# Re: [EVDL] How many Farads are needed?



## EVDL List (Jul 27, 2007)

*Re: [EVDL] How many Farads are needed?*

When I exchanged emails with "Kelly" they said that they filter the INCOMING
current with large caps... limiting the spikes in the current... I just
wonder about that.

I am thinking that the PWM square wave has some nasty current spikes at the
leading edge that are lost to most (slow) instruments and probably don't
cause much heating due to their short duration, but can't be doing anything
nice to the battery physically... if those were provided by the Supercap the
batteries might be happier in the long term...

I am not talking about any appreciable power, but much more than standard
inverter filtering...Maybe full load current for 5-10 cycles..or 5/16,000
sec or 0.0003125 sec...

So a very very low impedance 1F cap would be more than sufficient...

Am I even close in my thinking?

Bob Sisson
1993 Geo Metro Convertible Project
Gaithersburg MD


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Bill Johnson
Sent: Wednesday, December 02, 2009 5:12 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] How many Farads are needed?

One Farad will charge (or discharge) one volt when the current is one amp
for one second.

Or one Farad will charge to one volt when one Colomb has passed thru it.

If you charge a 1000F Capacitor to 96V and begin drawing 1000A from it, it's
charge voltage will drop one volt each second.

Wow, I was spouting that off the top of my head. But I think these three
statements are correct.

In general the energy formula major quotes is probably more useful for your
needs if you happen to know how much energy you want to store. My point was
that as you draw that energy out, the voltage will drop linearly with
supercaps and may become unusable by most of our circuits when it gets to
about the 60% level.

Bill Johnson.



> Jeff Major <[email protected]> wrote:
> 
> > Hi Bob,
> >
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] How many Farads are needed?*



> Bob Sisson <[email protected]> wrote:
> > ...
> > I am thinking that the PWM square wave has some nasty current spikes at the
> > leading edge that are lost to most (slow) instruments and probably don't
> ...


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