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My camera, which is powered by two AA batteries in series, would not power on. I removed the batteries, exchanged their locations, and the device worked again - for another 15 minutes or so.

The temperature of the batteries did not change, and only about few minutes passed before I discovered this.

Is there a physical explanation for this phenomenon?

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What probably matters most is that you waited for a fwe minutes. A battery can "revive" itself by distributing the material more conveniently. You may think that a nearly dead battery suffers from congestion, hiding some useful material behind the material that is discharged, but it gets cured when you wait for a while. –  Luboš Motl May 16 '11 at 4:37
    
@Lubos I waited a few minutes after they died, but the device still would not turn on. Do you think that three minutes of rest would enable a battery to run for 15 more minutes? –  David May 16 '11 at 14:11
    
perhaps the motion of removing the batteries disturbs the electrolyte and causes a redistribution. I have noted this effect routinely in using my camera. I frequently get additional usage from the AA batteries by removing the batteries, and then reloading them into the camera. It might be possible also that the camera requirements for power are borderline with the remaining charge of the battery, so you are achieving a slight boost from a redistribution, putting you back over the edge of operability with your camera. This is all empirical though, so I'm curious for real answer. –  Jen May 16 '11 at 15:13
    
I am curious why this question got a downvote? –  David May 16 '11 at 15:18
    
Were the batteries or connections old at all? I've seen this happen for devices/batteries that had a little bit of corrosion/oxidation. As you remove the batteries, the metals contacts can get scraped and cleaned just enough to provide you that little bit of extra current, due to less resistance. –  farrenthorpe Jul 5 at 13:48
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4 Answers 4

I've noticed this many times, too. In many devices, the batteries are in series and two of the terminals are just a short-circuit to connect them to each other. In these, the total voltage is the sum of the individual voltages, and the order of the batteries should have absolutely no effect on anything.

I can think of 2 possibilities:

  1. The act of removing them and putting them back in is causing the effect. It might remove power from a microcontroller or other "always-on" part of the circuit and cause it to re-measure the battery voltage when they're re-inserted, for instance. You could test that this is the cause by removing them completely and then putting them back in the same order, and see if it has the same effect.
  2. In other devices, the middle terminal is actually used in the circuit, such as a circuit that has ground, +3 V, and −3 V powered by 4 batteries:
    Battery split supply schematic
    In this circuit, it's very unlikely that both sets of batteries (positive supply and negative supply) will be drained at exactly the same rate. Maybe a power LED is connected between + and ground, for instance, which draws a little more than the ground to − side. By removing the batteries and replacing them in different places, you could be putting the less-depleted batteries in the more-hungry slots, which would squeeze a little more life out of them.
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I made up an explanation, please comment if this could be correct.

Living batteries might look like this:

enter image description here

Where red stuff is not the labels but the charges.

When you attach an electric chain, the potential of batteries drives current through the chain. And by current we mean charges. So battery loses charges, which are restored by the chemical (?) stuff inside. When it's used up and the stuff can't restore anymore charges, you just use up the remaining charges creating picture like this:

enter image description here

Then David swaps the batteries:

enter image description here

And voila - we have the charges to maintain potential and drive current for 15 more minutes.

The pictures shouldn't be taken to literally, by using up charges I mean you drain the stuff in one side of the battery.

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A battery does not accumulate charges, but potential energy. If your drawings were correct, in the second case, the charges would flow from one to the other cancelling out. –  Davidmh May 30 at 14:22
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First, your camera is not designed to work with batteries below a certain voltage. When it detects an excessively low battery voltage it turns itself off. That circuit stays in the "off" state until voltage is completely removed from the circuit.

When you operate your camera, the current required by your camera varies according to what you do with it. So the typical current required by your camera is less than the maximum current. Theoretically batteries are supposed to have a voltage which doesn't depend on the current you draw from them, but this is only an approximation. In fact, all batteries have a negative V-I curve; if you increase the current taken from them, their voltage decreases. This effect is like having a resistor in series with the battery. It is called the "internal resistance" or impedance sometimes. Usually it is a very small effect but the dead voltage sensor in your camera can be very sensitive and so small effects can make a big difference. In addition, as a battery runs out of juice, its internal resistance increases [also see] and so the slope of its IV curve increases.

Battery V-I curves

As you use a camera, the current requirement goes up and down. As the batteries run out of power, eventually the maximum current usage will give a voltage that is too low for your battery's voltage detection circuit. At that time your camera will turn itself off.

Your swapping the batteries had no effect on their voltage. (Okay, there might have been a very very slight effect from your hands warming the batteries slightly, but this effect is probably quite small compared to the effect I'm describing here.) Instead, what you did was to reset the dead-battery circuit. After 15 minutes, either you did something that increased the battery current enough to again trigger the dead-battery circuit, or possibly the small current drain caused the battery to become even more dead than it already was.

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Sometimes dead batteries will return to life if the are shaken. This only works for batteries that have liquid energy storage chambers. I have never heard of it working with AA batteries before, but I haven't heard of a lot of things.

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I've had this work before with D-cell alkaline batteries in a flashlight before. Shaking them would make them work for a bit longer. So even though the electrolyte is more like a pasty goo than a liquid, I guess it still helps somehow. –  John May 15 '11 at 22:33
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+1 for "but I haven't heard of a lot of things" –  Richard Rodriguez May 16 '11 at 0:52
    
I have also read that squeezing them with pliers works, but neither of these cases is relevant to my situation. –  David May 16 '11 at 14:11
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protected by Qmechanic May 30 at 15:14

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