Can someone Explain to me Internal Resistance?

While I have used the word a few times myself during quotes, I dont know what it actually means.

its resistance to electron movement. like a "resistor" has resistance, other materials and objects do, too.

It is resistance within the battery itself. As more current is pulled, the voltage supplied drops. The amount of internal resistance depends on the chemisity and construction of the batteries. In rechargeable batteries, internal resistance increases over time.

Here is a good informative link.

I also remember hearing that a spring on a tailcap can give resistance. Is that true? It was in a X1 mod I believe..

Resistance is an electrical property of virtually all material, so it is almost never a question of whether it has resistance or not, but how much. In most devices like flashlight you'd like the battery internal resistance, the electrical path to the lamp to be as low a resistance as possible. So there is certainly electrical resistance in the tail cap, you just try to make is make it very small relative to the lamp itself. Other materials have very high resistances, they are called insulators, the difference between a good insulator, and a good conductor is often a factor of 100 million. Special devices are used to test insulators, and they work by applying a high voltage, and measure the resultant current. One of the simpler ones is called a 'Megger'. The low range starts at about a million ohms. . .

There are some simple relative exoctic material that exhibit True ZERO resistance, the simple ones do so a near absolute zero (mercury,tin and niobium for example are super conductors near absolute zero, they have true zero resistance), and few materials have been developed that have true zero resistance at temperature around Liquid Nitrogen, however the physical properties of these materials make them difficult to use. The high magnetic fields used by most Nuclear Magnetic Resonance Imaging (MRI) system are produced by magnets with super conducting winding, so the losses in the coils due to I^2 R heating are zero, since R is truly zero.

Another discussion of this from Wikipedia

RESISTANCE AND IMPEDANCE

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