Let's Learn About Super Capacitors! (A Practical Guide to Super Capacitors)

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                                                    LET'S TALK ABOUT SUPER CAPACITORS!
Thanks for looking at my instructable!  This instructable will be a little bit shorter than some of my others, and it will be written from the perspective of a technician, not an engineer.  This instructable will be followed with similar super capacitor related instructables.  I'm not going to blast super capacitor noobies with a ton of flashy math.  However, I will be providing links to sites where math can be found for those of you who are interested.  I want to keep this document as practical as possible. There are some fun videos in STEP#8 and some links to my hobby electronics stores in STEP#9.  For those of you who don't know much about super capacitors, here is a little bit of fun theory:

THEORY:
Super capacitors act like any other kind of capacitor, only they can store tremendous amounts of energy.  Many capacitors that you'd have seen in audio circuits have capacitances such as 470uf or 680uf (micro farads).  Capacitors used in high frequency RF applications can be as small as 1pf (pico farad).  The farad is a measure of capacitance (or storage capacity).  They are often used in filtering applications, coupling or decoupling applications, or AC-DC smooting applications (there are some large caps in your standard AC-DC power supply that acts to smooth out the ripple on the line). 

Super capacitors can be used in solar power applications, battery back-up applications, battery applications, flash-light applications, etc.  Aside from the fact that the super capacitor can be charged very quickly due to their low internal resistance, which is known as ESR, but they can just as quickly be discharged.  Batteries contain harmful chemicals, and die over time.  If you handle your super capacitors carefully, you will die before they do...Seriously!  Howver, there are rules...

Super capacitors do not give off gas like lead acid batteries, but they cannot store as much power either.  You can place capacitors in series or in parallel to either up the maximum charge voltage, or total capacitorance.  We will talk about this later. 

Really, there is a lot to be said about capacitors, and you're not going to want to spend your entire day listing to me, so let's get down to the basics.  You can go fourth and choose which tabs you;re interested in.   Here is a video just for fun of me starting my car with super capacitors!

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Capacitances: Capacitors Vs. Super capacitors!
Have you ever heard someone talk about nano this or micro that?  These terms can be used for voltage, power, current, resistance, inductance, etc.  When we talk about the capacitance of a capacitor, we  will do the same.  The below explanation will also help you to understand just how much capacity a super capacitor has in relation to a standard capacitor. 

Understanding Capacitance Terminology:
1pf (pico farad) = 0.000000000001 farads
1nf (nano farad) = 0.000000001 farads
1uf (micro farad) = 0.000001 farads
1mf (milli farad) = 0.001 farads

The table in the image is much more detailed.  This page is an attempt to demonstrate just how much capacity a super capacitor has.  A one farad super capacitor can store one million time more energy at a common voltage, than a 1uf capacitor, one billion times more than a 1nf capacitor, and one trillion times more than a 1pf capacitor.  Cool, huh?  

However, super capacitors have very small voltage ratings, such as 2.5v, 2.7v and 5.5v (Some common values).  This makes things difficult, as in order to make our capacitors capable of charging up to a higher voltage, we need to place them in series, which brings a bunch of other variables into play.  There are sections coming up on Series/Parallel configurations, as well as charging methods, and balancing methods.

You can also employ DC-DC voltage boosters.  Typical DC-DC boosters take a voltage of around 3.4-5VDC and are capable of boosting the output voltage.  We sell all sorts of boosters, super capacitors, and solar panels here:
Visit our ebay store here: http://www.electroniclessons.com/
Please check out our hobby electronic store here: http://engineeringshock.com/

Ah, the fear of super capacitors.  Both capacitors and super capacitors can be dangerous, but in different ways. 

Capacitors *Potential Shock Hazard*:
If you have a 500v capacitor that has a capacitance of 100nf, and you touch your finger to the positive and negative lead, you can get a nasty shock.  If you have one hand grounded, and you touch the positive lead with your other hand, your body will act as a path to ground, and you will get a nasty instantaneous shock.  The capacitor will instantly discharge through you and IT WILL HURT.  I know.... It has happened to me.  
Capacitors with a lower capacitance commonly have a higher charge value, often in the 100v-1000v range or higher.  However, they can't store much power.  If they're discharged, the voltage will go from 100v to 0v in an very quickly, depending on the size of load that is discharging it.  Since the voltage is so high, you can give yourself a nice shock.  Most of the time, you're not going to have to worry, but it will seriously scare the crap out of you.

