Technically all batteries are E=mc^2, it's just that nuclear fission converts more of the mass, but still, not all of it.
Lithium batteries do get ever so slightly heavier when charged, but it's basically undetectable.
Lithium batteries do get ever so slightly heavier when charged, but it's basically undetectable.
The mass associated with those chemical bonds is miniscule compared to the protons, electrons, and neutrons, but it is not zero.