Originally Posted by ItsJustMe

That is exactly the wrong capacitor to use. I bet you got that from a microwave.

High voltage means it can HANDLE high voltage, not that it automatically has high voltage. it's not an amplifier. An extension cord can handle 110 volts, but you only need to be concerned about it if it's plugged in to 110 volts.

It says "danger high voltage" because it's DESIGNED to be part of a high voltage circuit. When in a high voltage circuit, it has high voltage in it. If you hook it up to 5 volts, it will have 5 volts in it.

High voltage capacitors are not magic voltage amplifiers. It simply means that they have thicker plates and are able to have more voltage applied to them without the insulation breaking down and arcing internally. The thicker insulation (or dielectric) also radically reduces the capacity of the device to store electricity. The capacity is related to the square root of the thickness of the plates. Go with thinner plates, you get far more capacity (in watt hours) but it can take less voltage before it breaks down and fails.

So what you want is a capacitor with a lot more capacity than this (this probably has maybe 1 microfarad, maybe enough to run a taillight for a tenth of a second or less). I'd guess that what you WANT is more like a few thousand microfarads or more. To get this in a reasonable package size, you want the voltage rating of the capacitor to be as low as possible - double what you expect to see in your circuit is a good starting point.

Also you need to put in diodes and current limiting so that the capacitor charges off the residual power and doesn't hog all the power and keep your light from working until it's charged.