Charging super capacitors.

Discussion in 'Power Electronics' started by Gavin Brunton, Aug 17, 2016.

  1. Gavin Brunton

    Thread Starter New Member

    Jul 21, 2015
    Hi all,

    I'm a layman so be gentle. Basically I live on a boat and I'm off grid. I have a bank off batteries that I use to power my boat. I managed to pick up 6 3000f maxwell capacitors used from eBay. I want to use them in parallel with my 12v battery bank so that when any sudden high current draw motors turn on the capacitors take the strain. I've been told that this should extend the life of my expensive batteries and also extend the ah of the batteries as the relationship between the average current draw and Ah is reversely proportional. The problem I am having is that my solar charger and alternator will not charge the battery or cap bank when it is connected. I'm assuming the the very low resistance of the caps is confusing the changing systems. I've also been told that in hybrid busses that the caps would be isolated from the batteries via a dc to dc Buck converter. This would be very expensive and to be honest beyond my ability to make. My alternator is 175 amps. I was wondering if I could use a high powered resistor between the charging systems and the caps to match the resistance of the caps to that of the batteries so that he charging systems would work? I could then use Buck convertors between the caps and loads which I could get from eBay.
  2. mike_g

    New Member

    Aug 21, 2016
    Maybe it is a problem but those big resistors are costly, you could just get so Ni-Chrome wire, and use it as a heater.

    did you pre-charge the caps with a power supply?, even a 5 amp model would work but take a very long time..

    Then when caps are same voltage a battery (or cell) voltage hook up and no problem... I hope

    I got a 80 volt 100000 uF cap in parallel with mine a no problem

    I got a 48 volt sys and had many problems with 12 and 6 volt battery's mostly from under charging..
    Make sure you got a very volt meter also as I had problems with them showing wrong voltage..
    measure at the battery posts.

    Too bad you got 12 volt it would be easy just to have 2 volt cells and put a 2.7 volt cap on each one..
    That could reduce the balancing problem. Equalize the cells ~2.6V... or end up with "Sulphated battery's"
    1mA per amp hour or your battery is headed for problems , so 100 amp battery should only take 0.100 amp to keep it on float...(@13.2V)

    You should have the battery's and caps ESR tested you may find something there.
    I'm going to try the same thing you are doing just have to find some good caps, just for motor starting
    as it's only a few milliseconds of energy needed. Like the caps energy will not last very long...
    it's in the milli-second range 3000/6=500F. Depends on the voltage drop, a 0.1 drop per cell (2.4-2.3)
    would give you ~170 amps for ~300 mS (with each cap 0.00023 ESR) single cap * 6 maybe .014 ohms?
    Your battery should be around that or less. My 12 volt marines are ~5 milli-ohm. (0.005 ohm)

    And got a small LiFep04 they are much lower 0.45 milliohm per cell and very hard to measure with standard tester.
    But got no problem with them and the lead acid after ~2 years but the LiFep04 can not be charged like the LA
    they need at least 25% of C of current or big problems...
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
    What? o_O

    The bigger the battery the greater the average current draw it can handle.

    That's an odd one? What type of alternator do you have and what is the typical system voltage when you are bringing it online?
  4. wayneh


    Sep 9, 2010
    You don't want to use the "big resistor" strategy, because it's so wasteful of the energy out of the alternator. Whatever you gain from the supercaps in your system you would be losing in lost energy from the "big resistor".

    There are know configurations of super-cap assisted batteries, and even commercial products using both. I suspect you can find one with some searching.