50 Farad Capacitor - DIY Electric Car Forums

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#1
11-11-2007, 07:40 PM
 ohio Member Join Date: Jul 2007 Posts: 51
50 Farad Capacitor

if i put 1 of these on each of my batts 13 of them do you think it worth it ?

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#2
11-11-2007, 09:14 PM
 Mr. Sharkey Senior Member Join Date: Jul 2007 Posts: 207
Re: 50 Farad Capacitor

What, exactly, do you expect it to do?
#3
11-12-2007, 09:39 AM
 CPLTECH Senior Member Join Date: Sep 2007 Location: Cincinnati, OH Posts: 293
Re: 50 Farad Capacitor

One of the purposes of a capacitor is to merely "store" voltage. In electronics, it is used to smooth out the DC peaks & valleys of a power supply & mimic a battery in smoothness. The voltage quickly decays when input power is removed, as when batteries removed from a playing radio. I understand that a 50F must be "huge", but consider the "huge" amp draw of your motor. Explain any benefit in using the capacitor in this application.
#4
11-12-2007, 03:15 PM
 ohio Member Join Date: Jul 2007 Posts: 51
Re: 50 Farad Capacitor

faster take off and easier on the batts
#5
11-12-2007, 07:34 PM
 mattW Super Moderator Join Date: Sep 2007 Location: Sydney, Australia Posts: 882 Blog Entries: 18
Re: 50 Farad Capacitor

The capacitors will only kick in when the voltage across the batteries drops and only until the charge held in them is dissipated. Those capacitors on a 12V battery could store 3600J each of energy, the rate of discharge is dependant on the change in voltage according to the equation i= Cdv/dt so current is equal to the capacitance (50F) times the rate of change of voltage (or the gradient of the voltage curve). You should whack a voltage meter across the terminals of you batteries and see if the voltage changes under take off etc.
#6
11-17-2007, 12:28 AM
 mattW Super Moderator Join Date: Sep 2007 Location: Sydney, Australia Posts: 882 Blog Entries: 18
Re: 50 Farad Capacitor

If i was very clever and was willing to experiment i would have a relay which when the brake pedal was applied would disconnect the motor from the controller and connect it to the string of capacitors with a resistor to regulate it. They would then charge up pretty quickly while slowing the car down. Then once braking stopped i would have another relay(s) that would open and allow the capacitors to slowly discharge into the batteries. This would be a very efficient way to do regenerative breaking. If I was going to use capacitors that would be a much better use.
#7
11-20-2007, 06:10 AM
 mattW Super Moderator Join Date: Sep 2007 Location: Sydney, Australia Posts: 882 Blog Entries: 18
Re: 50 Farad Capacitor

Here is an email from the EV discussion list about this topic:

Say you have a 144V pack that sags to 110V under heavy acceleration and
the caps are directly in parallel and you pull that heavy acceleration.
The caps are great at t=0, but the cap voltage decays as they provide
current. As cap voltage drops the current will shift back to the batt.
Eventually- time depends on capacitance- all the current is coming from
the batt, batt's sagging to 110V, and the caps do nothing. In fact the
batt voltage will stay low for an equally long period as the batt puts
out equally high currents to raise the cap voltage on these
low-impedance caps.

Furthermore, at 110V the cap still has 58% of its initial energy, it's
just not usable. It might be desirable if your car can keep moving at
say 70V to disconnect the batt and just run on the caps since they don't
wear out or suffer ill effects from high current charge/discharge. But
that is quite an extra level of complexity. In fact once you separate
them you and let cap voltage drop below batt voltage they can't just be
reconnected, it's almost like a short circuit on the batt and the batt
will see the highest currents possible which isn't improving the lifespan.

Don't forget such a cap bank is, today at least, 1) phenomenally
expensive, 2) very large, 3) very heavy. So adding something similar in
size in addition to the main battery bank to stiffen it isn't currently
reasonable and practical.

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