Charge invariance refers to the fixed electrostatic potential of a particle, regardless of speed. For example, an electron has a specific rest charge. Accelerate that electron, and the charge remains the same (as opposed to the relativistic mass and energy increasing). The key word here is relativistic. Some particle characteristics are relativistically invariant (charge, spin, and magnetic moment). Others are relativistic (mass, energy, and de Broglie wavelength). The key is the Lorentz factor,
where
where the first equation describes relativistic energy, the second – relativistic mass, the third – charge invariance, and fourth – spin invariance. Notice that
The property of charge invariance follows from the vanishing divergence of the charge-current four-vector