how-to connect 1F cap to car without danger of draining it after 3-4weeks?

Discussion in 'The Projects Forum' started by rasto, Dec 4, 2008.

  1. rasto

    Thread Starter Member

    Dec 1, 2008
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    Currently I have 1F cap connected to my car battery directly (40A cable) and the problem is that it drains 100mA constantly. If I don't use my car for 3-4 weeks my battery is drained so much, that it wont start my car.
    It's a diesel which needs a lot of juice at start for preheat and starter.

    I was thinking of getting some kind of disconnecting circuit, but this cap creates quite a big sparks when connecting and I'm afrait that these sparks will fry regular 20A relay contacts.

    What would You recommend?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You could use a power MOSFET in the cap's ground path that receives voltage to it's gate only when the accessory switch is on. Use several high-wattage 12v bulbs in parallel between the battery's + terminal and your capacitor.

    There will be a high initial surge current, but the bulbs will rapidly heat up increasing their resistance and decreasing the charge rate. As the cap becomes charged, the voltage across the bulbs will decrease, so they will cool and their resistance will decrease.

    You really don't want to charge/discharge those big caps too quickly. You'll melt them.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    How about a solar charger? It would make up your loses as it sits?

    It's amazing how many places those light bulbs can be used though.
     
  4. rasto

    Thread Starter Member

    Dec 1, 2008
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    I'm going to test how many light bulbs I'll need to not get them too hot.
    I don't want to set my car on fire, bulbs will be in spare tire location in my trunk...

    Solar panels?
    I would like to see a solution that would not severely uglyfi my car. Otherwise an idea I haven't though of.

    Thanx
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    A simple 12 V bulb will do it, and not get that hot. It's self limiting.
     
  6. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    It doesn't seem to me that a capacitor, unless defective, should NOT have any kind of continuous DC leakage current pass through it once it charges up to the battery voltage. I know that when using series connected caps to come up with a overall cap with a higher effective working voltage, they sometimes wire up a string of series 'equalizing' resistors in parallel with the caps, to make sure each individual cap has the same voltage across it, and that would result in a small continuous DC current draw through the resistor string. But that wouldn't apply for a 1F cap designed for a 12volt system, would it? Or might this 1F cap be made from say 4 4F 5 volt 'super caps' in series and there is a equilization resistor string being used? I have no idea how these super large car audio caps are constructed.

    Anyone have any ideas why the poster is seeing a continuous DC leakage current through the cap?

    Lefty
     
    Last edited: Dec 4, 2008
  7. Audioguru

    New Member

    Dec 20, 2007
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    Haven't you seen those fancy huge capacitors in car radio stores?
    They have a current-hogging LED display of voltage.
    Maybe the display can be connected to the car lights so it turns off when the lights are turned off.
     
  8. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    No, I've only seen brief pictures in magazines and didn't notice they were also light displays. That would explain a continous current drain. Can they be disconnected as LEDs serve such a silly purpose in this application?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    These huge caps are used to supplement crappy wiring to crappy and unnecessarily loud stereo systems in pimpmobiles nowadays.

    "See how cool I am, cruzin' wit' mah jams" :rolleyes:

    That's OK, my Avenger system is nearly back online. I'm going to change targeting parameters from heat signatures to excessively loud audio signatures. :eek:

    Oh, and tailgaters, too. :)
     
  10. leftyretro

    Active Member

    Nov 25, 2008
    394
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    Yes, I've frequently wished I had front and rear mounted AIM-7s on my ride :cool:
     
  11. rasto

    Thread Starter Member

    Dec 1, 2008
    10
    0
    Yes this cap has voltmeter on it, so maybe disconnecting it will alleviate som mA...

    but I'm going to disconnect it when not in use.
    Maybe putting MC14541B to delay shutting it off by some 10minutes.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    How often if ever does somebody look at the LED display on a huge capacitor?
     
  13. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    I guess it would depend on how often and active a dater one its. Standard line might be: " Hey honey, I need to pull over and park here to check the charge level on my big capacitor" :cool:
     
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