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I have a flashlight which I use when I have to get up in the night in the pitch-black room. It takes three AAA batteries.

Recently, it has been rather dim, so I assumed that I would have to switch batteries soon. First, I took the current batteries out, put in brand new ones, and tested it. Much brighter, as expected.

Then I took out the new ones again and put the old ones back in, to compare. Now it was almost as bright as with the new ones again.

This is not the first time in my life that I've had similar things happen. A remote controller for a television set might seem dead, but once you open the battery hatch and "roll around" the batteries a bit, or take them out and put them back in again, it again works.

How is this possible? How can this (apparently) cause new energy to flow into the batteries? Surely this cannot be what is happening?

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    $\begingroup$ You may have disturbed some high resistance contact oxidation. You may have given the batteries a rest and they will bounce back chemically for a short while. $\endgroup$
    – Transistor
    Jan 1 at 1:12
  • $\begingroup$ next time spin the cells in place $\endgroup$
    – jsotola
    Jan 1 at 2:54

2 Answers 2

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It is common for both the button contacts on the ends of a battery and the springy metal fingers that make contact with those buttons to get a thin film of oxide forming on them. It does not take much of this to impede the free flow of electrons across those contacts, and simply shaking the flashlight or rapping it against a tabletop is often enough to rattle the batteries around a little, thereby scrubbing the oxide and regaining good metal-to-metal contact, which yields a brighter light.

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In addition to @niels ' discussion of contact quality, for some battery types agitating the battery can cause the dielectric gel to rebalance its distribution (and quite possible pull resistant crud off the electrodes). This will allow extraction of remaining energy from the cell internals.

A more obvious example is found in a lead-acid battery of the old-fashioned car type. If most of the water evaporates, very little contact with the electrodes is made (even tho' the remaining fluid is a high pH acid). Add a bunch of distilled water back in, and the increased surface area on the electrode-fluid contacts leads to more output.

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  • $\begingroup$ "A high pH acid"? $\endgroup$ Jan 5 at 11:32
  • $\begingroup$ @DanielHatton How did I guess someone would freak at that. "A very strong acid" ; my intent was to describe an acid far from 7; but I can see that it's misleading. $\endgroup$ Jan 5 at 11:46

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