Car Related, not specifically a Car Question.

Discussion in 'General Electronics Chat' started by cjdelphi, Jun 17, 2011.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    272
    2
    Basically I just had a thought, I'm sick of Car 12v batteries, the last one Died on me after being on float charge for almost 2 years solid... now the car battery just died!

    So, forgetting about parasitic drain (not that it's an issue anyway)... is there any real reason one day a super capacitor could not replace a battery?...
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Yes. A battery internally regulates voltage. A supercap doesn't. An electric car might have a supercap with 8KV on it.
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    The starter motor on an IC engine consumes hundreds of amps at 12 volts. That would be a pretty big cap. If you converted to a 42 volt system you might have a better shot. One novel arrangement converted the flex plate into a dual purpose 42 volt starter/alternator.
     
  4. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Umm... what?

    You cannot easily overcharge a capacitor. Ever tried it?

    I put 35V on a 6.3V electrolytic cap quickly. With the multimeter attached it showed 35V. But within ~100ms it had dropped to just 12V and slowly decayed.

    A super cap will never get 8kV across it, it would simply arc over or damage the capacitor. Similar for a battery.

    I think the main problem is that super caps are expensive. You'd spend $500 on a super cap starter battery for a small car when you could just use a lead acid for less than $100 (I think... I have never bought a battery.) A small car might require 60 to 100 amps to start for 5 seconds at the most. At 12V, that is 1.2kW, or 6kJ over 5 seconds, required to start a car on a cold day. A 2500F 2.5V supercap cell has about Q=CV joules in it, or 6.25kJ. You'd need 6 cells to get 15V. 6 cells may just about be able to manage it. Now upgrade your charging circuitry and battery monitoring circuits. Then you are ready. But as I said, at $80 a pop, that would be almost $500.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If a float charger that is not temperature compensated is used on an automotive battery where the storage temperature is not controlled, it can kill the battery.

    Lead-acid batteries need to be float-charged at a higher voltage when the temperature is low, and a lower voltage when the temperature is high. The float voltage is nominally 13.65v @ 25°C internal temperature, but this changes at a rate of -18mV/1°C offset.
     
  6. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    With car batterys Ive found when purchasing them i check the spesific gravity of all cells before using the battery. The reason is ive found a lot af batterys will have at least one cell well down on the others. I charge these batterys un till all cells are equal before fitting the battery to the vehicle. In winter time with short runs i check battery ocasionally & give a topup charge. Both my vehicles there batterys are 6Yrs old & still going, I think if a battery gets a good start in life & looked after should give good service.
     
  7. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    I had to store a mini-pickup truck for a year, so I attached a wall-wart to charge the battery. After the battery was charged full, I added resistance until the bubbling stopped. Checked the water level every couple of months. Just a few ma kept it healthy through both summer and winter. Not very "millivolts per centigrade", but it worked.
     
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