Using Ultra Capacitors To Extend Battery Life

20 Posts
Jul 24, 2011 02:32 am
Using Ultra Capacitors To Extend Battery Life

I ran across an article in my travels (see link below) detailing the use of ultra capacitors in forklifts to extend the battery life.  This got me thinking about how this could be applied to solar systems.

I am considering setting up my system so that the battery bank feeds the ultra capacitor then the loads are connected to the ultra capacitor.  My idea is that when heavy loads are applied the start-up current would come from the ultra capacitor then the batteries would be constantly charging the ultra capacitor.  This would spare the batteries the high current bursts needed by things like motors starting and similar "burst" loads.

I figure that if it works in a forklift it should work for our applications also.

I have found pricing on ultra capacitors and they don't look that outrageous. I am considering 2600 Farad 2.5V ultracaps.  I would be running these in a series parallel 6 per string to give me 15 volts @ 5200 Farad.  According to my math this is 271 watts.

That 271 watts could be delivered in a fraction of a second (or longer) as needed and the batteries would charge them at a modest amperage.  From my brief experience with my solar system it seems that the higher the load the less you can get out of the batteries.  Also the higher the load the more the voltage sags which makes it require even more amps to keep up (cascade effect).  My hope is that this would stop all voltage sag and a nice steady low amperage load would be "seen" by the batteries.

Has anyone considered/tried this?  I figure anything to extend the usefulness of existing batteries would be worth it.

Quote from the article:

"In a typical forklift application, the equipment can draw up to 700 A from the battery under heavy load conditions. But when the battery is operating with a parallel EDLC pack, this battery current can be reduced anywhere from 30 to 45 percent depending on the effective series resistance (ESR) of the EDLC pack and its interconnecting bus. The lower the ESR of the EDLC bank and its connecting bus, the greater the reduction of current drawn from the battery pack."

Please also see the nice graph they have provided detailing the benefits of this system in a forklift application.

I am willing to be the guinea pig if I get some positive responses.  I have already found a source for reasonable priced ultra caps.
462 Posts
Jul 24, 2011 07:26 pm
Re: Using Ultra Capacitors To Extend Battery Life

Travis, an inverter is just a capictor also. It just has addecd features such as voltage and input/output capacities.
20 Posts
Jul 24, 2011 10:18 pm
Re: Using Ultra Capacitors To Extend Battery Life

Hi Tom,

Thanks for the response.

I went ahead and ordered these to do some testing.  They look like the answer to a lot of problems in the off-grid world.

Yes it is true that inverters have capacitors in them.  My goal though was to shield the batteries from seeing high current bursts.  The caps in the inverter are there to assist it without any real regard for the battery.  The battery is still expected to come up with these massive currents.  These caps are going to be physically larger than the inverter itself and much higher quality/capacity than anything inside it.

When the fridge goes to start there is a huge high current burst that the deep cycle batteries are forced to produce.  I can tell by the voltage drop that the batteries don't really like this to happen.  In my studies of how lead acid batteries work this is one of the main things that causes short battery life. 

I know that the batteries/inverter are capable of producing this current however it comes at a huge cost in terms of charge longevity.  It is a unsustainable chemical reaction that causes a runaway between voltage/amps.  The more amps the inverter draws the more the batteries sag which causes even more amps to be needed.  This is especially true when the batteries are close to 50% charged.

Those are the capacitors I went ahead and ordered. 

The article that I referenced in the first post is my reasoning for doing this.  These capacitors came out of a hybrid bus.  They are used to provide the burst current needed for the bus to accelerate from a dead stop.

There are videos on youtube of people starting large diesel engines and car engines from these caps.

There are even videos of go carts totally powered by these caps.  I didn't even know they had caps this large until I ran across them by chance.

I am going to be doing a lot of research and I'm going to post all my finding in a scientific way.  I ordered a 500amp shunt and I'm going to be using data logging software to graph the difference in amp draw seen by the battery.

If I can "smooth" the current draw seen by the battery by 25% (or more) I should be able to achieve a lot longer run time on my battery.

Sorry for the long winded reply.  I have been really enjoying my solar system and learning about all this.

