Step 7: Balancing Your Series Banks

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:

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.  

Many thanks to David A. Johnson P.E . (Professional Engineer) for this circuit.  The document behind this circuit can be found here:

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.

<p>i want to create a 48v supercapacitor pack. so i put 18 supercapacitor in series. how to do balanced charging for this?</p>
I'm building a battery tab spot welder. Lot's of info out there and people using all sorts of capacitor banks pumping out loads of energy. Consensus says 1farad at the weld is what we want for a good weld. This has to factor in a guesstimate of what the circuitry resistance will be from the capacitor bank to the probes. Amy way to ballpark guess what the loss of farads due to resistance may be? Trying to save money and be efficient in capacitor selection. Do I need to have 58F at the bank to get 1F at the weld or would I be safe with 10F at the bank to get 1F at the weld?
<p>ERROR!</p><p>Just an awesome write up thank you for sharing. </p><p>Not sure if it has been commented upon,but Step 4 Example 2 has an error. </p><p>best.</p>
<p>i have 100 farad 2.7 volt capacitors so i need to charge it with 2.7 volts right?</p>
<p>DenizG1 Most of the time you need to limit the current. </p>
<p>question:</p><p>I have a pre assembled boost cap board which has 6 maxwell boostcap capacitors wired in series and each rated for 350 farads at 2.7 Vdc. Each capacitor has an led to indicate when they are charged. What type of charger should I use to charge the capacitors.</p>
can you help me find the polarity of this super capacitor please?
<p>The arrows (&gt;&gt;&gt;) printed on the side will always point to the negative terminal.</p>
<p>Thaanks!. That was what I was looking for!. BTW, any problem charging a <br>super cap with a battery? ie a 15 volt bank with a 12 volt <br>battery? or do I have to limit the current. Thank you very much.</p>
<p>Probably late, but anyway:</p><p>You have to limit the current. Otherwise the battery will act as if it is shorted out. (this will damage it; it might even explode.)</p>
<p>Please sir, which resistor can i use to charge a 2.7 volt, 500f capacitor in a 12v supply</p>
<p>Hi. I am using a 15W solar panel and letting it feed a BUCK CONVERTER to charge USB devices. When a cloud passes the buck converter shuts down and a button must be depressed to turn it back on. Id like to use a capacitor to maintain the &quot;ON&quot; state of the buck converter, which is this:</p><p><a href="http://www.ebay.com/itm/DC-6-5-40V-To-5V-2A-USB-Charger-DC-DC-Step-down-Buck-Converter-Voltmeter-Module-/321787604751?hash=item4aec092f0f:g:0CIAAOxygPtS4cu5" rel="nofollow">http://www.ebay.com/itm/DC-6-5-40V-To-5V-2A-USB-Ch...</a></p><p>It has a wide input range but my cap will be charged from around 15V-22V and at max about .9 amps. </p><p>Id imagine you would recommend a 25V Cap. What is the max amperage that could charge a cap. like that? What would be the ideal farad value and # in parallel that would allow for longer &quot;cloudy moments&quot;? thanks so much, very helpful instructable.</p>
<p>My hat is off to you sir, thank you for clearing 'first light' on this intriguing subject. <br>I for one am greatly anticipating your announced instructables, especially the charger and balancer.</p>
<p>Well done!</p><p>Good job explaining a subject that does not have a lot of solid information (at least that I could find) out there. Yours has been the best.</p>
<p>what does a ciruit for charging a photo flash capacitor look like with a 4 pin transformer? or how do i even do it and is it possible</p>
<p>Thank you, I've learned a lot from your Instructable about super-capacitors</p>
<p>is there any possibility to use supercapacitor as battery backup for electric car.... if possible please say how to proceed so.</p>
<p>if we are charging a capacitor with a fixed voltage source then there is no point of overcharging because capacitor will automatically stop charging when voltage across it will match with the supply voltage.</p><p>so would you please explain my doubt regarding your statement that &quot;we should care not to overcharge&quot;.</p>
<p>hello sir, </p><p>we are working in a solar car project,our problm is:-</p><p>1.we have to ride the car with solar power without using batteries,for a 50-70(meters),the car weighs 250kg(max).</p><p>we r using 3 solar panels and we r getting approx. 470w from it.</p><p>is it possible to drive d car with that power.</p><p>if NO, for initial torque we r going to use super capacitor,is it possible.....</p><p>please give d super capacitor values,nor possible suggestions......</p><p>i will be very much thankfull to u .....................</p>
<p>If you're actually getting 470w from your panels, that's about 2/3rds of a horsepower, With correct gearing it shouldn't be any problem... Supercaps could help it start up quickly though</p>
<p>I want to use a supercapacitor in a circuit charged by an inductor and magnet (energy harvesting coil). How can I limit the charging voltage coming out of the coil so it won't blow the supercapcitor (a diode to ground? or something) and what happens if I keep adding more current after it's fully charged (but still at the rated voltage)? Thanks for a very informative ible.</p>
