Light Weight Question

I have a brace of questions regarding the ubiquitous power cells (commonly called batteries) we use in our portable electronic devices.

As a battery discharges, powering a flashlight, for instance, does it lose mass? Is there any appreciable difference in weight between a fully charged battery and the same cell completely discharged?

In the same vein, why do the batteries in my flashlight develop that corrosive greenish corruption around the terminals when I haven’t used the device for a long period of time? This only seems to happen to batteries that are installed in a device. I have never seen it in an unopened package of batteries. And how does that stuff move? I’ve seen entire stainless steel contacts encrusted with the stuff. Is the battery gak as deadly a poison as my mother used to tell us growing up?

I Am Not A Physicist, but…

My guess is that there is no appreciable decrease (or any change, for that matter) in mass (or weight). Personally, I would say that there is no change in mass. When a battery “discharges”, and produces light (let’s go with the flashlight example), the light produced from the flashlight isn’t really coming from the battery in the first place (it’s coming from the fliament, which is basically just glowing from the power from the battery).

I’m sure someone else can explain this much better than I’ve tried…

As for the “buildup” on batteries not used in a while, it’s called a “sulfation buildup”. Battery acid is (usuallu? always?) composed of sulfuric acid, and this acid is involved in a chemical reaction which produces the power for the battery. My guess is that you only see the buildup on batteries installed in a device because the chemical reaction I mentioned above only happens (or rather happens “appreciably”) when installed in some kind of device.

That’s my take on it, anyway. :slight_smile:


The batteries do not lose an appreciable amount of mass because nothing leaves the cell, and the only reactions going on are chemical.

As to why the electrodes should corrode, I have no idea. Inside the common dry cell is zinc chloride, ammonium chloride, and manganese(IV) oxide, so don’t eat it. I think the older cells were drenched with sodium hydroxide, so your mother may have been right. Contact with that stuff would have been detrimental to skin. Someone check me on this.

Yeah, I’m pretty sure that batteries don’t appreciably lose mass. It’s just basically a chemical reaction in a sealed cell, so unless you’re an alchemist (as my high school chemistry once accused me of being) you’re not going to see a loss of mass.

I could be wrong… but I really don’t think I am :slight_smile:

Someone always posts while I am typing my post. The batteries you place in your flashlight are not acidic; they are basic, but not so strong. The batteries in cars are strongly acidic: sulfuric acid. I am a physicist, but these chemical questions can be really tricky. Why should an electrode corrode when there is no current going through it? It shouldn’t. The real answer is that I would have to see it and investigate before giving an opinion.

Thanks for the reply LilShieste-

I realize that the light and heat of a flashlight is being produced by the resistance of the filiment of the lightbulb to the flow of current through the circuit, but that energy is being shed irretrieveably to the environment. I assume that this is what discharges the battery (otherwise you’d have, in effect, a perpetual motion machine). My question is whether that loss of heat and light has any measurable effect on the mass of the storage cell.

As for the crud question, why does the gak only leak out of batteries installed in electronic devices? When I turn my flashlight off, there is no circuit. Essentially, the battery in the flashlight is no different than the battery in the blister pack, other than the fact that the terminals have been “extended” a bit by the wiring in the flashlight.

Forgive my ignorance, but I’ve wondered about this before. I thought electrons flowed from the battery and provided the energy needed to heat the filiment, and that in this sense the battery lost some sort of mass, infinitesimal though it may be. (I’ve always wondered too whether current flowing through a power cord actually loses electrons at one end and receives the same amount coming in the other end.)

The light and heat that the filament of a flashlight produces is in the form of photons, which are by definition massless.

And electrons don’t really take off on a trip from the battery to the filament and back to the battery again. They stay more or less in place, but affect the nearby electrons which affect others and so on to produce the current. So there is no loss, even infinitesimal, from moving electrons. Even if they really traveled, the necessity that a complete circuit be formed would force the outgoing electrons to equal the incoming electrons.

And batteries corrode just from simple leakage. If even the tiniest of pathways exist for the chemicals to be exposed to the air, a chemical reaction will form. Your average mass-produced battery is about as leak-resistant as a cardboard box.

See A Note on Battery Corrosion for more detailed info.

Einstein showed that mass and energy are equivalent. Thus if the chemical energy is depleted as it powers an electrical device, the battery must have less mass. In practical terms, however, as for any chemical reaction, this decrease in mass would not be measurable. (WAG: in the nanogram range or less.)

True, but IIRC, while photons have zero rest mass, they do have momentum (and energy, of course).