462 Posts
Jul 25, 2011 10:49 am
Re: Using Ultra Capacitors To Extend Battery Life

Travis most inverters are designed to take this surge into account. That is what capacitors do. They build up charge and disperse it accordingly, saving the surge action from happening on the batteries themselves. Remember, keep it simple. Adding more and more devices increases possible problems.
  I've seen some real old PV systems that used the type of capacitors you are exploring. The system took up an entire wall. But this was before modern inverters were around.
« Last Edit: Jul 25, 2011 10:51 am by Tom Mayrand »
97 Posts
Jul 25, 2011 04:16 pm
Re: Using Ultra Capacitors To Extend Battery Life

This is an interesting post, Travis, and it caught my attention.  A couple of practical factors also occurred to me.  Since the operating voltage may not exceed 2.5 volts for each capacitor, If you are trying to match them to a 6 volt battery for example, you would then need to place 3 capacitors in series before wiring them in parallel with the 6 volt battery, so that each capacitor doesn't see much over 2 volts across its terminals.  As with any series capacitor arrangement, equalizing circuitry must be used to maintain an equal voltage across each capacitor.  This is done simply by wiring a resistor of equal value across each capacitor, or by some other more complex means (the sophisticated control circuitry mentioned in your first link).  Resistors represent a power loss since they would tend to drain the capacitors (and battery) without any other load on the cells.  And, when you place three capacitors in series, the net overall capacity is reduced to one third, so the 2600 farad rating would drop to 867 farads at a rating of 6 volts......for a 48 volt system with 24 capacitors in series, the rating would drop to 108 farads of capacity. At this reduced capacity, would they actually help the batteries with surge current problems, or just add a lot of expense and complexity to your system?  I'm not trying to rain on the parade, but rather, just expressing curiosity at this concept.  Jon C.
20 Posts
Jul 29, 2011 05:42 am
Re: Using Ultra Capacitors To Extend Battery Life


These capacitors are very new technology.  Capacitors in this capacity were typically out of reach of everyone (except insanely rich).  New they were insanely expensive.  I bought these used from a Hybrid Bus application.  They provided the current needed to move an entire bus.  These are not your typical capacitors. 

Jon C,

I need your help with picking the correct resistors for this project.  I'm new to electronics and currently I have them running without any balancing circuitry.  I will report all my findings in the most scientific way I can.  I have a computer driven scope and a shunt on order.  I am going to record total amps drawn and also battery voltage.  If I can achieve anything close to the quoted increases for a forklift in a renewable project this is very big news.

Random First Night Operating Notes.

End of night voltages @ 12.53 pack voltage

Cap 1 - 2.05
Cap 2 - 2.10
Cap 3 - 2.07
Cap 4 - 2.07
Cap 5 - 2.08
Cap 6 - 2.11

I have made and connected a pack.

I have been testing without any equalizing circuit.  I am going to be watching these cells closely tomorrow during the charging cycle.

I decided the best way to do this would be to charge each cap up to 1/6th my battery voltage.  So I started the LONG process of charging each capacitor.  It over 30 minutes to charge each capacitor (12v @ ~10 AMPS!!!) and I had to monitor them the entire time to ensure they didn't go over my 2.18 target... ARRGH... Took forever.  I used a 10 amp light bulb in series with my 12 volt battery to control the amperage of the charge.

When I assembled the pack I accidently touched my wrench across 3 caps wired in series.  All I can say after seeing that is WOW.  It took a pencil size chunk out of the wrench.  I would like to know how many amps that was.

Then I had to re-equalize the pack.  The voltages on those three cells were down .3 volts from where they should have been.  Imagine all that damage and it didn't even discharge the caps.  I could have done that over and over and they still would have had a charge.

After I connected them to my battery (pre-charged to near perfect voltage match) I turned on my radio equipment.  I did some high power transmissions to test the voltage sag difference.  The improvement is nothing short of amazing.  This is the perfect application for this setup.  Typical transmissions are 50% duty cycle. (1 minute transmit 1 minute receive)

The first thing I noticed is that the voltage sags much less and much slower.  I cannot wait to assemble the other pack and connect them in parallel with these.  Previously I would see a voltage drop of about 1 volt.  Now that is down to about .3 volts and slowly sags but never past 1/2 volt during a 2 minute transmission.

The second odd thing I noticed is that after the voltage sags it doesn't bounce instantly back up like normal.  Its obvious when a battery has these caps on it.  The voltage returns to open circuit voltage very slowly.  It shows they are doing their thing.

At the beginning of the night I started with 12.6 volts on the batteries.  Its currently at 12.53 volts.  That is a huge improvement over what it was however I don't have any nice graphs to back it up yet.

