 Geroch has a clever way to answer this question. Suppose we use the amount of energy \(E_o\), as measured in the rest-frame of the battery, to accelerate the battery (instead of charging it) so that it eventually reaches a final speed of 670 mph. Since 670 mph seems like something moving pretty fast, at least by the non-relativistic standards of human travel, one might think that one is using quite a bit of energy to accelerate the battery to that speed. Now suppose that instead of using the energy \(E_o\) to accelerate the battery, we use that same amount of energy \(E_o\) to charge the battery. The increase in the mass of the charged battery is \(E_o /c^2\), but because the speed of light is such a large number, approximately 670 million mph, “the mass of the battery would be increased by about 1 part in a million-million (i.e., by a fraction \(10^{-12}\)…)” (Geroch 2005, p. 199). 