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How does a battery only send the amps amount the device needs, and not more?

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It doesn't. The device won't accept more than it needs. – Ignacio Vazquez-Abrams Aug 30 '14 at 4:02
@IgnacioVazquez-Abrams - Um, doesn't that beg the question? – nsayer Aug 30 '14 at 4:08
up vote 2 down vote accepted

Simply put, the premise of your question is faulty in just one word. A battery does not send current, a circuit pulls it.

Now for the water analogy! (Sorry, because water analogies fall apart eventually, but this one works well enough.)

When you get a drink of water, you aren't obligated to consume the entire water supply, even though it is technically available. The same is true for a circuit, or a simple light bulb or motor. It only pulls what it can use.

But let's say you were a zombie with gaping holes! Now you can drink and drink and drink, but the water just keeps running out onto the floor. So, in a way, this would be similar to a short circuit or a device with a problem; it draws much more current than it should, and might trip a circuit breaker, blow a fuse, or (if those safeties are not in place) cause a fire or other damage.

Check out the other questions I linked in a comment earlier, they'll help you gain a bit more insight:

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Ohm's law: E=IR. The circuit has a certain resistance and the battery has a certain voltage.

Now, that said, the battery is not a perfect voltage source. As the current goes up, its ability to produce voltage will hit a plateau and the voltage will sag. But on a theoretical basis with a perfect power supply, Ohm's law is the answer to the question.

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