« Last Edit: Jul 29, 2011 06:03 am by Travis Austin »
97 Posts
Jul 29, 2011 10:55 am
Re: Using Ultra Capacitors To Extend Battery Life

Travis.  Any resistive equalization would represent a power drain on your system and would be counterproductive in my opinion.  You have already seen a sample of the kind of fire you are playing with when you dropped the wrench across capacitor terminals, but do you realize the worst case scenario......The 2.5 volt rating on the capacitors is an indication of the dielectric strength of the separator material in the capacitors, and if this voltage is exceeded, the capacitor may experience a separator failure which usually means instant death for the capacitor.  Capacitors do not die quietly however, and discharge all of the stored energy instantaneously along with fire and smoke.  If one capacitor in your string shorted, the next ones would see an elevated voltage and would not be far behind, setting off an avalanche effect through the string.  You would see a virtual lightning storm with explosions and the likelihood of intense fire.  I have seen lead acid batteries explode as a result of heat igniting the hydrogen gases, and they are very messy.  So, that being said, I wish you success with your project, but you may want to consult an expert about this.  My knowledge about this is very limited, but I do know about smoke and fire.  Jon C.
20 Posts
Jul 29, 2011 12:19 pm
Re: Using Ultra Capacitors To Extend Battery Life

A photo for anyone interested in the project:

For size reference that is a Group 27 Battery. Notice how the caps are nearly as large. Yes I know a lead acid battery inside isn't smart. Yes I also know my interconnects are too weak. Yes I know my wiring is a mess.  Notice the discolored interconnect from the wrench incident.  That happened in a fraction of a second.

Jon C,

Thanks for the reply. I didn't realize the worst case scenario. I am going to be watching these really close. What I'm going to have to do is design a emergency disconnect circuit that monitors the voltage of each cell and disconnects the pack at say 2.49 volts. The manufacturer left a 10% surge voltage designed in. They can be used at 2.75 volts per cell however that is the absolute max before bad things can happen. They de-rated them for safety. I am going to have to put some serious thought into this before I can leave them unattended.

Last night before I went to bed I went through and equalized them using that same light bulb that I charged them with. I brought them all to within a .01 volts of each other.

They have been charging all day and the cell voltages are as follows.

Cell 1 - 2.17
Cell 2 - 2.17
Cell 3 - 2.18
Cell 4 - 2.17
Cell 5 - 2.18
Cell 6 - 2.18

Yesterday when I charged them I don't think I got them close enough to being at the same voltage. I was pretty close but discharging them (individually) with the bulb to get them exact seems to have been the trick.

I can tell these are adding capacity to the system because now when the sun passes in and out of clouds the voltage doesn't swing up nearly instantly as it did. In fact typically I would be at 14.2'ish volts right now. I am currently at 13.8 so there is a big difference. I am curious to know if these will help with the charging of the lead acid battery. I'll have to do some testing on that also.

The exciting thing is that overnight they didn't fall out of balance. If I can continue this for an extended period of time without them falling out of equalization I'll feel a lot better.

I am going to start building the other pack of caps today. I cannot wait to see what happens with 2 of these packs connected.

I will make another post tonight with any new findings.

(I hope these types of posts are acceptable)

Disclaimer: As Jon pointed out this can be VERY dangerous. I am not responsible for anyone duplicating my setup. I am just an average person with a multimeter and a screwdriver. I have no formal electrical experience/education (Computer/Network Technician). If anyone is crazy enough to try this please post your findings here.Smiley
« Last Edit: Jul 29, 2011 01:52 pm by Travis Austin »
20 Posts
Jul 29, 2011 03:07 pm
Re: Using Ultra Capacitors To Extend Battery Life

I cannot resist posting again.  There are DEFINITE increases in the abilities of the solar system.  I am currently sitting at 14.45 volts.  If I key up the radio on high power previously the voltage would do a nose dive all the way to 13.2'ish.  The capacitors are doing their job and during short transmissions the voltage isn't dropping.  This means a few things.

1> The battery is still charging even while I transmit

2> The solar panels are going to put out more since they are closer to their Vmp rating.  I have a PWM charge controller so this brings me a little closer to MPPT efficiency.  Actually I think I am over MPPT efficiency because the battery is still seeing nearly the same charge during short cloud cover.  Lead acid batteries seem to resist charging and capacitors don't.

3> The battery will last longer (more daily capacity).  Lite usage while the sun is out isn't going to discharge the battery.  I will be able to make transmissions without my batteries even knowing I'm doing it.  They are even still charging while I transmit.

4> It seems to make the charging more efficient.  I don't have any scientific proof however when the sun goes behind a cloud the voltage is lingering high almost staying flat during short cloud cover. The battery is still getting a charge even with dropping solar voltages.  The caps are holding the voltage higher.

