If I have an electronical device that runs on 8 AA 1.5V batteries. 8 X 1.5V = 12V. Could I power that off a 12V car battery without damage
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M$4 Answers
So, assuming you know for sure that the AA batteries are supposed to be hooked up in series (e.g. because your user manual states that the device works on 12VDC), you can safely hook up the device to a car battery. The current in a DC device will be determined by the voltage (the same 12V in either case) and the resistance of the device (which is also the same in both cases). Thus, the current, measured in Amperes, or more colloquially - Amps, would be the same in both cases, and would not damage the device.
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M$
The current (or Amperage) is determined by the voltage of the battery, and the total resistance of the circuit, which is going to be mostly the resistance of the device. Thus, you are mistaken in stating that the current will be ~30 times higher with the car battery. Certainly the car battery has the ability to provide much higher currents, but your circuit would need to have a lower resistance to draw that larger current.
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M$
You risk damaging the electronic components by doing this. If you do this, you'll probably hear a pop, and notice that a capacitor is leaking a white fluid. Amps are as important as voltage in sensitive electronic equipment, as you can see from most schematics:
http://www.electronics-wiki.com/Infrared_Remote_Control_Tester
http://www.techspot.com/vb/all/windows/t-8532-Amperage-vs-Voltage--Which-is-more-dangerous.html
AA batteries are 1.5v at 2.4amperes when it's in perfect condition.
http://www.youtube.com/watch?v=8cXN7ic_L9ACar batteries are about 12v at up to several HUNDRED amperes.
When you stick a 9v battery, which is a bit more beefy than a AA, on your tongue you get zapped a bit. Connect the terminals with a wire and they get warm. In the attached video somebody connects the terminals of a car battery by using jumper cables to attach it to a saw and a metal rod.
That's a bit more than a little zap.
"you make it sound as if this is an AC circuit, which it is not."
That would be because the only time I would expect something like this to work is, like I said, in a situation designed to handle that much juice. Take DansData's DIY car battery UPS for example, which is exactly what I was referring to. Anything else I still think you place far too much faith in the ability of consumer electronics to not be fried by excessive current. Things never designed to run from something with that much available could very well be prone to thermal runaway with anything juicier than their original AA's and it's reckless to assume that suddenly using wildly out of spec parts won't cause problems one way or another just because it's the same voltage.
And that's just ONE potential pitfall you're ignoring in this scheme that I can think of off the top of my head at 8 in the morning. I very much doubt that it's the only problem with throwing several hundred more amps than something's expecting at it.
@shadowex3, you say "On paper you would expect that result, yes, but in the real world you can't just hook up some 300amp cold cranking car battery to a device that is expecting 8 AA's and not see something melt or explode."
However, unless you reinvent the laws of physics, if you execute something correctly, based on a sound design, it will work, and not just "on paper." Since you now acknowledge that the design would work, it is a question of execution.
Obviously, if you play around with electricity, and don't know what you're doing, you could damage your equipment and potentially hurt yourself. This is why many folks get electricians to replace light fixtures, which is a simple task to do yourself. Even a simple electrical task, if you are not careful, can turn horribly wrong.
As for your statement that it would be ok with a regulated item if you "keep the waveform clean (modified square probably would work)" you make it sound as if this is an AC circuit, which it is not. The "waveform" in a DC circuit is (nearly) step-function. You have very brief transients as you turn it on, and then it settles (within micro-seconds) to a flat DC voltage.
Regardless, I think we now have finally arrived at an agreement. The answer to the question is that yes, you can use a car battery and it should work fine. However, I'll certainly accept the proviso that given the car battery's capacity to provide extremely high currents, this is not advisable if you do not know what you're doing, and if you are not careful about making sure you don't short-circuit the battery.
It would be nice if you did not simply click fact refuted without more support than that. It so happens that you are simply less than fully knowledgeable in electrical engineering.
In the examples you give, the items connected have very low resistance, so that the current can be very high, unless the battery is incapable of providing such high currents without dropping the voltage. Obviously a car battery is designed to provide high currents (so it can turn the engine over when you turn the ignition key) so it can provide the "zap" you see.
In the case of your tongue, or the wire, being connected to the 9V battery, again, the resistance is low, so a high current results.
However, in the case of an electronic device designed to work with 12VDC, the resistance (or more properly, the impedance) is high enough that the current drawn from any 12VDC source will not damage the device.
The current is determined by the resistance of the device (plus the usually much lower internal resistance of the battery) and the voltage of the source. In the examples you give, the resistance of the portion of the "circuit" outside the battery is very low, so you can get very high currents. If you hook up a set of 8 AA batteries in series and connect the "+" end of the stack to the "-" end of the stack with a wire (i.e. you short-circuit the batteries), you will tend to get a high current, but since the ability of AA batteries to provide high current is limited, the voltage of the cells will soon drop below 1.5V each, reducing the current. This is a very different proposition than what the asker is talking about.
On paper you would expect that result, yes, but in the real world you can't just hook up some 300amp cold cranking car battery to a device that is expecting 8 AA's and not see something melt or explode.
If we were talking about some kind of regulated item like a computer or whathaveyou I'd say it might work if you're careful and keep the waveform clean (modified square probably would work) but if you're talking about a 12V RC car or something I'd be very suprised if doing that didn't burn out several components.