Battery Logic
A fully charged battery is heavier than a discharged battery, but not because it has gained any material.
A lithium-ion cell, like those used in a Tesla, is a sealed system: the same lithium, cobalt, nickel, graphite, and electrolyte remain inside whether it is empty or full.
When you charge the battery, you are forcing lithium ions to leave the metal oxide cathode and insert themselves into graphite layers at the anode.
This rearrangement of matter stores energy in the chemical structure of the molecules, and energy has mass according to Einstein’s equation.
A Tesla Model 3 pack stores about 60 kWh, or roughly 216 megajoules, of energy. Dividing this by the square of the speed of light yields a mass increase of around 2.4 micrograms when it charges from empty.
That is one-twentieth the weight of a typical grain of salt, spread across a 480 kg battery pack.
The weight increase comes entirely from energy stored as chemical rearrangements, not from accumulating extra electrons. The electrons are just carriers; they help rearrange lithium ions and bonds inside the battery. Electrons circulate and are balanced by ionic motion; no net electron rest mass is added.
They didn’t teach me this at uni but every molecule is lighter than the sum of its isolated atoms, and every chemical reaction changes the total mass slightly. We ignore it in chemistry because the changes are at least a billion times smaller than atomic masses.
So much for the periodic table!
The molecules that make up all living things on Earth weigh roughly a few hundred thousand kilograms less than the same nuclei and electrons were they to be taken apart and separated to infinity. The exact number depends on composition and bonding, but it is in the 100-1000 tonne range (says GPT).
offshore westerly 