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Using Capacitor in the Main Traction pack

2620 Views 9 Replies 5 Participants Last post by  Zemmo
I haven't had my EV for very long and was thinking about some possible improvements. I was wondering if anyone has built a capacitor pack to match the system voltage and run them in parallel? The capacitors I think would help level out the power demands from the batteries. It wouldn't extend the range but it should in theory level out the power surges. You can find some what large farad capacitors on ebay fairly reasonable (50 bucks a piece for 3 farad capacitors). Has anyone tried using capacitors in the main battery pack? If so, any improvements, longer battery life?
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have a look at this conversion. He used Li-Ion and supercaps to give extra power
That is an interesting website. I was trying to find one where people used some sort of capacitors. But am looking at the cheaper capacitors, the 3 farad ones that people use in stereo installations. My EV runs a 120 volt pack so I would only need 12 of these caps which would run me $600 bucks. If I talked to a distributor, I might be able to get them cheaper by ordering them in bulk. But I was wondering if anyone has any experience with those capacitors. Would having those 3 farad capacitors help the life of lead acid batteries?
I have seen those large capacitor. But they are the size of a battery. I would only be able to fit about 3 or 4 of those in my car in the left over space. Thats why I was looking at those 3 farad capacitors, can easily get them to fit. But I am trying to find out if it is even worth doing. I haven't seen data on this yet to know if it would expand battery life or if it would do damage. I don't know either way, in theroy, it would seem to help because of the amperage draw differences.
I took out my 12 volt battery because it was drawing too much power from the DC/DC converter. But the Sevcon DC/DC converter doesn't product enough power to run the car at night. (Lights dim, slow blinkers etc) So I may try a capacitor in place of that battery, it would be lighter weight, smaller and wouldn't require any maintenance. It is cheaper to replace that battery with a capacitor than replacing bulbs with LED equivalents (6 to 9 bucks a bulb). But I am surprised no one has experimented with this yet. Maybe not enough people have run across this forum yet.
It is cheaper to replace that battery with a capacitor than replacing bulbs with LED equivalents (6 to 9 bucks a bulb)
But will a capacitor discharge slowly enough for all the 12v equipment? Or is it just to compensate for the DC-DC converter's low current?
The unit of capacitance is a farad. Capacitance is given as Q(coulombs) = C(farads) x V(volts). A 1-farad capacitor can store one coulomb of charge at 1 volt, and will deliver 1 amp-second at 1 volt. Charged to 12 volts, the 3F capacitor built to sustain this voltage could theoretically store 36 coulombs, and would discharge to provide 36 amp-sec at 12 volts, or 432 amp-sec, or 0.12 amp-hr to a 12 volt load. Unfortunately, a capacitor's discharge voltage also drops exponentially with time to zero, depending on the load's circuit resistance.

Might be good for some dc motor-starting applications or other excessive (very short-term sub-sec) peak transient load requirements that exceed a storage battery's normal discharge capabilities, but methinks this is hardly a valid candidate for outright battery replacement.

For most applications in traction packs, it seems to me a much better expenditure would be for additional batteries instead of any "supercaps."

Just my $.01 worth, your mileage may vary...

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This is an interesting thread. Am I right in assuming that the energy contained in a capacitor would bare a linear relationship to the voltage across it so in order for a capacitor to discharge some of its energy the battery voltage would have to sag? If so to half discharge your capacitor, battery voltage would have to sag to half which would require a huge current draw. What would be the leakage current across such capacitors? Could they be left sitting across the battery bank when the car is off so that they don't unduly stress the batteries on start up? Surely the initial connection of the capacitors to the battery would need to be via a resistor to limit the current.
Those are some good points. I have only seen the use of the caps inline for stereos. They are connected full time to the battery source. They also counter act the amps surges pulled by a stereos amps to push large stereo's. So the DC/DC converter can supply the volts and amps to run the car but the surges of using the brakes, headlight, etc along with the draw on the 120 volt pack to make the car go. So I figured a cap would help alleviate the surges on the DC/DC converter. I don't have a clue on the formulas, just thinking of some improvements.
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