Super capacitors *Potential Fire/Burn Hazard*: 
Are you afraid of touching the leads of your 12v car battery?  You need not be, and for two good reasons.  The DC voltage on a car battery is so low that you're not going to have to worry about being shocked.  Secondly, your skin has such a high resistance.  This means that next to no current is going to pass through your body.   However, if you short the leads of a battery with a conductor, you will create a TON of heat, which can burn you (Or blow up the battery).  This is due to current, not voltage.  If you create a dead short across a battery, you will create heat.  Why?  Because a dead short will mean next to no limiting resistance between the positive and negative lead, which means that current is not being limited, which means that the battery will supply as much current as it can, which will create a ton of heat.  Car batteries will blow up if you do this.  Super capacitors will not.   If you have a 12v capacitor bank with a 20 milli ohm (0.02 Ohms) internal resistance, and you short the leads, you're not going to hurt the caps.  They are built to discharge much faster than batteries, as batteries have a higher ESR.  However, if you short the leads with a conductor, the conductor is going to go RED with heat, which can set fire to the insulation of the wire, or severely  burn anyone who touches it.  Believe me, as again it has happened to me. 

Example#1
I have a 12v super capacitor bank, and I touch my right hand to the negative lead, and my left hand to the positive leads.  The resistance of the path from my left hand through my body to my right hand is more than 2,000,000 Ohms.  The current being passed through my body will be:
Current(I) = Voltage(V) / Resistance(R)
I = 12v / 2,000,000 Ohms
I = 0.000006 Amperes (6uA)

Example#2
I have a 12v super capacitor bank, and I short the leads with a conductor that has a resistance of 0.2 Ohms. The current along that conductor will be:
I = 12v / 0.2 Ohms
I = 60A (60 freaking amperes)
Means that there will be a TON of heat dissipation along the leads.  This happened to me with a 12.5V 96 farad capacitor bank,and the insulation on the wires set fire.

NOTES:
1) If you short a car battery, it will likely explode. 
2) If you short a capacitor bank, a ton of heat will be created along the conductor, but they will not be damaged.  They are created for extremely fast discharges, and high current applications at lower voltages.  Just be careful not to burn yourself.

Treat your capacitors with respect, and you're going to have a good day =)

Super capacitors are like gremlins, or perhaps like the Mogwai!  You must follow certain rules or you're going to regret it!  If you follow the rules, you're going to have happy capacitors that lead extremely long lives, and give you more bang for your buck!

The Rules:
1) You must never charge past the capacitor voltage rating.  If you have a 2.5v super capacitor, you must NEVER charge it at a higher voltage.  If you do, you risk damaging the integrity of the capacitor, or worse, an explosion.  Personally, I never charge past 80-90% of the rated charge.  If you have a capacitor bank of say 15v, never charge that bank past 15v.  I would personally charge that bank to 13-14v MAX.

2)  If you have a 5.4v capacitor bank, and you are charging that bank with a 5v-5.4v charge, you can leave them unsupervised and go have a snack.  Unlike batteries, you don't have to worry about the capacitors having a charge memory issue.  When the capacitor bank is full, it will stop accepting energy.  It will NOT hurt the capacitors if you keep a charge on them at all times, as long as you are not charging the super capacitors at a voltage higher than they are rated for, or that the bank is rated for.  Batteries on the other hand, will lose integrity if left in a charger for too long.  Another reason why super capacitors are so awesome!

3) If you are charging super capacitors, you will  have to be careful.  Not for your own safety, but rather the safety of your power supply.  Super capacitors will take in as much current as humanly possible, and will look like a dead short on your power supply, which can cause fuses to blow.  You will have to limit the charge to the super capacitor(s).  See the "HOW TO CHARGE" section for more information.

4)  FOR THE LOVE OF GOD, DO NOT REVERSE POLARITY!  Some smaller capacitors have are non-polarized, while some are (electrolytic capacitors).  When a capacitor is polarized, it means that it has a positive (+) and negative (-) lead.  The positive charge must be applied to the (+) lead, which is typically longer than the (-) lead.  the (-) lead should be connected to DC ground.  If you reverse polarity, you will not only risk losing your super cap, but you will risk an explosion.

