I was changing my battery when I noticed the new one was heavier than the old one

It is because batteries have chemicals inside them. It is chemical energy that is converted into electrical energy so I guess that the chemical created was lighter than the chemicals at the start of the reaction.

The OP must have been using a different type of battery.

There IS an actual mass change, any time you remove potential energy from a system. However for the amount of energy in a battery you will never be able to measure it by any ordinary means, let alone feel it.

I agree that the OP probably used a different type of battery, because there will be no change in weight or mass between a charged and a discharged battery.

The stored energy is in the form of chemical energy, and none of the chemicals are lost in the use of the battery, but they are simply combined into different chemicals. The sealed case holds everything inside it.

[quote]because there will be no change in weight or mass between a charged and a discharged battery.[/quote]

None that you can measure with any ordinary instrument. But yes, there's a change in mass.

There's also a change in mass when you compress a spring. You've stored energy in the spring, and energy has mass.

It doesn't have to be a nuclear reaction for E=MC^2 to apply. But the amount of mass equivalent to the energy stored in a battery or compressed spring is *extremely* small.

Hi,

I have a more simple answer. What is mass, after all. Mass is composed of molecules, molecules are composed of atoms, atoms are composed of subatomic particles like neutrons, positrons and electrons, those are composed of quarks, and so on and so forth.


In short, mass is a multi-level complex and dense structure of energy. So, enery is mass.

When a battery is charging or discharging, amounts of energy are either coming in or going out, respectively, so do the mass.

Maybe the car battery was low on water?

The weight of the battery stays perfectly the same. The electrons coming out of the negative terminal enter the positive terminal. Also the matter inside stays perfectly the same except when the battery leaks.

Cheap batteries from a dollar store (often called "heavy duty") have 1/10 the weight a really expensive one that lasts more than 20 times longer has. Maybe you were replacing a cheap battery with a good, expensive one.

There is an effect that makes you believe that a used up battery is lighter. The casing (shell) is part of the chemical reaction. Atoms from the shell are bound into the chemicals inside, the casing becomes thinner. While the weight of the battery doesn't change, the shell becomes softer, it sounds different when dropping the battery. Also the softer casing makes the battery feel lighter when you bounce them in your hand.

When I was a kid, I also had the impression that the battery was lighter, my physics teacher challenged me claiming that there's no difference. But I won by sorting the batteries by charge by bouncing them in my hand. The teacher took a scale and the scale showed the same reading on all of the batteries. It took a while until we discovered that I didn't feel a difference in weight, I felt a difference in "hardness".
Buy a pack of 4 batteries, use up two of them (flash light, walkman, ...). Now drop the batteries onto a hard surface and you will hear a difference in the sound. By bouncing them in your hand you might feel a difference, too. But putting them onto a sensitive scale will give you perfectly the same reading!

Makes sense to me. I have noticed the same thing about really old A&D cells, especially when they leak but I never tried to sort them by charge.

I tried it again recently, but I failed to sort them correctly. I still can feel bad and good, but I can't separate the medium ones correctly any more. Either the design is different or I lost the ability with age.

For Power Wheels batteries which are 6v sealed lead/calcium gel cells, I observed that fully charged ones weigh about 5 pounds each, while dead ones are only about 4.5 pounds each. Coincidently, the dead ones were usually slightly indented on the sides. In a car battery, the electrolyte has a higher specific gravity when it is charged than when dead. This is true with the water level in the battery remaining constant. Material is vented into and out of the battery to and from the air. They release hydrogen which is why car batteries can catch fire.