I know that the Peak output voltage in a ignition coil is between 10,000-45,000 V "Or so I heard" but how many amps does the ignition coil draw from the 12Volt side ? I heard it was small. Why is that?
I read that modern coils draw about 4 amps while the points are closed. Either 12V across a 3-ohm primary coil or 6-9V across 1.5 ohms in a ballast coil. Either way, somewhere in the 4-6 amp range while the car is running. If you have a ballast coil (e.g. runs on 6 or 9V) that bypasses the ballast resistor when starting the engine, you could get higher amps (12V over 1.5ohms = 8Amps).
I imagine that if you average the draw over one cycle of all cylinders it would be considerably less, depending on the duty cycle of how long the points were closed (dwell time) vs the points-open time (when the spark happens and no primary current is flowing, except into the capacitor across the points.
Ideally, the points would only close and dwell long enough to build up what was needed to create a nice magnetic field in the coil.
Smart electronic systems will adjust the dwell time to ensure that enough magnetic field is built up for a good charge. Dwell times are usually in milliseconds (3-10ms), I think. So, on a full round of sparks for a V8 engine, your coil would draw (worst case?) 8cyl x 10ms = 80 or 100ms or so. 100ms/rev x 1000rpm = .001sec/rpm x 1000 rpm = 1 sec of current draw at 4 amps per minute while running at 1000 rpm.
Suppose a battery had a 50 amp-hour rating. That would be 50 x 60ming = 300 amp-minutes, or 300 x 60 = 18,000 amp-seconds. Using 4-amp-seconds per minute from above, 18000/4 = 4500 min. So, the battery might last 4500 minutes/60 = 70 hours or so (2-3 days @ 24hrs/day) of running time.
That assumes that all the battery does is run the ignition - not the headlights, radio, heated electric seats, not starting the car, etc.
Because of the electrical and magnetic characteristics of a coil - but that would be best explained in the Electrical or Physics stack. Also, the capacitor and points / contacts are only designed for the small current flow and they have to operate at 1/2 * engine rpm (or one per two revolutions for a four stroke).
Usually 10 amps or less - the wires to the coil tend to be 14/010 same size as used for indicators etc.
See some detail here: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/ignition.html[How a coil works]