5) If a super capacitor is discharging a ton of current, do not touch the leads.  They may not be red, but they can still burn you!

Super capacitors can be placed in parallel to up the capacitance of the circuit.  You can even place series banks in parallel with one another.   However, there are a few things to consider.   

1) If you place two capacitors in parallel, the voltage ratings do not have to be the same.  However, the parallel bank should never be charged past the rating of the smallest capacitor.  For instance, if you have a 2.7v super capacitor, and you place it in parallel with a 2.5v super capacitor, you will never want to charge the bank past 2.5v.  We will call our maximum charge voltage VT.  If you have two strings of capacitors in series, you can place them in parallel with one another, but if the voltage ratings of the banks differ, you have to make sure that you regulate the charge voltage to be just under the voltage rating of the smaller bank. If you have a 5v capacitor bank, and you want to place it in parallel with a 10v capacitor bank, you MUST NOT charge past 5v.  Personally, I never charge past 80-90% of the lowest charge.

2) To determine the total capacitance of a parallel bank, You simply add the capacitances together!
 
Example#1
You have three capacitors in parallel with one another.  The first has a capacitance of 6800uf and has a max charge voltage of 25v. The second is 1f capacitor rated for 2.7v, and the final capacitor is 3000f rated for 2.5v.  You simply add the capacitances together:
CT (Capacitance total) = 0.0068 + 1 + 3000
CT = 3001.0068 Farads
VT (Charge voltage) = 2.5v (The smallest capacitor charge voltage)

Example#2
You have two series capacitor banks.  The first is rated for 200f at 15v, and the second is rated for 1000f at 12v.
CT = 200f + 1000f
CT = 1200f
VT = 12v (The smaller capacitor bank charge voltage rating)

Example#3
You have two series capacitors in parallel.   The first is rated for 12f at 5.5v, and the second is rated for 2600f at 2.5v.
CT = 12f + 2600f
CT = 2612f
VT = 2.5v (The smaller capacitor bank charge voltage rating)

Series / Parallel Capacitor Calculator: http://www.electronics2000.co.uk/calc/series-parallel-capacitor-calculator.php

A lot of the theory behind placing super capacitors in series will tie in to STEP#7, which talks about balancing circuitry.  Most of you guys are going to want to place your super capacitors in series, so that you can create higher voltages for your projects.  When you place capacitors in series, you can up the charge voltage.  However, you sacrifice some of your capacitance when you do this.  As well, you will need to consider balancing options.

The Simple Math:
The maximum charge voltage (VT) of a series capacitor bank is found by simply adding the voltage ratings of the series capacitors together.  The total capacitance (CT) of a series bank is found using a special formula.  If you're familiar with paralleL resistor theory, you're going to have no problems here.  The foruma for your capacitance total in series is:
CT = 1/[(1/C1)+(1/C2)+....(1/CN)]     NOTE: CN is the last capacitor in the bank.  Okay, that looks complicated....  Let's go through a few examples.

EXAMPLE#1
We have two capacitors in series.  The first capacitor is a 100f 2.7v capacitor.  The second is also a 100f 2.7v capacitor.
VT = 2.7v + 2.7v
VT = 5.4v    
So we can charge our bank up to a MAXIMUM of 5.4v.
CT = 1/[(1/100f)+(1/100f)]
CT = 1/[(0.01)+(0.01)]
CT = 1/(0.02)
CT = 50f     That's right!  If you place two capacitors of the same calacitance value, you're total capacitance will be half!
So your capacitor bank will be rated for 5.4v at 50f!

EXAMPLE#2
To keep things simple, let's add a third capacitor of the same value into the equation.  We now have tthree capacitors in series. All three capacitors are rated for 2.7v at 100f.
VT = 2.7v + 2.7v +2.7v
VT = 8.1v
So we can charge our bank up to a MAXIMUM of 8.1v.
CT = 1/[(1/100f)+(1/100f)+(1/100f)]
CT = 1/[(0.01)+(0.01)+(0.01)]
CT = 1/(0.03)
CT = 33.3f
So your capacitor bank will be rated for 8.1v at 33.3f!

