EM waves drain batteries

[MRC_Hans]

*snip*Actually, that can, in principle, contribute to draining the battery. Typically there is some leakage current, though it's usually quite small, created by the electric field of the battery (even if it's only an internal leakage current inside the battery). Add on an additional static electric field and you can increase this leakage current. Adding on an alternating field, however, will produce no additional net current.

Sorry, no. The leakage is due to the potential over the battery, not the field. In effect, there is a tiny constant load built into the battery, and current runs throught it, driven by the battery voltage, eventually draining the charge. An external field will have no effect on this.

On "Zapping": Anything that can burn off the whiskers that form in certain battery types, which means any sufficient current source. The whiskers don't require a lot of current to disappear, but the battery itself will pose a considerable load to any external source, and this brings the need for a fairly heavy current. A good rule of thumb is that you need at least ten times the normal charge current.

Zapping is a typical kill or cure treatment.

Hans

 

[Ziggurat]

Sorry, no. The leakage is due to the potential over the battery, not the field.

Back up a second. Voltages (potentials) don't drive current, electric fields do. Voltages are just the path integral of the electric field between two points, and do not, in and of themselves, tell you what the electric field is anywhere. Because of some nice properties of most resistors, you can figure out the electric current through a resistor based on the voltage, without having to calculate what the electric field actually is, but in all cases, it is STILL the electric field that is driving the current.

In effect, there is a tiny constant load built into the battery, and current runs throught it, driven by the battery voltage, eventually draining the charge.

No, it's driven by the electric field inside the battery. The voltage is just the path integral of that electric field through the battery.

An external field will have no effect on this.

It's not that simple. When you set up a charger, for example, you're just applying a larger electric field to the battery. When you add a load to the battery, you're just dropping the internal electric field inside the battery. When the electric field inside the battery changes, current flows.

The REAL question is actually different: does applying an external electric field translate to a change in the internal electric field inside the battery? If it does, then it will contribute to discharging the battery. If it doesn't, then it won't. It's quite possible that the accumulation of screening charges on the surface of the battery will prevent an external field from changing the internal field, but that's really the issue.

 

[Ziggurat]

Got any sources for that? I'm not entirely convinced a battery will suffer leakage in a static electric field.. According to wikipedia (the only source I can find at the moment) the self discharge is non-current-producing, and is just the chemical reaction of the battery occuring. I can imagine some current due to conduction in the electrolyte, but is that current a significant part of the total leakage?

The reference within that article is specifically to alkaline batteries. I'm not familiar with the reactions involved, but if it's a reaction that isn't producing current, then it's got to be a different reaction than what produces the voltage. Since the reaction isn't producing a current (or voltage), I would therefore also not expect the reaction to be reversible by applying a current, so it's a one-way street: once it's reacted that way, the battery is dead for good. Rechargeable NiMH and NiCd batteries seem to have shorter shelf lives (annecdotal), but recharge quite a bit more often than alkalines. The self-discharge process should therefore be dominated by something reversible, and so I don't think it IS the same process as whatever dominates alkaline battery self-discharge. I suspect that, instead, that it is just the leakage current back through the electrolyte, and that the electrolyte resistance in rechargeables is just much lower than in alkalines.

 

[Meffy]

Zapping is a typical kill or cure treatment.

Yuppers, the project instructions said as much, and to be safe I put a strong plexiglas cover over the charging compartment. About one in six, maybe one in eight didn't survive the zapping treatment.

 

[casebro]

Yes, but it worked for Klaatu, the day he make the whole world Verada Nikto.

But it ain't worked since.

 

[jj]

Back up a second.

Yes, let's. MOst any battery is in a conductive case. That will shield it completely from a static external field, yes?

 

[Ziggurat]

Yes, let's. MOst any battery is in a conductive case. That will shield it completely from a static external field, yes?

It may provide some shielding, but it cannot shield it completely. Why? Because there has to be a gap in the conductive part of the casing, because if there wasn't, then current would just flow through the casing between the two terminals, and you'd REALLY have self-discharging batteries.

The casing can dampen any internal field created by an external applied field, but it can't eliminate it completely. To do that, you'd still need a buildup of screening charges between the two different terminals, which has to flow through the battery itself. I'm starting to think this won't help discharge the battery, though, because it'll just flow back the other way when the voltage is removed. But it's not quite the same dynamic as screening charges in a good conductor, because it really will have to flow through a bad conductor (the battery electrolyte).

 

[jj]

It may provide some shielding, but it cannot shield it completely. Why? Because there has to be a gap in the conductive part of the casing, because if there wasn't, then current would just flow through the casing between the two terminals, and you'd REALLY have self-discharging batteries.

Um, consider the average battery. Now where are you going to apply the field, in which direction, and how will it cause current to flow?

 

[Ziggurat]

Um, consider the average battery. Now where are you going to apply the field, in which direction, and how will it cause current to flow?

First off, voltage is just the path integral of the electric field. If the top terminal and bottom terminal of a battery are at different voltages, then there's an electric field between them. Casing and an internal electrode bend this electric field so much of it ends up radial, but it's there, and it's still between the top and bottom terminals. If you apply an electric field along this direction, then the voltage differential between the two electrodes will change, and the internal electric field will change too. Depending on which way you apply the field, the response will change.

Inside the battery, you have a chemical reaction that pushes charge in one direction, driving the voltage difference. This chemical reaction can only procede when the voltage drops below a certain amount, and may proceed in reverse if the field gets much above this value (the recharging process). There is also a small leakage current within the battery, as charges get pushed back because of the electric field it creates. That's the slow self-discharge. Current flows, decreasing the voltage, and the chemical reaction can then proceed until the voltage is re-established. It's an equilibrium process.

If you apply the field in the same direction as the battery's existing field, then this higher electric field will create a higher leakage current. But this extra leakage current will also build up a screening charge in the process, and so you only get a little bit of extra leakage current before the field is screened, and then nothing more. You will also get a bit of the reverse of the chemical process, which will recharge the battery slightly. When you remove the field, the voltage will be too low, and the chemical process will go forward again until it re-establishes the voltage difference. These will all be minor changes, though.

If you apply the field in the opposite direction, then the voltage difference between terminals is lowered and leakage current is discouraged. But the chemical reaction faces a lowered voltage and so will proceed until it, too, builds up a screening charge that re-establishes the voltage difference between the two sides. When you remove the external field, the voltage will be too high, and you'll again get a combination of leakage current and reverse chemical process.

In both cases, the net charge/discharge from the whole process might be zero (compared to the normal leakage current that's happening continually). But instantaneously, you can still be driving currents around, which really shouldn't surprise you.