So I was wasting time on the interwebs and came upon this interesting techniqueto figure out whether an AA battery was good or empty.
Puzzled, but always having tons of batteries around the computer, I did the experiment and it does indeed work. It’s even easy to figure out why it does : the discharged batteries are noticeably lighter than the new ones. Hence the higher bounce. Which begs the question : why ?! And where does the mass go ? These batteries aren’t *that *old, so I’d assume leakage is not a factor. Or is it ?
It must be one of those stupid questions, and I’m sure I could answer it myself if I remembered high school chemistry but I’ll admit, I’m a bit stumped.
I can’t believe that the discharged batteries are truly any lighter than they were when they were charged. At least, not due to charging or discharging. There is an electrolyte which is aqueous (but a thick paste), maybe it evaporates?
Perhaps it has something to do with the bounciness of zinc (in a fresh alkaline battery) vs zinc oxide (in the discharged battery), or the corresponding properties of the manganese oxides.
I’m declaring shenanigans on that. Did you actually weigh them using a digital scale? If not, did you at least do a blind test, i.e. you didn’t learn whether a batter was charged or not until after you decided whether it was “light” or “heavy?” If you just picked up a known-dead battery and a known-charged battery and decided “hmm, yes, the dead one is lighter,” then I don’t have much faith in your assessment. Not saying you’re absolutely wrong, but the reason instruments and blind experiments exist is because it’s extremely easy for bias (even unintentional bias) to creep into data.
I’d advise repeating the test, but taking some steps to eliminate bias/error:
start with several identical, new batteries from the same package.
Weigh them using a scale.
Do the bounce test. Drop from a measured height, and measure the height of the bounce.
Discharge them. Use a fairly high-powered device so you can empty the batteries within an hour or two.
Weigh again.
Do the bounce test again.
If you want to go even further, keep half of your group of batteries in a fully charged state. Label all of the batteries with randomly assigned numbers, and keep a reference sheet so you know which ones are charged and which ones are discharged. Hand the batteries one at a time to a volunteer, and ask him to determine via the bounce test whether they are charged or discharged. At the end of the experiment, you should have a pretty good idea whether it’s possible to reliably determine the state-of-charge of a battery thorugh this bounce test.
This was an experiment that Yankee fans used to perform. If the battery bounced off the head of the outfielder, it was no good but if the outfielder went down, it was good.
Er, lighter things don’t bounce higher. It depends on their stiffness and their density and weight distribution. Try bouncing a ping-pong ball and a golf ball.
The OP’s linked image doesn’t mention the weight of the battery. As pointed out above, is is very very unlikely that the weight will change. Dry cell batteries are sealed, and should not vent unless they are very badly mistreated.
The claim is that a good battery won’t bounce about as well as a dead or dying battery. This isn’t a claim about the mass of the battery, it is a claim about the mechanical properties of the battery. In particular what amounts to the mechanical Q of the battery. So, it might be conceivable that the chemistry of a drained battery changes enough that the morphology of the contents is modified enough for this to be true. Maybe it forms crystals, or some other change occurs so that the Q increases.
So, the experiment needs doing. And doing well. Incubus has a great suggestion in making it the subject of a kid’s science project.
Pedantry alert. Yes, simple AA and AAA batteries are not really batteries. They are single cells. Further, yes, any cell does weigh some minuscule, utterly unmeasurable amount less when depleted of its stored energy.
Regrettably, I don’t have any scientific or even cooking equipment at my work desk :). But yes, you’re right, it was a “hmm, this feels heavier” thing, not a controlled setting and such.
I shuffled them around in one hand, then correctly figured out which was the dead one just by doing a by-feel weight comparison afterwards, one in each hand. Only a couple of times though - I didn’t do it 5000 times in order to absolutely eliminate human error or statistical aberrations.
So, all right, assuming the weight thing to be a red herring, what could be the actual trick behind the bouncing trick ?
Not about to discharge batteries for fun, or even for Science!. Those things are expensive !
[QUOTE=lazybratche]
Perhaps it has something to do with the bounciness of zinc (in a fresh alkaline battery) vs zinc oxide (in the discharged battery), or the corresponding properties of the manganese oxides.
[/QUOTE]
I could imagine batteries losing mass over time, maybe depending on whether they are used or maybe only depending on time. Also, batteries that you’re not sure about whether they have a charge are most likely to be the ones that have been laying around for a while, not ones used up quickly. So I’d add
2b. Discharge them slowly, over the span of at least several months.
2c. Store batteries unused, for at least a several months.
Unfortunately, that makes the experiment take much longer. But on the plus side, it could just be a matter of labeling all the batteries prior to use/sitting around, and performing the measurements when the ones in normal use need to be replaced.
Its because of a density change. Google up US Patent 5567541, Method and apparatus for measuring the state of charge in a battery based on volume of battery components.
