CaSS Forum | Capacitor Basics

Frequently Asked Questions and Answers by ECaSS®

~ Part 1: Basics Topics~

Edited and answered by: Michio OKAMURA

Capacitors have been becoming more popular recently. The world must be noticing, though slowly, the advantages of capacitors for electrical storage. A lacking of correct understandings of capacitors and their system technology must be major reason for such slowness.

The "Frequently Asked Questions and Answers" pages on this website are divided in two sections; one is here and the other is in the member pages. The former one deals with basic topics and the latter discusses more in-depth issues. To enter the member pages, registration is required. It is free of charge and we will never sell your data; so do not hesitate to register if you need further information.

Q1	Can we replace batteries directly with capacitors?

A1	No. Capacitors are totally different storage by nature.

I know this answer might be disappointing for you, but a fact is a fact. Batteries and capacitors are two different things. For temporary experiments, replacing batteries with capacitors may be all right. However, to obtain the advantage of capacitor storage, you need to have a charger, an output converter and probably an overall system that is specially designed for capacitor applications.

You might protest that both batteries and capacitors are for storage and serve the same purpose. While a deluxe carriage with eight horses and a Mercedes are intended for the same purpose; you would not try to modify one into the other.

Q2	ECaSS® papers say, "Do not try to evaluate capacitors only". Why not?

A2	Improvements are made 2-5 times by capacitors and 4 times by electronics.

Those are coarse values and will vary with applications, though it is clear that more than a half of the improvements of ECaSS® are provided by electronics portion.

Evaluating by capacitors only is something like tasting a sandwich just by the bread. So try to evaluate or incorporate all the accompanying electronics included. Without properly designed electronics using the ECaSS® method, those capacitors could be no more than a pile of lossy parts.

(More numerical discussions appear in FAQ part 2 in the member pages.)

Q3	Are capacitors inherently more expensive than batteries?

A3	Not inherently, though more costly now because of limited production.

There are good reasons. The materials used for, at least ECaSS® capacitors, are not special and do not require particularly costly manufacturing process. Right now, we are buying quite expensive organic electrolyte of special vintage, but it is clear that the price will go down to the level of insulation oil if purchased regularly in large quantities.
(Numerical analysis is available in our member pages. Refer to Q58.)

Q4	Ultra-capacitor, super-capacitor and EDLC, are they same?

A4	Such expressions could cause confusion. Study the following table.

They are also called electro-chemical capacitors and include several different ones. The major difference is whether of not the storage principle works with chemical reactions. For true capacitors, there should be no chemical reaction involved. An electric double layer capacitor (EDLC) uses EDL capacitance on both positive and negative electrodes; therefore, this is a true capacitor and somtimes called as "symmetric" EDLC.

Since it is usually difficult to obtain high enough energy density with EDLC, people looked for another way and produced some capacitor and battery hybrids. One uses a single electrode for a battery and another for a capacitor, and is called "asymmetric" EDLC. Another method uses pseudo capacitance. The word pseudo is not meant to mock such capacitors but is an authorized terminology as capacitance appears not from EDL but redox, or reduction-oxidation chemical reaction.

In a strict sense, "2." and "3." in the table are not capacitors, because they rely on chemical reactions in the major charge/discharge process. As a result, they should generally obtain higher energy densities than symmetric EDLCs together with life, efficiency and charging time limitations as batteries.

"TABLE of ELECTROCHEMICAL CAPACITOR"


1.	EDLC symmetric (pure EDLC)
	1-1. Aqueous (water solution) 1-2. Non-aqueous (organic) <------ ECaSS® capacitors are here
2.	EDLC asymmetric (battery-capacitor, 50% EDLC) (battery-capacitor, 50% EDLC)
	2-1. Aqueous eg.: NiOOH - carbon 2-2. Non-aqueous eg.: Li ion - carbon
3.	Pseudo capacitance (5 -10% EDLC)
	3-1. Metal oxides eg.: RuO2 3-2. Conductive polymers eg.: p and n doped polymer

Q5	Are EDL capacitors much safer than batteries?

