# B Choosing a capacitor based on desired output pulse

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1. Jan 4, 2016

### Undacuva

Hi folks,
I need to get a super capacitor which will be charged by a car battery and then connected to let out a massive pulse. (Yes a bigger pulse than my 450CCA battery can give). But first I need to work out how much capacitance will be needed. I'm not sure of the formula.

Thanks

2. Jan 4, 2016

### Merlin3189

Well I suppose you need Q = CV the charge stored is the capacitance x voltage
and I t = Q the current x time for your pulse is the amount of charge needed.
(Edit: or for varying current Q = integral I dt )
Though you will need to think about the fall of voltage during discharge, as only the difference in charge between the starting and finishing voltage, will be available.
Maybe some figures for your requirements (and description of task?) would be useful for people trying to help?

I have no special knowledge nor experience of super capacitors, so I don't know what their internal resistance is, nor whether there are any other limits on their discharge rate. (I assume you are looking at a very short pulse here?)