# Thread: String Trimmer Batteries - same weight charged and empty

1. ## String Trimmer Batteries - same weight charged and empty

I weighed these guys empty and fully charged. There are two different batteries, one is stronger and heavier so it lasts longer. But when I weigh each battery fully charged and empty it's weight doesn't change, at least to the nearest gram. Do I need a more precise scale?

I know electrons have no mass so is that the answer?

Okay, so I googled and found that the electrons just move, they don't leave the battery as electricity. Cool.

2. nm

3. Electrons do have mass (I didn't even know that they were catholics); But electricity isn't electrons - it's the movement of electrons. And they move quite slowly - the drift velocity of electrons in a standard domestic copper wire at 15 Amps is only about walking pace - and it's alternating current, so they don't flow into or out of your home - you pay the electric company to jiggle the electrons that you already own, they don't give you any new ones.

In a battery, electrons flow from one side to the other as the battery discharges; Then when you recharge it, you push them back to where they started. The total number of electrons doesn't change, so their mass isn't relevant. The energy of a charged battery does appear as mass, in accordance with Einstein; But as c is a huge number, and c2 is the square of a huge number, and E=mc2, you need a metric fuckton of E before you get a measurable amount of m.

A 12V, 15Ah battery stores 648kJ of energy. 648kJ masses about 350 nanograms, or 0.35 micrograms (a hundred millionth of an ounce, for those watching in black and white) - You will need some VERY sensitive scales to detect that. If you leave a fingerprint on the battery, it's going to outweigh the mass gain from recharging it.

4. Originally Posted by bilby
A 12V, 15Ah battery stores 648kJ of energy. 648kJ masses about 350 nanograms, or 0.35 micrograms (a hundred millionth of an ounce, for those watching in black and white) .
Check your math, cause it's less than what you got.

5. Originally Posted by barbos
Originally Posted by bilby
A 12V, 15Ah battery stores 648kJ of energy. 648kJ masses about 350 nanograms, or 0.35 micrograms (a hundred millionth of an ounce, for those watching in black and white) .
Check your math, cause it's less than what you got.
I have just finished a(nother) twelve hour day and a(nother) six day week. Given how tired I am, I wouldn't be at all surprised if I have dropped several orders of magnitude. I have a tendency to do that even when I am fully compost-mentis.

Suffice to say, the mass involved is far too small to measure with a domestic balance that has divisions of plus or minus 1g

6. Originally Posted by bilby
Originally Posted by barbos
Originally Posted by bilby
A 12V, 15Ah battery stores 648kJ of energy. 648kJ masses about 350 nanograms, or 0.35 micrograms (a hundred millionth of an ounce, for those watching in black and white) .
Check your math, cause it's less than what you got.
I have just finished a(nother) twelve hour day and a(nother) six day week. Given how tired I am, I wouldn't be at all surprised if I have dropped several orders of magnitude. I have a tendency to do that even when I am fully compost-mentis.
Not several, merely 1.68613444433554 orders of magnitude

I did not know slavery was still legal in Australia.

7. That is actually not a bad question. The Laws Of Thermodynamics have to apply.

Consider recharging a plain old lead acid battery. Plates immersed in an electrolyte. What happens is the reversal of a chemical process. Heat is generated because the reversal process is not 100% efficient.

You should be able to look up lead acid battery.

8. Okay,

Two similar flashlights are on opposite pans of a balance beam scale. Weight is added to bring the flashlights to exact balance. One flashlight is turned on and left to shine until the battery is disharged.

Are the two flashlights still in balance?

9. Laws of thermodynamics must apply, the starting point on any energy-mass problem.

Consider a charged battery. Put a bulb across it and energy is consumed X watts are used.

To restore the battery to the same state x + %efficiency must be added via current through the battery.

The short answer is I do not know off the top of my head, like I said it is a good question. I am weak in chemistry. You can look up the equations on the net for a simple common battery. The chemical charge and discharge equations will show energy and mass balances.
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Offhand I'd say the weight remains the same. because of conservation. Put a bulb across a battery and the electrons leaving one terminal must equal the number of electors returning through the other terminal. If not you would be creating or destroying energy in the bulb.

10. Originally Posted by steve_bank
Offhand I'd say the weight remains the same. because of conservation. Put a bulb across a battery and the electrons leaving one terminal must equal the number of electors returning through the other terminal. If not you would be creating or destroying energy in the bulb.
There is no law of conservation of mass, there is a law of conservation of energy.
So correct answer is "Yes, charged battery is a tiny bit heavier".
Charged capacitor is heavier because electric field in charged capacitor has energy, hence mass.
With batteries, they they change their chemical composition when charged/discharged, which again corresponds to change in total energy.

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