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Charging a car battery with dry cells?

  1. Nov 14, 2008 #1

    TOD

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    If eight flashlight batteries in series have an emf of about 12 V, similar to that of a car battery, could they be used to start a car with a dead battery? Why or why not?

    My friend posed to me this question and then I just sort of stumbled and was dumbfounded. I answered it by saying the internal resistance for 8 batteries would be too high to be useful for charging, but I'm not quite sure if that's exactly right or if it's the reason why you can't charge a car battery like that. Can someone clarify?
     
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  3. Nov 15, 2008 #2
    My chevy blazer takes about 180 Amps to turn over the engine they tell me (It's a V6). That was easy to find on the web, but looking up battery ESR, or current rating of D cells only got me knocked around by endless insipid ads. So I looked up the carrying capacity of copper wire for 180 Amps. .350" diameter wire was the recommended value for chassis wiring. I can't imagine all that funneled into an alkaline D cell battery, but that's just me.
     
  4. Nov 15, 2008 #3

    rcgldr

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    You could hook up a bunch of batteries in parallel. There are some RC aircraft that pull 80 amps or more with NiCad type batteries. I don't know the limits of Li-Po batteries, but they can be hooked up in parallel without self-drainage issues, to deal with high current applications. However this is a far cry from 8 D cells.
     
  5. Nov 15, 2008 #4

    mgb_phys

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    You are correct it is due to internal resistance.
    A lead acid battery has an internal resistance of20-50 mOhm, so you can pull 100A to start a truck and only drop 100*0.02 = 2V leaving you with 10v output.
    An alkaline battery has an internal resistance of a 200-500 mOhm and will drop to 1.3V output if you take 500mA.
    You could put a few 100 AA batteries in parallel to give you enough current but that's not very practical.
     
  6. Nov 17, 2008 #5

    Danger

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    Wow! You must live in a warm place.
    At the other end of the spectrum, I followed the chart for the Roadrunner to select the right battery. That's 446 cubic inches, 12 1/2:1 compression ratio, power steering, with temperatures down to -50 C. Ended up with a regular 350 CCA (cold cranking amps) automotive battery up front paralleled with a 650 CCA tractor battery in the trunk via 00 welding cable. At -40, I have about 2 minutes to get the thing going before draining them dry. (And yes, I had to have a custom starter built for it; the regular one ended up with fused coils after the first attempt. :grumpy:) I'd hate to think how many 'D' cells it would take to spark that sucker up.
     
  7. Nov 18, 2008 #6
    Good grief, Danger. Maybe you need third battery and a larger engine to haul the extra lead around. I have a friend who had a Corvette in Oklahoma, at one time. He installed racing cams and some other stuff. Told me it took 2 batteries in series to turn it over.
     
    Last edited: Nov 18, 2008
  8. Nov 18, 2008 #7
    Ya Know. The hook asks "Charging a car battery with dry cells?" How did it become cranking an engine with dry cells?
     
  9. Nov 18, 2008 #8

    mgb_phys

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    The title is charging, but the question is starting.
    You could eventually charge a car battery using dry cells - might be a bit expensive though.
     
  10. Nov 18, 2008 #9
    With 12 volts trying to charge another 12 volt battery, it will not work, because to charge a battery you need a higher voltage than the voltage of the battery. The automotive alternator put out between 12 to 14 volts for this reason. Plus it must be able to maintain the higher voltage.
     
  11. Nov 18, 2008 #10

    Danger

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    :rofl:
    You can get regular size car batteries of over 1,000 CCA these days, but back then ('70s) the strongest was about 450 or so. Anything that increases resistance (physical, not electrical) to cranking demands more amperage. That includes higher compression, belt-driven accessories, low temperature, heavier oil... It can sure add up.
     
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