When you charge a battery, the overall mass of the battery does not change significantly. The charging process involves transferring electrons from one electrode (the anode) to another electrode (the cathode) through an external circuit. This transfer of electrons is accompanied by chemical reactions within the battery that store energy in the form of chemical potential energy.
The chemical reactions during charging involve ions moving between the electrodes and changes in the oxidation states of the materials involved. However, these reactions do not typically result in a significant change in the overall mass of the battery.
The energy gained during charging is stored within the battery in the form of electrochemical potential energy. It is not primarily associated with an increase in mass but rather with changes in the chemical composition and the distribution of charge within the battery.
It's worth noting that over time and repeated charge-discharge cycles, batteries can undergo chemical changes and experience small changes in mass due to the formation of solid electrolyte interface (SEI) layers, electrode degradation, or other factors. However, these changes are generally minimal and not directly proportional to the energy stored in the battery.