Figuring out the charge left on a battery.

Just wondered, obviously with a dvm you can make a good prediction
of the life left in a battery, like the AA cells for instance. Is it possible
to figure out the charge left if you had a sufficiently accurate
scale? Like Picograms or some such. Wouldn't a battery get a tiny
bit lighter as it used up charge? Hmm, its hermetically sealed mostly
but maybe gasses could escape. What do you all think?

Originally posted by sonhouse
Just wondered, obviously with a dvm you can make a good prediction
of the life left in a battery, like the AA cells for instance. Is it possible
to figure out the charge left if you had a sufficiently accurate
scale? Like Picograms or some such. Wouldn't a battery get a tiny
bit lighter as it used up charge? Hmm, its hermetically sealed mostly
but maybe gasses could escape. What do you all think?

Practical application of knowledge is for the birds.

Originally posted by sonhouse
Just wondered, obviously with a dvm you can make a good prediction
of the life left in a battery, like the AA cells for instance. Is it possible
to figure out the charge left if you had a sufficiently accurate
scale? Like Picograms or some such. Wouldn't a battery get a tiny
bit lighter as it used up charge? Hmm, its hermetically sealed mostly
but maybe gasses could escape. What do you all think?

As far as I know most batteries (NiMH, NiCad, Li-Ion) aren't giving off gases as they are used. If this is true then conservation of matter states that the weight won't change as charge does.

Originally posted by XanthosNZ
As far as I know most batteries (NiMH, NiCad, Li-Ion) aren't giving off gases as they are used. If this is true then conservation of matter states that the weight won't change as charge does.

Do electrons not "move out" of the battery? If so, a tiny loss of mass would occur?

Originally posted by Alcra
Do electrons not "move out" of the battery? If so, a tiny loss of mass would occur?

They also 'move into' the battery at the same rate, conservation of charge and all that.

Originally posted by XanthosNZ
They also 'move into' the battery at the same rate, conservation of charge and all that.

Thats for sure! Maybe one or two strays would go in or out but I don't
think we can weigh one electron yet. If there is energy used,
molecules will swap mass and mass is usually lost. If it isn't, THAT
is breaking the law of conservation of energy. Just like a fusion
reaction, some atoms whack together fiercely enough to fuse but
some of that energy is given off in various forms and the system
losses mass, even if say an electron is converted to a photon,
the mass goes down because the electron is now a photon which is
massless but the total energy will be conserved, ala E=MC^2.

Originally posted by sonhouse
Thats for sure! Maybe one or two strays would go in or out but I don't
think we can weigh one electron yet. If there is energy used,
molecules will swap mass and mass is usually lost. If it isn't, THAT
is breaking the law of conservation of energy. Just like a fusion
reaction, some atoms whack together fiercely enough to fuse but
some of that energy ...[text shortened]... lectron is now a photon which is
massless but the total energy will be conserved, ala E=MC^2.

I could be wrong, but I don't think any mass is converted to energy in batteries. Energy is conserved because, for each unit of electrical energy produced, the battery loses a corresponding amount of chemical potential energy.

Originally posted by richjohnson
I could be wrong, but I don't think any mass is converted to energy in batteries. Energy is conserved because, for each unit of electrical energy produced, the battery loses a corresponding amount of chemical potential energy.

This is correct.

A ball would gain a lot of energy if you took it to the top of a building. It doesn't lose mass though.

Potential energy is nonsense.

Originally posted by XanthosNZ
This is correct.

A ball would gain a lot of energy if you took it to the top of a building. It doesn't lose mass though.

Good point, relativistic effects aside.

Originally posted by Bowmann
Potential energy is nonsense.

Useful nonsense though.

Originally posted by Bowmann
Potential energy is nonsense.

So kinetic energy is nonsense as well?