Capacitor input output help with car battery

[burtonboy]

Hey, I'm in grade 12 and electricity is only a hobby so my knowledge is limited.


THEORY BEHIND MY QUESTION
I have a science fair idea. Basically when a car battery wont supply enough power to start the car, it usually isnt dead, its just too low to supply enough power to the ignition coil. So I was thinking if I figured out some sort of capacitor system, maybe I could charge up a capacitor with the half dead battery to the point where the capacitor will supply enough power to send a surge of electricity to the ignition coil just like a full power battery would.
Kind of like the flash of a disposable camera. The low powered battery charges up the capacitor, then you press the button and flash!

BOTTOM LINE OF QUESTION
so basically my question is, what kind of capacitance would i need to use to be charged by a car battery with not enough power so that the capacitor sends the required power to the ignition?

is there some kind of formula with capacitors where a certain voltage and current into it equals a certain voltage or current out of it depending on its capacitance?

I guess with capacitors the capacitor will charge up to its "capacitance". But then what kind of power is put out when it is discharged, and how would you control that.

thanks for any help![/b]

[pebe]

It is not the current for the ignition coil that matters when you start the engine, it is the current drawn by the starter motor - and that can be around 100A

My rule-of-thumb for calculating charge voltage for capacitors is that 1A flowing into a 1Farad capacitor for 1sec will raise its voltage by 1V. The same rule applies for discharging.

To work out the capacity required we can assume that the battery voltage when starting should drop by no more than 2V and the car should start in 5secs max. That gives a required capacity of 100A x 5sec / 2V = 250Farads.

1Farad = 1million microfarads. So if you were to use 500uF/16V electrolytic types (a typical value smoothing capacitor) you would need half a amillion of them! So I'm sorry to say that your idea is impracticable.

[I_Daniel]

The only way you can achieve this is to use an ignition booster system.
It consists of a dc-dc converter to charge a 8000 micro farad capacitor.
These used to work fine on an ignition system that use contact breaker points, but for electronic systems it wont work for it is placed between the contact breaker and the coil. The electronic systems I have seen have the special coil built into the distributor..

CDI (Capacitor Discharge Ignition) systems work very well down to about 8 Volts but this is merely an aside and will not solve your problem..

If the ignition system is merely an electronic assisted system, i.e a transistor that switches ten to twenty Amps, ouch for the load on the electrical system, then the capacitor ignition booster system should work.

I have a circuit diagram in an ancient Practical Electronics magazine (February 1972) but I don't think I am, because of copy right, permitted to publish it here. It uses OC28 power transistors (PNP Germanium!). The secondary winding is fifty turns centre tapped and the primary is 32 turns centre tapped. using 20 s.w.g. Two separate tickler (feedback) windings are 5 turns each of 28 s.w.g.