Does the weight of a phone increase by a few mg when your phone is fully charged? It is the question needs some detailed explanation. Coz, It depends upon various things like the Mass of an electron or something. The General Answer to this question is, Yes , the total mass of a battery increases when the battery is charged and decreases when it is discharged. The difference boils to Einstein's E = m c 2 that follows from his special theory of relativity. Energy is equivalent to mass and c 2 , the squared speed of light, is the conversion factor. I would omit the scenario I. If the lithium is leaking from a battery, or if any atoms (and it's the nuclei that I am talking about) are moving in or out, the mass of the battery is obviously changing by the mass of these nuclei (or whole atoms). That probably doesn't need an extra explanation. So we will continue with the scenario II in which the atoms inside the battery are ...