Super Capacitors - Real or not

Discussion in 'General Electronics Chat' started by Gavin Brunton, Jul 21, 2015.

  1. Gavin Brunton

    Thread Starter New Member

    Jul 21, 2015
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    Hi, I live on a boat in London and im completely off grid. Ive recently bought 6 3000F 2.7v Maxwell super caps from ebay that are wired in series to give 16.2v capacity. This bank is wired in parallel with my 550ah battery bank. The thing is i bought the caps used and they are plain silver and unbranded. the seller has good reviews and said that the reason they were unbranded was because they were part of a much larger bank. I have not found any other unbranded maxwells on the internet. My question is how do i test them? I only have basic multi meters and a DC clamp meter. The caps are in the system and are accepting voltage. Infact there was a hell of a spark when I connected them to the batteries. Should of probably used a resistor to get them up to voltage.

    Also will the supper capacitors cause the Alternator or Solar charger to not function properly owing to the reduced resistance of the capacitors vs a lead acid battery?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Capacitors are a poor substitute for batteries for power applications. The primary reason for this is their discharge curve. It is exponential, fast at first and then slowly as the charge is exhausted. Batteries on the other hand have a nearly flat discharge curve until just before they are exhausted. If the capacitors have not been damaged, I suggest you exercise greater care in experimenting with them.
     
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  3. dl324

    Distinguished Member

    Mar 30, 2015
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    You might find this blog entry enlightening.

    Or this

    and this
     
  4. Gavin Brunton

    Thread Starter New Member

    Jul 21, 2015
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    Thanks Papabravo but I don't really see how your response answers my question. My capacitor bank is not intended to replace my batteries it is in parallel with my Trojan lead acid batteries. My intention is to use increase the life of my batteries by allowing the cap bank to supply high amounts of current when needed to take some of the strain off the the batteries. My understanding is that lead acid batteries do but like high current draw.
     
  5. nsaspook

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  6. Papabravo

    Expert

    Feb 24, 2006
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    Your approach is flawed, since the rapid draw down in capacitor voltage puts a considerable load on the battery. Also there are different types of SLA batteries, and the ones you want are the marine deep cycle batteries instead of the automotive starter batteries. I think you're wasting your time with the capacitors for power applications. But you go ahead and do whatever you want; far be it for me to dissuade you from pointless endeavors.
     
  7. Lestraveled

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    May 19, 2014
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    Trojans are some of the finest deep cycle batteries in the world. Gavin, I understand what you are trying to do, spread out the current peaks so the batteries see more of an average. In theory it makes sense, lead acid batteries supply more amp-hours at lower currents.

    Edit: I looked at the wrong cap. The internal resistance of the Maxwell caps are at or lower than the Trojans batteries. I removed the part of my post that was wrong.
     
    Last edited: Jul 21, 2015
  8. Lestraveled

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    May 19, 2014
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    OK, how to test. The first parameter that changes when a cap is at the end of life is the ESR, it starts to increase. The ESR of these caps is an astounding 0.3 Milli-ohm. This value is very hard to test. A result of increased ESR is the caps will run hotter when they have to provide current.
     
  9. GopherT

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    Nov 23, 2012
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    So, a 550 amp-hour battery is like a 1,980,000 amp-second battery.

    Connecting 6 capacitors in series, each with 3000 F is like one 500 F capacitor. That is equivalent to 500 amp seconds (if you ignore the huge voltage drop vs. time so it is really something significantly less).

    So, you have added something significantly less than 500 to 1,980,000. Was it worth the effort?

    Also, do you have some resistance present when charging that capacitor bank, otherwise you are not taking any load off of the batteries. Charging a 500F load at 0.3mOhm internal resistance will be a spike if you batteries have ever seen a spike!
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    While Gopher's math is correct, it must not tell all of the story because of all of the diesel-battery-supercap-hybrid buses running around. Part of that is the capacitor's ability to accept charge from the regenerative braking system much faster than the batteries do, but what good is it if it can't be dumped out just as fast? So I think capacity and transient capacity are two different things, and the two storage systems each excel at one and not the other. I agree with NSA that the only way to manage this is with some DC/DC converters. What is the peak current you are trying to relieve the main batteries of?

    ak
     
  11. Lestraveled

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    May 19, 2014
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    So, far we are doing a good job of not answering his questions.

    How to test them
    As I said in my earlier post, it is difficult to test this low of a ESR. With only a meter and a clamp on amp meter, probably impossible. Also, as I said, they will start to run hotter the older and, more ineffective, they get. A substantial fuse would be a good addition. Also, the older the caps get, the more leakage current they will have. This means they will actually start to drain (dis-charge) your batteries. You can use your clamp on amp meter to measure the leakage current. (Your meter must be able to read DC.) Clamp on to either wire connecting the caps. Leakage at 20 deg.C is 0.005 amps. Most clamp on meters can't read that low of a current.