<p>This is not something you need to worry about . Although for a microsecond your coil will generate high voltage this is too fleeting to cause harm ( unless it's the size of a toaster) . The high voltage is immediately dissipated, it's better to think in terms of power . Just connect the coil with a diode in series to the cap, but don't overcharge, put a voltmeter across the cap , or a single white LED, which will light up at around 2.4V.. if it's a moderately sized coil it will take quite a while to charge up.</p>
<p>......An improvement on my above reply.... The coil will not generate high voltage when connected to an empty capacitor, it's the same as if you were to short the coil, voltage low, current 'high'... the coil only produces high voltage on open circuit... the power can't get out , so 'pressure ' builds until it can, (so to speak)...but as the capacitor fills up you could reach high voltage, limited only by how leaky the capacitor is, and the efficiency of the diode....hope that helps. </p>
<p>Nice informative instructable.</p>
Nice tutorial! I've been looking around the web for some practical information about super capacitors, and this seems to be the best source so far.<br><br>I have a few questions though (which I haven't been able to find the answers to on any site thus far). <br><br>I'm working on a project that requires a temporary backup power source, and the simplest and most cost effective method seems to be through the use of super capacitors. The idea is that the caps will provide power to a microcontroller which can detect the transition to backup power. From there, it will enter a low power state (low current display, RTC, backup registers, and backup SRAM). <br><br>The current consumption is rather low, probably 100mA at most at 3.3V. So here's my questions:<br><br>A capacitor basically stores energy via an electric field, which is created by charges accumulating on separated plates (generalizing here). The voltage across these plates is proportional to the amount of charge stored and so forth. As a battery discharges, the voltage remains relatively constant until it's nearly depleted. But because of how capacitors store energy, the loss of charge will result in a loss of voltage. So if I use super capacitors as a power source, is there any rule of thumb to predict how long the voltage will remain roughly around 3.3V? (I probably could work it out by hand, but this seems rather application dependent).<br><br>Also, do you recommend any current protection when using these caps? In my project, everything is primarily digital, so it'll only draw however much current is needed, but I'm a little concerned just based on how quickly capacitors can discharge versus a regulated power source.
<p>When salvaging components from circuit boards are all capacitors that look like the ones pictured at the top of this page super capacitors. If not how can I tell a super capacitor when I see one?</p>
<p>There is not much difference really between run of the mill electrolytic caps and supercaps. A rule of thumb is one farad and up, i.e. 1F. 100,000 uF is 0.1F</p><p>In all likelihood the caps you salvage are not supercaps but nevertheless useful components.</p>
<p>I want to use a super cap or several, to light a half dozen LEDS. I am going to charge the caps with small solar cells. My question is how many Farads do I need to keep the lights going for 5 or 6 hours. </p>
<p>If you don't do to the math you will have to experiment. The math involves knowing the current and voltage of your LEDs, the power of your solar panel, and of course capacitors discharge exponentially (not anything like a battery) so perhaps you should experiment with small devices first.</p>
<p>which size battery can i use for charging a 450 volts and 100uf(m) capcitor.</p><p>help me plz</p>
<p>Anything up to a 450 volt battery will charge your capacitor to the voltage of your battery, but only 100uf.</p>
<p><strong>Can I replace a 1000uf 6.3v for a 1000uf 10v capacitor ? please help me </strong></p>
<p>Yes you can.</p><p>Your circuit will only charge it to the voltage that it is supplied with though...</p><p>the 10V cap will only store max 6.3V that the origanal Cap would have stored.</p>
<p>I've looked at this circuit (balancing circuit show above) about 30 times now, and I really don't get a few things. First, assuming the circuit as drawn, how did Johnson come up with the two resistors that go along with the reference diodes? I've looked up the datasheet for those diodes, run the values through some calculators, and I get minimum shunt resistor values more like 900&Omega; instead of the 33k&Omega; he used. I'm assuming the high values go along with the 10k&Omega; resistor to drop the current for the comparator, but I can't get the number he used.</p><p>Next, do you really need three voltage references for this? Couldn't you use just the first one, since they all seem to be generating the same reference voltage? Clearly you need all three comparators/op amps, but the references are just there to set a lower value to compare to the voltage divider output for each comparison. (I'll be using 5.5v supercaps, and I have a 5v rail available - I can't see why I'd need the voltage references at all. Just tie the negative values into the 5v rail.)</p><p>Finally, given the low voltages and currents involved here, is there some reason he couldn't just choose a suitable zener diode to balance these supercaps? 2.4V 1W? Or if you're feeling risky, 2.7V 1W? A 2.7V zener should start to breakdown just below 2.7, right? With 300mA max current, it doesn't seem like you'd be risking your wattage limits. I'm thinking about 5.1V, 5W zeners for my circuit, and wondering if I really need the comparators and MOSFETs at all. (30V max, 500mA max, coming from a bike hub generator, so there won't be a big in-rush issue.) This is a terrific circuit for more serious stuff, but it seems like serious overkill for the circuit that's there.</p>