A change in density would not make batteries noticeably lighter or heavier. The difference would only be weight of a volume of air equal to the difference in battery volume.
You understand, I hope, that particular patent dealt with larger batteries where you can see a measurable deformation of the battery.
The patent doesn’t apply to small AA batteries. A moment’s thought reveals that density changes don’t pass a sniff test. The volume doesn’t change (if they are, why are you using visibly deformed batteries at all?) and the mass certainly doesn’t change (or there’d be leakage on the supposedly sealed cells). So, the density of the battery doesn’t change, either.
A better question is if the battery bounce trick works at all. Nobody really gave a scientific demonstration in the first place.
This was brought up and demonstrated at a luncheon of ham radio operators and we all saw the discharged batteries bounce. All were of the same brand and expiration date. A few witnesses thought they could feel a weight loss in the discharged cells, but no one had a scale. That would seem consistent with a water loss from the electrolyte paste if true, but i don’t know if or how those cells were vented.
The Lockheed inventor described sealed cells that couldn’t lose mass, hence a density change instead. If these commercial alkaline cells are similarly sealed, possibly some gas is released during discharge, albeit a tiny amount, that expands the negative can and makes it more elastic when dropped.
As mentioned above, the qualifier was “scientific”. A double blind study would work. A bunch of guys testing random batteries is more anecdote than evidence. The problem is confirmation bias. Whether or not it’s true, people will know the difference exists and that will bias their observations.
Rather than some electrochemical principle, the simpler answer is that a used battery is used, i.e. it’s got wear and tear on it. Rather than weight, I hazard (with no evidence) that people can feel the minor scuffing and wear and translate that to a feeling of less weight. Also, confirmation bias. You know which cells are discharged before you bounce them, so you look for even minor differences in behavior and may even bounce them differently (kind of like how ouiji boards work).
If they’re sealed, they don’t vent gas. The Lockheed inventor was referring to real batteries, not single cell. They didn’t lose mass but they did change volume - the patent refers to a deformation of the battery, i.e. a volume change. You don’t get those kinds of volume changes in single cell AA. If neither mass nor volume changes you cannot have a density change. It’s the definition - density = mass/volume. If mass doesn’t change and volume doesn’t change, you can’t have a mystical density change suddenly appear.
“Just-so” stories are a reasonable place to start. But at some point, they have to be properly tested to make any sort of confirmation. A story that “makes sense” is fine, but it’s hardly proof.
Measuring the open circuit voltage (V[sub]OC[/sub]) is better than nothing, certainly. But while a low V[sub]OC[/sub] value is certainly indicative of a discharged cell, it is not necessarily true that a “good” V[sub]OC[/sub] value (e.g. 1.55 V) is indicative of a good cell. I have seen cells that have a good V[sub]OC[/sub] value, and the voltage quickly plummets when you draw even a few milliamps from it. This was due to the cell’s high internal impedance (which a simple V[sub]OC[/sub] measurement can’t detect).
A much better way to test a cell is to measure the voltage when a load (a wirewound power resistor, usually) is applied across it. The load should have the same (or a little bit lower) resistance as the device powered by the cell. As an example, if a 1.5 V cell is used in a device that draws around 300 mA, connect a resistor with a resistance of between 4 Ω and 5 Ω to the cell and measure the voltage. If the cell’s voltage droops an appreciable amount when you connect the resistor, the cell is bad.
Here is a plot to give you an idea of the voltage as a function of current for a fully-charged AA alkaline cell.
An even better technique is to connect two different resistors to the cell, and then calculate the cell’s equivalent V[sub]OC[/sub] and internal resistance.
I brought up the patent, http://www.google.com/patents/US5567541 because it described a density change in one or more electrodes as a known consequence of discharging *any *electrochemical cell, not just a battery of cells. The inventor didn’t explain the electro-chemistry, but neither was he required to.
The examiner cited an earlier patent as “prior art,” http://www.google.com/patents/US5244754 It disclosed a variation on a “standard alkaline battery” (technically a single cell) having a compressible chamber filled with a viscous gel and exposed to the expandable cathode of the battery. As the battery discharged, the cathode expanded, causing the gel to be extruded out through an outlet into a viewing channel. The amount of gel flowing into the viewing channel was said to be proportional to the amount of expansion of the cathode and, thus, indicates the amount of discharge and, conversely, the remaining life of the battery. That inventor described the expansion as irreversible and accumulative, and usually a linear function of the total current taken from the battery.
So both patents exploited a density, and given constant mass, a dimensional change in the cathode of a single cell to measure its state of charge *without *measuring cell voltage. Each patent claimed a different method to amplify the effect so as to make it detectable outside the cell without a micrometer or the like.
What we saw were three AA Alkaline cells at 1.6V O.C. that dropped onto a granite restaurant table with a thud and fell over, in comparison to two otherwise identical cells at 1.36V O.C. that bounced a few times before falling over. Just a little elasticity in the bottom of the can.