Obviously a capacitor isn't going to help with steady continuous loads to this degree.  But for my application I can already see a HUGE benefit.

My radio draws 114 watts during a high power transmission.  My panel is a 110 watt panel.  To me this is the true test of how well this is all working.  Previously on transmit the voltage would fall to the batteries working voltage and I would bleed off the surface charge almost instantly.

Now if the capacitors can stay balanced without maintenance then this is the PERFECT setup. (So far they are staying balanced)
« Last Edit: Jul 29, 2011 03:31 pm by Travis Austin »
20 Posts
Jul 30, 2011 05:31 pm
Re: Using Ultra Capacitors To Extend Battery Life

New and Improved Version 2

I have all 12 caps wired in series parallel.  See pictures above.  I will be doing some further testing and I'll post some results here when I get my test gear.

So far this seems to be twice as good (makes sense)
20 Posts
Aug 1, 2011 12:35 am
Re: Using Ultra Capacitors To Extend Battery Life

Version 3 of my capacitor bank.  I decided I needed to beef up the connections between the caps.  I used some aluminum stock that I picked up at home depot.  I also used a sheet of plexiglass to hold the cells together.  This seems to work wonderfully.

I still haven't gotten my shunts so I still don't have any scientific testing done however at the end of the night my voltage is much higher than previously.  These are making a huge difference from what I can see so far.

I have 12 more of these caps on order and I'm going to be testing 24 also.  I am going to see what capacity creates the greatest increase.

(I hope I'm not boring anyone with my posts)
« Last Edit: Aug 1, 2011 03:56 am by Travis Austin »
97 Posts
Aug 1, 2011 11:21 pm
Re: Using Ultra Capacitors To Extend Battery Life

Travis, I am pleased that your project is living up to your expectations.  I do not see any fuses between the capacitors or between capacitor and battery, and quite frankly, this scares me.  I know you do not have a death wish, but you are tempting the electrical demons as I see it.  Just a friendly reminder?  :-)  Jon C.
20 Posts
Aug 2, 2011 12:44 pm
Re: Using Ultra Capacitors To Extend Battery Life

Jon, This is actually beyond my wildest expectations.  I think the relationship between the battery and the capacitors is like the relationship between concrete and steel. 

Neither the battery nor the capacitor is perfect by itself.  The capacitors have limited (compared to a large battery) power storage however can provide nearly unlimited amounts of surge amps.  The battery holds a lot of power yet cannot provide any surge amps without a nasty sharp reduction in amp hour capacity along with destroying the plates inside the battery.  Together they produce a "super battery".

That is a good call on the fusing.  I appreciate your friendly reminder.  I hadn't even considered protection and was focused on trying to minimize losses of any kind.  I never even thought about fusing this because its tied to the batteries directly.  After reading your post and giving it some thought I really should have some type of fusing installed.

I'm not sure how I would fuse cell to cell in the capacitor bank.  If I use a lead acid battery as an example there is no fusing between the individual cells inside the battery itself.  Is fusing between cells necessary?  A friend suggested that I alter the aluminum I have currently hooked up to make it thin in the center.  Then the capacitor to capacitor links would physically melt/blow and should provide some level of protection.  The question would be how thin should they be made in the centers.

I don't know what the proper value would be to select for a fuse between the capacitor bank and the battery.  This capacitor bank is able to provide insane levels of current however I never expect to draw those levels of power.

Since I am getting 100amp shunts I guess 100amps would be the proper value for my testing.  I would have to replace these fuses once I attach more batteries and loads in excess of 100amps however.  I am open to any input on proper values for the fusing.

Please everyone keep in mind that I'm very new at this.  As I stated before I'm just an average person with a screwdriver and a multimeter.  If anyone sees anything I am doing wrong please let me know.  Chances are I honestly don't know the proper way.  This is especially true if you notice anything that is a fire hazard or a personal safety concern.


Some people have questioned why I am doing this at all.  I have had lots of people tell me that simply adding another battery (or batteries) would do the same thing.  The reason I am doing this experiment comes down to something called Peukert's Law.

This law defines the relationship between current draw and overall capacity of the battery.  It would be impossible (impractical) to design a battery bank around the 20 hour rate that most battery manufactures use.  My battery is a 100AH battery so the example given on wikipedia works perfectly for my system.

I am attempting to violate this law and produce a practically sized battery that can be charged completely daily for maximum battery health yet can provide all the power that I need.

I was doing some math on this last night and currently I'm seeing approximately a 40% increase in battery capacity.  This cannot be due entirely to the capacity of the capacitors and must have more to do with the peukert effect. 