    I don't think the caps in parallel with the batteries will have any affect on the charging system. The caps will only make the internal resistance of the batteries look lower and most chargers look at the voltage for state of charge.
     
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  12. Gavin Brunton

    Thread Starter New Member

    Jul 21, 2015
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    Ok Some great stuff here and some food for thought. First of all I sense some people are focusing on the power density of the super capacitors . Lets make this clear I am not trying to increase my overall power capacity of my system. If I wanted to do that I would buy more trojans which I am assured are excellent "true deep cycle" lead acid batteries. What i am trying to do is prolong the life of my batteries. My system runs my lights and and lots of pumps within the boat ie fridge (compressor i know) bilge pump, shower waste pump(its a whale pump which is an actuator), Water pressure pump, Central heating pump diesel Hurricane heater (full of pumps). Also I have a 2.5kw 240v Ac (UK) pure sine wave inverter that I run power tools from and a vacuum cleaner. As I understand it lead acid batteries dont like the sudden draws you get from from unregulated motors in pumps and over time they cause wear on the batteries so I was advised from a well respected Off grid specialist in the UK to look at super capacitors to take the strain of the high peak power draws. No I read http://vtb.engr.sc.edu/vtbwebsite/downloads/publications/APECdigestfinal.pdf

    that NSAspook posted and although that active version seems superior the passive system is quoted to be able to provide 2.3x the max power capacity.

    im going to leave defending my rational for a moment and return to my origanal question. how do i test the things.
     
  13. Gavin Brunton

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    Jul 21, 2015
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  14. Gavin Brunton

    Thread Starter New Member

    Jul 21, 2015
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    What I'm really worried about is that they might be fake. I was thinking I take a pump and measure its current draw. then I charge the caps to a defined amount ie 12.7v. Then using a the pump try and discharge the cap bank whilst timing how long it takes to get down to say 12v(due to the pumps all being ignition protected i.e cuts out at 11.5v ish) This far from being a highly controlled scientific approach should give me enough numbers to crunch to work out a very rough capacitance no? Could some one maybe help with the numbers?
     
  15. nsaspook

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  16. Brownout

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    Jan 10, 2012
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    I think the fact that you got a big spark tells you they are not fake.
     
  17. Gavin Brunton

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    Jul 21, 2015
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    When I say fake i mean Chinese copies that are say 50F not 3000f
     
  18. AnalogKid

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    Aug 1, 2013
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    t = R x C
    P = E^2 / R

    The test you propose is a variation of an R-C time constant test. Charge a cap up to a known voltage, discharge it through a known resistor, time how long it takes for the terminal voltage to decrease to 36 % of the initial value. That time divided by the resistance equals the true capacitance at that current level, temperature, etc. The problem with using anything other than a pure resistance is that nothing is like a pure resistance. Light bulbs, motors, whatever, they all have non-linearities, but trying it won't hurt anything. If nothing else, it will tell you differences among the individual capacitors if you can separate them for testing.

    If you go the resistor route, you can start with how long you want to wait for the result, use t and the assumed value of C to get the resistor value, pick a close standard value, use that and E to get the estimated initial power level, and double that. That's your test load.

    ak
     
  19. GopherT

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    Nov 23, 2012
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    @Gavin Brunton

    If you pick a 1k ohm (1/4W) resistor to discharge, you can follow this very exact table

    upload_2015-7-22_9-36-56.png

    If you pick a 100 ohm (2 W or more) resistor, you can follow much faster

    upload_2015-7-22_9-39-52.png

    Or a 10 ohm (20 W or more) resistor

    upload_2015-7-22_9-42-6.png

    NOTE: if discharge is slower than stated, your capacitance is HIGHER than 500F. Also, if the trend is correct after a few hours, then it will continue as posted on the table - you don't need to follow to the end because physics is physics.
     
    Last edited: Jul 22, 2015
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  20. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Would recording (logging) currents in each the battery and in the capacitor leads during 'normal' boat operation tell any enlightening clues on what is going on, by comparing and interrelating time-stamped readings ?
    After all, a supercapacitor in parallel to a battery does make sense to absorb drain peaks. There is no true 'discharge' event in such operation, would be just like a drain current-smoother for the batteries.

    Just keep in mind supercapacitors are very lossy in self-discharge, if that is of any importance when continuously energized. I would try just for peace of mind, to compare the behavior to a single 16 volt supercapacitor that you can gets your hands on for diagnosing, instead of a series of several 2.7V ones. If one of the series has problems, the whole thing can cause the headache.

    If they are unbranded, how do you say they are Maxwell ?
    Please measure the voltage on each of the capacitor series when connected to the battery to check if values are equalized.
     
    Last edited: Jul 22, 2015
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