As the battery discharges, the sulphuric acid breaks down into positively and negatively charged ions that are deposited on the electrodes. Mass does not into and out of the battery during the charging and discharging cycles. I mean, where is it going?
https://academy.gs-yuasa.eu/wp-content/uploads/2019/05/Lead-Acid-Battery-Operation.pdf
Battery discharge process
Negative plate
The discharge process begins as soon as an electrical load is applied to the battery. This takes the form of a chemical reaction between the negative plate and the sulphuric acid in the electrolyte solution. The sulphuric acid begins to break down into positively charged hydrogen ions (H+) which move to the positive plate and negatively charged sulphate ions (2-) which move to the neutral porous lead atom on the negative plate. The porous lead atom on the negative plate becomes ionised and negatively charged as the sulphate ion attempts to bond with it. To complete the bonding process and achieve a neutral charge state the lead atom must become positively charged. It therefore releases 2 negatively charged electrons. These 2 free electrons can now flow through the electrical circuit through the applied load and on into the positive plate.
Positive plate
The two negatively charged electrons from the negative plate arrive at the positive plate and bond with the
positively charged lead ion (4+) in the neutral lead dioxide molecule. This causes the charge of the lead ion to change from (4+) to (2+) as there are now only two more protons than electrons in the lead ion, altering what is known as the oxidisation state of the lead ion. Negatively charged sulphate ions (2-) produced from the breakdown of the sulphuric acid bond with the positively charged lead ion (2+) on the surface of the plate creating a neutral molecule of lead sulphate and releasing negatively charged oxygen ions into the electrolyte. The two hydrogen ions (H+) approach and bond with the negatively charged oxygen ion (2-) creating a neutral molecule of water (H2O). The battery is now discharged.
Further, there are not nuclear reactions going on inside the battery. You are not changing one element or isotope into another. Just look at the energy equivalent of one gram of mass.
https://www.quora.com/What-is-the-energy-equivalent-to-the-mass-of-1gm
So 1 g of matter is about 90 trillion joules of energy. According to
this page it’s estimated that humans generated (& used) about 5.67e20 joules in 2013, which would be about 6.3 million grams worth (which is 6,300 kilograms).
More here:
https://www.reddit.com/r/AskPhysics/comments/op5kxm/eli5_in_what_sense_does_1_gram_of_mass_relate_to/
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