Keep in mind that the capacitors are constantly charging and discharging as the battery voltage changes.  The capacitors don't work like a battery and only respond to changes in voltage.  The more the voltage sags (or increases) the more current the capacitors produce or accept.  I key up the radio and the voltage sags which causes the capacitors to discharge.  When I quit transmitting the battery attempts to return to its unloaded voltage and in the process of doing so it recharges the capacitors for the next blast.  Unlike a battery these capacitors can do this millions of times at nearly 100% efficiency.
4 Posts
Aug 11, 2011 08:04 am
Re: Using Ultra Capacitors To Extend Battery Life

To check and see if the battery life will be extended you need a control for this "experiment".  The amount of energy storage you have in the capacitors alone can substitute for all the work the battery would do for small average loads.  At some point though if you have no charging supply hooked into the system the battery will be giving a large surge to charge the capacitor bank equal pretty closely to what the load draws because you do not have hardly any resistance between the battery and the capacitors.  If the idea is to extend the battery life, the only way to do that is to not use it and don't use it as hard.  So it seems to me that a small resistance between the capacitors and the battery would be a good idea and then have your load hooked up to the capacitors only.  Another way to do this would be to give the capacitors a head start. You can do this by hooking a Diode pair between the battery and the capacitor bank.  The resistor could then be used as a current limiter on the diode that returns the charge from the battery to the capacitor bank.  The diodes in a microwave are pretty rugged.  They handle about 1kw on a daily basis.  Be very careful not to hook the panels directly to the capacitor bank. Solar panels always make more voltage than the battery.  That is why you nave a controller.  I would be tempted to put this booby trap in the ground outside with a plastic lid over it be cause above ground nuclear testing has been banned for a long time.     
4 Posts
Aug 11, 2011 08:31 pm
Re: Using Ultra Capacitors To Extend Battery Life

I went ahead and followed your links and the links embedded in the pages and from what I can see my initial impressions are correct. Two controllers set at  different voltages, and charge rates would take advantage of the system in the best way possible.  One for the capacitor bank and one for battery bank are needed if you are going to get the most out of the system.  If you use three or four diodes in series between the batteries and the capacitors in the system to set the capacitor bank a little higher than the battery bank then they can be used a as charging supply for the batteries. I would use a large power relay to shunt around every thing when the high demand load kicks in and then drop the diodes and the controllers back in after the high demand ends. A short run from a generator or a brief connection to the grid with a high rate charging system for just the capacitor bank at night could be used to kick the capacitor bank back up to 2 volts higher than the battery bank. It will rise that high because the shunting relay is disconnected and the diode block is back in the circuit. Then the battery charging controller would send the capacitor charge to the batteries at a much tamer rate than hundreds of amps which the capacitors could provide but is not so good for batteries.  All this stuff only seems worth it if you have a demand higher than you can normally get from your available battery supply.  I have just been using car batteries and playing around with this stuff but I got my education B.C. (Before calculators)  Back then they taught us to do the math and show the work.  You got a much better feel for a problem that way. Fork lifts have to use deep cycle batteries because it is a guarantee that they will be abused.  The caps are helping
avoid that abuse and providing an additional punch for the lift. Typically an electric fork lift will be rated at a 5000lb lift limit.  The graph on the page about the ultra caps shows the battery current but doesn't show the capacitor current.  I would guess that they made up the remainder between 720 amps and what the batteries provided.

After the lift you will notice a slow smooth decline in the battery current.  It looks like to me that when the demand is off the caps have a small resister between them and the battery so the batteries can charge the caps back up gradually.  I saw a car battery blow the hood open on a car once. Nobody was anywhere near it but a charger that had switched to a 35 milli-amp trickle had just been removed from it. It made a mess. I wonder how big a bang those forklift batteries would make.

4 Posts
Aug 12, 2011 05:28 am
Re: Using Ultra Capacitors To Extend Battery Life

Jon, This is actually beyond my wildest expectations.  I think the relationship between the battery and the capacitors is like the relationship between concrete and steel. 

Neither the battery nor the capacitor is perfect by itself.  The capacitors have limited (compared to a large battery) power storage however can provide nearly unlimited amounts of surge amps.... [/quote/]

I found some more info about those Capacitors and some formulas and graphs to go with them.  The next thing I would look for would be a schematic of the forklift application.

Your practical application tells you what your system will do. About a minute of transmit time means you start to seriously use the capacitor bank and after two minutes the battery is probably delivering some current.  I still think a half volt head start would set your capacitors up for more usage before the battery started to contribute any thing.

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