EXAMPLE#3
Let's try something a little harder, shall we?  We have four capacitors in series. 
CAP#1 = 2.5v @ 10f
CAP#2 = 2.7v @1f
CAP#3 = 5.5v @ 0.47f
CAP#4 = 2.7v @ 3000f
VT = 2.5v + 2.7v + 5.5v + 2.7v
VT = 13.4v
So we can charge our bank up to a MAXIMUM of 13.4v.
CT = 1/[(1/10f)+(1/1f)+(1/0.47f)+(1/3000f)]
CT = 1/[(0.1)+(1)+(2.1276)+(0.000333]
CT = 1/(3.2279)
CT = 0.31f
WHAT THE HELL?  Yes ladies and gents, your total capacitance will ALWAYS be lower than the capacitance of the LOWEST capacitor in the bank when working with series banks; in this case 0.47f.  Try a few more examples for yourself!  Here is a very simple super capacitor calculator:

Series / Parallel Capacitor Calculator:  http://www.electronics2000.co.uk/calc/series-parallel-capacitor-calculator.php

PLACING SERIES BANKS IN PARALLEL:
You can place two series banks in parallel. To compensate for lost capacitance! I've done this on several occassions. If you have two series banks or more, look at each bank as a single capacitor. If you place two banks in parallel, think of them as two separate capacitors, and follow the rules of parallel capacitors. For example, if we have two series banks of 1000f 12v capacitors and we place them in parallel, we will have a 2000f 12v bank. Here is another example. You have three series banks:
Bank#1 = 1200f 14v
Bank#2 = 3000f 12v
Bank#3 = 4000f 10v
When you place these banks in series, you will have a 8200f 10v bank. Simple, eh?

PLEASE NOTE THAT THERE ARE PAGES THAT ARE DEDICATED TO CHARGING SUPER CAPACITORS, AS WELL AS BALANCING SERIES CAPACITORS!  CHECK THEM OUT!

Ah, the never ending debate about balancing your super capacitors.  This is a tough one, and I'll tell you why.  There are many different methods of balancing for super capacitors, but it seems that everyone has a different preference.  This information can be hard to find.  As well, most methods of balancing will limit the charge and discharge limits of your super capacitor.  Meaning that you will possibly negate the function of the balancing circuit, or damage your balancing circuit if you charge at a high current, or discharge at a high current.  Custom balancing circuits are available, but they are expensive, and still have limitations.  Yes, even for Maxwell balancing circuits.

Personally, I prefer to make a bank and charge it to only 75-80% MAX of the total charge voltage.  For instance, if I have a 15v bank of capacitors in series, I will only charge to 12.5-13v.  This will slightly waste on capacity, but you're not going to have any over-charging issues, as all of the caps in the bank will all be charged to 75-80% of the maximum charge.  You can try this for yourself.

There are many other options, such as using a resistor divider network, diodes, and active bleeder circuits.  I've found a wonderful little forum discussion (Thanks to ultracapacitors.org).  The problem with zener diodes is you will need high wattage zeners that will likely require heat sinks.  The problem with the resistor divider network is that you will either have to implement high resistance resistors and charge EXTREMELY SLOWELY, or use high wattage low-resistance resistors, and you will bleed a TON of energy off in the process.  There really is a lot to it.  The problem is that most balancing circuitry theory is based around capacitors that are extremely small in comparison to super capacitors.
 
See this link for forum discussion:
http://www.ultracapacitors.org/index.php?option=com_fireboard&Itemid=99&catid=8&func=fb_pdf&id=2247

If you are willing to sacrifice some capacity, then my preferred method is the way to go.  I plan on doing some experiments in the future, but instead of providing direct information that I have not personally verified, I suggest having a look through the document posted above.  This is a very heated topic of conversation. 

When in series, the voltages on each capacitor will vary mainly due to each individual  leakage current.  It is HIGHLY recommended that you use the same capacitance values in your series banks.  This is because if you have a capacitor with high capacitance and a capacitor with low capacitance, they're going to discharge at different speeds based on the load.   Some have more than others which leads to voltage imbalance.  If you measure the voltage on each individual capacitor in a bank, you will see just this; different voltage on each of them.  Again, if you only charge to 75-80% of the maximum charge, you're going to have different voltages on each of the capacitors, but they will all be well within the charge limit range.  

