Do batteries get heavier when you charge them?

I was recently listening to a science show (Dr. Karl) for those of you in Australia. He said he thought that batteries got heavier when they were charged.
I believed that the charging process was the reversal of a chemical reaction and that no net change in the batteries weight should occur.
Who is correct?

When charged the batteries have more potential energy and therefore more mass.

In principle, there is a slight increase in mass (E = mc[sup]2[/sup]: It’s not just for nuclear reactions). In practice, though, the change is so small as to be completely imperceptible.

So if I carry a lead weight up from the street level to the top of a 15 story building and hold it over the edge of the building its mass will increase because its potential energy has increased?

E=MC^2 only applies if mass and energy are converted. Charged and uncharged batteries will weight the same unless the chemical reaction uses oxygen from the air, air zinc batteries or such.

It’s not a matter of conversion. Mass is energy, just energy that’s arranged in a particular way. If you put energy into a container of some sort, the mass of the container increases.

Doesn’t the energy from a cell come from electrons moving, as a result of chemical reactions, from the negative half of the cell to the positive half? Seems as if the mass of electrons lost from the anode is gained by the cathode. Recharging is the same, in reverse. Beginning chem classes all teach that mass is conserved in chemical reactions. So, according to traditional chemistry, no change in mass (except for hydrolyzing water and losing hydrogen gas in wet cells, I reckon- mass is still conserved, but some diffuses out of the battery).

Right. delta M = 0 in a chemical reaction.

Since it’s never going to be perfect, won’t charging a battery result in a net loss of mass over time, since the conversion from usable to depleted (and the vice versa) will naturally be imperfect and lose some small amount?

I imagine the increased mass is almost undetectable to any human, almost miniscule.

A fully charged AA battery weighs about 0.0000000001 grams more than a discharged one due to the extra energy it contains. However, if one scrapes even a single atomic layer off the battery contacts when removing the battery from the charger, one will have removed ten times that mass in raw metal material. Then there are the oils from the hand adding even more mass than that, dust, etc.

Take the familiar lead acid battery. It will lose mass to the extent water is electrolyzed on charging. This has nothing to with the energy gained. Batteries also accumulate lead sulfate in the plates reducing their capacity.

To reiterate what** Chronos** and Pasta said: Charging a battery is a chemical reaction, powered by electrons being pumped through the battery. With a sealed battery there will be the same number of atoms and electrons after charging as before.
However, the molecules in their charged-up state have a tiny tiny tiny bit more mass, reflecting the chemical energy stored in the molecule. As Pasta said, it’s a very tiny bit, and basically unmeasurable (a couple dust grains or some oil from a fingerprint will have a bigger effect).

It’s my understanding that the system’s energy has increased, so the mass as measured from the outside the earth-weight system will have increased. However, the mass of the weight itself will not have increased.

(Of course any energy to lift the weight will most likely have come from within the earth-weight system and thus the mass will remain the same.)

as mentioned a zinc-air battery will take on oxygen.

having a link or transcript of the show would help.

The conservation of matter in chemical reaction is absolute. There isn’t even a small change in mass. The only way the mass of a battery will change is if it gains or loses material.

No, you are misunderstanding what is being said. Mass is energy. It may be a small amount, but a charged battery has more energy so it has more mass, no matter what happened chemically. Thats assuming it does not gain or lose matter.

This is a relativistic effect, and it is very tiny. A closed container (here, a battery) weighs more if you increase the energy content inside.

For some definition of “material”, sure. Hook a battery up in a flashlight, and let the flashlight run. It’s losing photons, which are carrying away mass from the flashlight.

Please correct me if I’m wrong but I believe that photons are massless, otherwise they would have infinite weight since they travel at the speed of light.