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Super Caps

piguy101piguy101 Posts: 248
edited 2011-12-11 06:52 in General Discussion
I want to get this cap: http://search.digikey.com/us/en/products/EEC-HW0D506/P11066-ND/300482 , it is 50F and 2.3V. But, here's the question: The voltage is too low to be useful to me, so could I put three of these in series to get a 6.9V 16.7F capacitor? If this would work, could I charge them all together in series at 6.9V or would I have to charge them each individually?

Comments

  • LeonLeon Posts: 7,620
    edited 2011-12-10 07:23
    Putting them in series will reduce the capacitance, and you might need to equalise the charge on them.
  • piguy101piguy101 Posts: 248
    edited 2011-12-10 07:28
    Does that mean I'd need bleeder resistors for the caps?
  • doggiedocdoggiedoc Posts: 2,246
    edited 2011-12-10 07:29
    I don't think that will work. For one thing, the capacitance in series will be less. I think the voltage is a rating, not a capacity like a battery. I could be wrong. I've been wrong before.


    http://www.allaboutcircuits.com/vol_1/chpt_13/4.html

    Paul

    EDIT:

    I'm too slow on reply. :D
  • piguy101piguy101 Posts: 248
    edited 2011-12-10 07:30
    Yes, I agree the capacitance will be less, they are rated 50F, so shouldn't it be 16.7F for three in series?
  • doggiedocdoggiedoc Posts: 2,246
    edited 2011-12-10 07:32
    I didn't find a quick answer about the voltage. But I am going to experiment to see.

    Will let you know.
  • piguy101piguy101 Posts: 248
    edited 2011-12-10 07:34
    Alright, I hope it will work. I have a super cap from the company Nesscap and it is two 2.7V 5F caps in series to make 5V 2.5F.
  • doggiedocdoggiedoc Posts: 2,246
    edited 2011-12-10 07:42
    I put 4 4700uF electrolytics rated at 6.3 v each and applied 7.5V - the hold the 7.5 fine. I'm going to attempt more voltage, but I fear I will let the magic smoke out.
  • doggiedocdoggiedoc Posts: 2,246
    edited 2011-12-10 07:52
    15.7V and no popped caps or bulged. That's more than twice the rating.
  • doggiedocdoggiedoc Posts: 2,246
    edited 2011-12-10 07:56
    24.1V !! ... they seem to be holding up. That's nearly 4X the rating of 6.3V
  • doggiedocdoggiedoc Posts: 2,246
    edited 2011-12-10 08:04
    Perhaps the next experiment should be with the caps in Parallel for increased capacitance.
  • davejamesdavejames Posts: 4,047
    edited 2011-12-10 08:16
    The voltage rating on a capacitor is the safe operating voltage. In the old days, that SOV was typically 50% of the maximum the capacitor could tolerate before exploding.

    Putting capacitors in series will decrease the capacitance, the same as putting resistors in parallel (same formula).

    Yes, the charging voltage will be "split" across the capacitors but the split will be uneven depending on the condition of the individual cap; that's why bleeder resistors are used (think voltage divider).

    Working with that large of capacitance - be careful!! There's a bunch o'energy there just watin' to vaporize something.
  • kwinnkwinn Posts: 8,697
    edited 2011-12-10 08:19
    piguy101 wrote: »
    ....here's the question: The voltage is too low to be useful to me, so could I put three of these in series to get a 6.9V 16.7F capacitor? If this would work, could I charge them all together in series at 6.9V or would I have to charge them each individually?

    Your calculations and conclusions are correct....in theory. Problem is the capacitance of the individual capacitors have to be closely matched for the voltages on each one to be the same. Not sure about this cap but most electrolytics are rated at +-20%. Voltage wise this would be the same as having 3 resistors (nominally 1000 ohms) in series. One could be as low as 800 ohms or as high as 1200 ohms. Putting 6.9 volts on them would not result in 2.3 volts on each one.
  • piguy101piguy101 Posts: 248
    edited 2011-12-10 08:35
    So can I charge them each separately, and not have any bleeder resistors and have about 6.9 V output?
  • ercoerco Posts: 20,261
    edited 2011-12-10 08:40
    Supercaps self-discharge very quickly, and their voltage drops off linearly with discharge (as all caps do). They are no substitute for a battery. Even NiCads are better.
  • kwinnkwinn Posts: 8,697
    edited 2011-12-10 08:40
    That would work. Of course the smallest capacitor in the series chain will determine the total power available. It might make more sense to use one capacitor and a boost switching regulator to provide the output voltage you want. Depends on your application.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2011-12-10 12:37
    The SparkFun super cap webpage says putting the caps in series increases the total voltage capacity (while lowering the capacitance of the system).

    I thought someone at Parallax has a super cap / solar powered robot?

    If it drove into a dark area, it would backup and look for somewhere with more light.
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2011-12-10 13:26
    to inject some data ,,


    Maxwell ( now general atomics ) uses them to 48v or more ! in series !
    and at kF ratings ...........
    so I know you can series them for charging and not have huge issues ..

    we know Super cap are VERY leaky .... so there is our resistor . no real need for a external one .




    and joules is 1/2C V^2 so the V is needed ..............

    Peter
  • sylvie369sylvie369 Posts: 1,622
    edited 2011-12-11 05:37
    Duane Degn wrote: »
    I thought someone at Parallax has a super cap / solar powered robot?

    If it drove into a dark area, it would backup and look for somewhere with more light.

    http://forums.parallax.com/showthread.php?124075-Solar-Panel-Boe-Bot-need-advise&highlight=supercap

    Go down to post #7 and following
  • TubularTubular Posts: 4,726
    edited 2011-12-11 06:33
    It's quite normal to charge them in series, but to have some kind of balancing network between the cells that gradually brings them into balance during use.

    This can be as simple as a resistor ladder across each cell, or active balancing circuits involving an op amp / comparator. I've pulled apart my maxwell boostcap (6 x 350F in series) and it has a small active balancing circuit between each pair of cells.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-12-11 06:52
    The biggest problem with supercaps is inrush current. It can be quite high, melt wires and blow fuses. A resistor needs to be inserted between the supply and the caps of appropriate resistance AND wattage in order to avoid problems.
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