That chemical reaction is reversed to power the car.
There are not weight changes you could measure on a scale that is capable of weighing something as heavy as a car. There are weight changes, but they are much to small to measure directly.
Does a ball weigh more when you carry it to the top of a hill? There’s a slight change in weight with batteries but it’s basically the same concept.
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