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www.piclist.com/techref/power/batterys.htm?key=battery
BY : Sean Breheny email (remove spam text)

Hi Tomas,

Yes, you will get sparks, but they will not be close to the explosive
gasses which may be leaking out of the battery.

The sparks can still be bad to look at (just from sheer brightness and
also from UV light they produce, just like an arc welder).

Actually, the question on what causes the sparks is a valid one. You
are correct that 12V is not nearly enough to break down any reasonable
amount of air.

What's happening is something called "inductive kick". The length of
wire (even if it is only the distance between the battery terminals)
has some inductance. When current is flowing, energy gets stored in
this inductance in the form of a magnetic field. When you try to
interrupt the current flow, the current cannot stop instantaneously
because of the energy stored in the magnetic field. As the field
"collapses", it forces some current to continue flowing for a short
time (perhaps microseconds) by creating a large voltage.

When you touch two wires together (or pull them apart), you get switch
"bounce" - that is, the wires actually connect and disconnect many
times within the first few milliseconds of contact. During this time
the inductance causes voltage spikes which are enough to cause arcing
across the tiny air gap. The voltage doesn't really need to get very
high because the air gap at that point might only be 0.1 millimeter
(it takes about 3kV per millimeter so 300 volts is enough in this
case).

Making the inductance greater (longer wire, thinner wire, more loop
area of the entire loop of wire, or wrapping the wire into multiple
turns) or increasing the current both make the arc bigger.

This effect is actually the same thing that happens in a boost
converting switching power supply. Current is drawn from the low
voltage side for a large fraction of the converter's cycle, put
through an inductor, and then that inductor is connected to the output
for a short time. Since the output draws less current than the input,
the voltage rises as the inductor tries to keep the current constant
(the extra current goes into the output capacitors). This is a more
controlled case because at no point is the inductor ever totally
disconnected - just being switched between a low impedance and a high
impedance.

Sean

On Mon, Jul 7, 2008 at 5:13 PM, Tomás Ó hÉilidhe <toelavabit.com> wrote:
>
> If I'm standing 10 metres away with heavy-duty cable, will I not get
> sparks at the point where I touch the crocodile clips together?
>
> Something I'm curious about: What causes the sparks?
>
> In the case of spark plugs, I realise that the voltage is so high that
> it makes the air conduct, but since a car battery is only 12 V, I don't
> see what causes the sparks.
>
>
<e726f69f0807071507r4306a984yc461782a462a5de5@mail.gmail.com> quoted-printable

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