# How much does electricity weight? Like if you charge your EV, does it weigh more than when the batteries are on “empty”?

284 views

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

Electricity or electric charge is essentially just a whole bunch of electrons. Electrons do have a mass but it’s so small that’s it’s pretty much negligible. At least in amounts of something like an EV battery.

The answer is yes, your car battery will weigh more when fully charged. But that difference in mass is tiny. So tiny to be meaningless.

Electricity is an abstract concept; a series of linked phenomena, related to moving electrons around. Electricity doesn’t have mass, although some of the things that make electricity work (like electrons) do.

When you charge or discharge a battery you are moving electrons around (and depending on the kind of battery, doing a bunch of other stuff). But none of those ‘physical things’ are leaving the system or being added to it. No things being added or taken away, no change in mass, right?

However, when you charge the battery you are “storing” energy in it (how depends on the kind of battery). And mass is an expression of energy; the more energy something has, the more mass it has. That is mostly what the famous *E = mc^(2)* formula means.

A quick Google says EV car batteries store somewhere from 40-100kWh of energy.
If we plug that into our formula:

> m = E/c^(2)

we get 1.6-4μg. So around a millionth of a gram. Across the whole battery. Not much.

Negligible but yess. based on https://physics.stackexchange.com/questions/34421/does-the-mass-of-a-battery-change-when-charged-discharged, your 100KW battery weighs 3.75×10^-8 kg less than when full.

Basically the weight of half an eyelash. (interestingly, about the same weight as the uncertainty in the kilogram before the watt balance redefinition)

It doesn’t.

All the material is already inside your battery, what charging is doing is essentially rearranging the positively and negatively charged particles so that later they can be again “unleashed” on each other.

In other words, using your battery is not like draining a bucket – particles are not leaving it, so the weight doesn’t change, and conversely charging it does not “pour” the material in. They both just rearrange the material that’s already there.

Yes It does. Charged batteries have more electrons in them then discharged and electrons weigh something.

Plus there is the fundamental concept E=mc^2. A full battery has an amount of extra energy and thus an amount of extra mass.

A single electron volt weights 1.8 * 10^-36 Kg. There’s 2.25 * 10^25 in a kw.

Assuming the car battery is 100 KW/h (100KW is not a storage quantity), then you end up with 0.00405 milligrams. Which is probably the weight of a short enough human hair.

I didn’t realize that was what was happening. As in even though the car is plugged in, nothing is going into it. The material inside the battery is being re-ordered?

Yes

And so does your digital data….people put a blank kindle on a very precise scale; weighed it; then filled it with books and weighed it again and the device registered heavier

A lot of wrong answers here.

A lithium battery is made of 2 layers; when charged up one of those layers is “missing” a ton of electrons, and the other has a ton of “extra” ones. If electrons can move between the two layers, they’ll naturally do so, until they’ve pretty much equalized. You can visualize this a lot like water flowing from a higher up bucket to a lower one, until the water level is the same in both. We keep the layers mostly separate, and only allow those electrons to flow from A to B through some metal wires, and they do useful work while they’re moving. Think, a man-made channel between bodies of water, that we put a waterwheel in. When the electrons have equalized, there’s no more force to move them and the battery is dead.

Charging the battery involves pushing those electrons back into one layer, using energy to do so. Like pumping water back into the higher bucket. So, energy was used to move things, no matter was added to the system.

…That said, according to general (or special, idk) relativity, energy isn’t just able to turn into matter, energy itself has mass. The water raised up higher against gravity *literally* weighs a tiny, tiny amount more than it did before. A spring literally weighs a little more when it’s compressed. And the electrons in a battery have just a little more mass when they have that electromagnetic potential energy waiting to be spent. It’s just so little that the most accurate scales on Earth couldn’t measure the difference.