IN REGARDS TO THE SCHEMATIC POSTED FOR THIS TOPIC:
Many thanks to David A. Johnson P.E . (Professional Engineer) for this circuit.  The document behind this circuit can be found here:
http://www.discovercircuits.com/DJ-Circuits/supcapvoltlim.htm

By far, this is the best balancing circuit I've come across.  It is a custom circuit for a 3v load, but it can easily be modified to suit other needs.  I'll go through the circuit theory, but be sure to check out the link above!  The 9v 300mA MAX solar panel is charging a set of three super series super capacitors.   The 1N5819 diode blocks power from entering back through the solar panel.  The charge off the super capacitors enters into a 3v regulator that powers the load (Load circuit not seen here).  When using solar panels, you don't necessarily have to limit the charge with a resistor, as you won't damage the solar cell if drawing ALL of the energy it is creating.  When using a wall transformer combined with say an LM317 variable DC power supply IC, it is EXTREMELY important to use a charge limiting resistor.

Each capacitor has its own charge limiter circuit, and I have to say that it is ingenius!   Each of the three capacitors is tied to a comparator circuit.  Each comparator circuit acts to drain the capacitor down to 2.65v if the voltage at the positive input surpasses 1.2v.  This is where customization comes into play. You can use this circuit as a reference, and really go to town with your modiification.  These capacitors have a charge limit of 2.7v.  The engineer who designed this wanted each cap to be charged to a maximum of 2.65v (Three of them is series would equal 7.95v).  That 7.95v is then fed into a 3v voltage regulator, which is redundant for this discussion.   There is a 33k ohm protective resistor in series with the 1.2v zener diode that sets the 1.2v reference at the negative input of the comparator.  At the positive input of the comparator, there is a resistor network that is comprised of a 75k and a 68k resistor. 

Some Calculations:
2.65v / (75 + 68) = 0.01853...
0.0185 x 68 = 1.26v (roughly)
This calculation means that when we see the voltage on the capacitor rise to 2.65v, we see more than 1.2v at the positive input.
When there is 2.65v or more on each capacitor, there will be roughly 1.26v at the positive input of the comparator.  When there is more voltage at the positive input than there is at the negative input, the comparator output is turned on, activating the FET, which drains the voltage down to less than 2.65v.  When the voltage on the capacitor is less than 2.65v, the voltage at the positive input of the comparator is lower than the voltage at the negative input, which then turns the comparator off.  When the comparator is off, the FET is not draining.   The current being drained along the FET is limited by a 2.2 Ohm 1W resistor.  I believe that the 10k ohm resistor between the 1.2v zener diode and the negative input is used to eliminate an offset voltage and is recommended in the data sheet.  

The operational amplifiers being used as comparators are micro powered.  This means that the VCC, or power supply voltage requirement is very low; in this case it is 1.6v-5.5v.  What is really cool about this circuit is that the DC ground is not used for reference in the top two comparator circuits.  The negative reference points are the negative leads of each super capacitor, which makes the working voltage for each circuit is the voltage on each of the individual capacitor.
 

Here are some fun videos that I created for my super capacitor designs.  Please note that we sell all sorts of hobby electronics and super capacitors at http://www.electroniclessons.com and http://www.engineeringshock.com.

In this video, we use super capacitors to power a spark transmitter that acts as a Tesla coil!

                                                  

GLAD YOU ASKED!
In addition to a huge number of different super capacitors, we also have a ton of unique and fun electronic hobby wares.  Check us out!  Feel free to ask us any questions you may have.  We also do some product design, and component sourcing.

Visit our ebay store here: http://www.electroniclessons.com/
Please check out our hobby electronic store here: http://engineeringshock.com/
 

DISCLAIMER:
Again, I want to add that this instructable was meant to be simple and fun.  There is a TON of theory that relates to both capacitors and super capacitors that have been left out for the sake of being practical.  For all of those engineering types out there, please do NOT break my balls.  I have a TON of experience with super capacitors, and I've designed many, many circuits that include them.

I WILL BE FOLLOWING UP WITH OTHER INSTRUCABLES RELATING TO SUPER CAPACITORS!

Over the next year, look for my isntructables on;
1) Super capacitor flashlight!
2) Super capacitor charger!
3) Super capacitor custom balancing circuit!


Thanks again to all of you who took the time to read this instructable!  I hope you found it insightful and fun =)