Russell McMahon wrote:
> Martin said:
> > Does rercharging  make them heavier...
> 
> Yes - charging adds about 20 nanograms of electrons per milliamp-hour. See
> below.
> 
> E&OE / YMMV.
> Electron mass to charge = - *e* / *m**e* = -1.758 820 ... E11 C/kg
> 150(44) [image:
> \times 10^{11} {C / kg}].
> 1 Coulomb = 1 A.s
> A 2000 mAh cell will gain 2000 mAh  / 1000  mA/A * 3600s/h ~~~= 7200 Coulomb
> when charged.
> This will mass 7200 / 1.76E11 kg or about 4E-8 kg or about 40 micrograms of
> electrons.
> Or, about 20 nanograms per mAh.

You are just being silly, right? Charging does not add electrons to a
battery, it just moves them around inside (true of capacitors, too).
Otherwise, the battery as a whole would acquire a huge negative charge
relative to its surroundings.

There is a tiny mass change arising from charging, but it's just the
relativistic mass-equivalent of the potential energy you've added to the
battery. (M = E/c^2).

2 Ah * 1.5 V * 3600 s/h = 10.8e3 J = 10.8e3 kg*m^2/s^2

10.8e3 kg*m^2/s^2 / (3e8 m/s)^2 = 1.2e-10 g

... or 0.12 nanogram for the entire cell, about six orders of magnitude
less than your estimate.

-- Dave Tweed
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