A5	Yes, especially the ones with PC or "non AN" electrolyte.

Several companies in the ECaSS® group have tested and received permission to publicly sell their products using PC (propylene carbonate) solvent for the electrolyte. There are many opinions, however, that capacitor performance drops if you avoid employing AN (acetonitrile).

Yes, it is true. Capacitance could rise about twice and internal resistance decrease to one-third by employing AN and optimized in place of PC. However, ECaSS® is looking forward for applying capacitors and their systems for public use. And we have cleared both performances and safety targets with hybrid trucks and fuel cell vehicles using PC.

For your further reference, you can watch movies or photos on safety testing of an ECaSS® capacitor under severe abuse such as nails hammered into a capacitor cell, heating with a torch, burning with an alcoholic lamp and overcharging to twice the maximum rated voltage, etc.

(More details are in our member pages. Refer to Q57.)

Q6	How can capacitors overcome insufficient energy densities?

A6	Capacitors are not always smaller in energy densities than batteries.

Storage batteries usually claim quite large energy densities. However, these values are one-cycle capacity disregarding cycle lives. To obtain pracitical cycle lives, they have to be operated in a shallow depth of discharge. For example, in a commertial hybrid vehicle in which 3,000,000 cycle lives are required, it is a standard schime for batteries to operate in a depth of 5-10% of full capacity. The fact means their effective capacities are muliplied by those value, or 5-10% of nominal capacities.

ECaSS® technique, there is a method to obtain higher energy density capacitors by sacrificing internal resistance. Everybody would suspect that the larger resistance induces greater amount of loss. Nevertheless, the losses can be conquered by current mode charging/discharging. This part is one of the important keys of the ECaSS® principle; so refer to the explanatory descriptions in any of downloadable papers 11 and 12 in the member pages or "Capacitor Storage BASICS, Applications Using ECaSS®".

Q7	Why does the serial connection of capacitors pose problems?

A7	Because the voltage distribution of those capacitors will not stay equal.

The voltage difference is mainly caused by the difference of leakage current and capacitance of each individual capacitor. There is almost no relation with other parameters such as internal resistance, series inductance or the number of charge/discharge cycles.

It will take a long time to reveal the problem; so this is overlooked sometimes until it causes a failure, resulting in a loss of faith in EDLCs.

This phenomenon and problem has been dealt with quite often in ECaSS® papers. A recommended reference is paper 12 on the download page. Look at the chapter titled "If you are lucky or impatient", and please read it carefully to fully understand what is happening.

Q8	Has the serial connection issues of capacitors been completely solved?

A8	Yes, perfectly. Even several thousands-volt strings of EDLCs are possible.

There are several steps to understanding this problem:

a) Noticing voltage imbalance problem
b) Looking for some method of equalization, balancing resistor, zener and active circuits
c) Realizing problems of passive and active equalizer, leakage, efficiency, power-loss and cost
d) Finding ECaSS® technique of initializing capacitor with parallel monitor instead of equalizing

In the beginning, no one would notice this problem. This is prior to stage a). This phenomenon and problem also has been dealt with quite often in ECaSS® papers. A recommended reference is paper #9 in the download page. Also, for more details, refer to Q56 in the member pages.

Q9	Are there any ways to experience the ECaSS® operation?

A9	Yes, there is an easier way than buying Honda FCXs in California.

Instead, purchase a capacitor hybrid truck manufactured by Nissan Diesel or UPS by Shizuki. That may be one good solution, but there is an easier, less expensive one, at least for the first step.

ECaSS®Forum does not sell anything, but you can jump via a link to the "Power Systems" website, where they sell an experimental set of ECaSS® with an English instruction manual. If their Japanese site is difficult to follow, send a mail directly to us.