<p>hello sir, </p><p>we are working in a solar car project,our problm is:-</p><p>1.we have to ride the car with solar power without using batteries,for a 50-70(meters),the car weighs 250kg(max).</p><p>we r using 3 solar panels and we r getting approx. 470w from it.</p><p>is it possible to drive d car with that power.</p><p>if NO, for initial torque we r going to use super capacitor,is it possible.....</p><p>please give d super capacitor values,nor possible suggestions......</p><p>i will be very much thankfull to u .....................</p>
<p>hello sir, </p><p>we are working in a solar car project,our problm is:-</p><p>1.we have to ride the car with solar power without using batteries,for a 50-70(meters),the car weighs 250kg(max).</p><p>we r using 3 solar panels and we r getting approx. 470w from it.</p><p>is it possible to drive d car with that power.</p><p>if NO, for initial torque we r going to use super capacitor,is it possible.....</p><p>please give d super capacitor values,nor possible suggestions......</p><p>i will be very much thankfull to u .....................</p>
Hi sir good day!!<br>Sir we are now working with our project, and we have some problem regarding it. Our project is a solar panel and we can only produce 500mv with a current of 1mA. Will we be able to store enough voltage and current on a capacitor for our converter to work which have a minimum input of .9V and 100mA? What rating of capacitor is necessary if possible?<br><br>By the way sir thank you very much for you're instructables, it was helpful.
<p>Is it possible to charge a supercacitor to store energy beyond the voltage of the input source. For example if I hook up a 1.5V battery to the supercapacitor with a diode in between to prevent discharge can the supercapcitor drain the battery and store the charge from it to 2V? I have tried and it does not work. I am trying to use the supercacpitor to store energy and then use it for light an LED, but the input source I have goes to a max of 1V. What circuit should I be using/modifying. Any thoughts.</p>
<p>The supercapacitor voltage will only go as high as your input source. That is to say if you have a 1V input source you will only be able to charge the super cap up to 1V. You can use a boost converter to make your input source 2 V. For example, the MCP1640 allows you to set your output to the needed voltage using a couple of external resistors. Note that you will also need an inductor. </p>
<p>hi i want buy the super capacitor charger where can i get it thanks </p>
<p>Please care enough to go through this article and correct the misspellings, please. <br>It's hard to take the copy seriously when you write 'listening' as 'listing'. <br><br>Please, from now on, always have another human carefully proof read your copy. <br>You want us to take you seriously, right? Care enough to do it right. <br>Thanks.</p>
<p>A few spelling mistakes? Really? This instructable may not be perfect, but it has been helpful to many. Instructables liked it enough to feature it. Take me seriously or don't. I didn't ask for you to read it. I didn't ask for your approval. I spent many, many hours of my own time on this, so please, if you have nothing kind to offer, then please don't post on my instructables.</p>
<p>These videos are in one word &quot;AWESOME&quot;</p>
<p>to sustain a circuit requiring 300ma almost for 10 hrs, how much charge should a supercapacitor store.</p>
<p>i meant 5v 300ma.</p>
<p>Errata, example 2: The total capacitance in series is: CT=1/(1/C1+1/C2), so then, CT=1/(1/200 + 1/1000) = 166.66f , Not 1200f.</p>
<p>can I use a 400V capacitor in a 5volt circuit using a 5 volt zener to limit to voltage going into the capacitor if i can how will it effect the cap thanks</p>
Conductor will go red??? Then discharging takes how much time??? And isn't it supposed to make a BIG spark, I do this with my 450v 220uF cap..
I salvage components from my retired electronic devices. How can I identify any of the capacitors are &quot;super capacitors&quot;?
<p>Very high farads in a (relatively) small package. you probably wont find them. The big ones are mostly used as regenerative breaking buffers in large vehicles, sometimes the little ones are used as backup power sources for small ICs (they keep the clock from resetting if the device looses power)</p>

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Bio: Hi there! My name is Patrick, and I am an electronics engineering technician who works full time as a lab tech, and part time as ... More »
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