A Batteries current is equal to its voltage?

Discussion in 'General Electronics Chat' started by fullNelson, Nov 21, 2011.

  1. fullNelson

    Thread Starter Member

    Nov 14, 2011
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    Hi guys,

    I have a d-cell sitting in my room and on its wrapper it only lists a voltage of 1.5v, and no current. So, I thought to myself: what is the current on a battery when I short it out? If I dont have an applied resistance via a resistor, and I am only considering the minute resistance of the copper wire, isn't ohms law saying that this battery's current through the wire 1.5v = 1.5amps?

    I am still waiting for my multimeter to come in the mail, so I wont know for sure, but theoretically, this is right: 1.5v = 1.5amps?

    P.S. I love this site, its information and this forum. Its a big help.
     
  2. Butterworth

    Active Member

    May 6, 2009
    135
    3
    Hello,

    I suggest reading up here. It is a section on Batteries and Power Systems under Volume I: DC at the top of the screen.

    It goes over all you need for batteries. :)
     
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  3. mrmount

    Active Member

    Dec 5, 2007
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    If the resistance of the copper wire you are mentioning is 1 ohm, then the current will be 1.5A as you say. If the copper wire resistance is less than that, the current will be greater.
     
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  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    No. The maximum current obtainable from a battery is determined by its own internal resistance. A combination of actual electrical resistance in the materials making up the battery cells, and possibly chemical rate-limiting effects restrict the battery current, so that it behaves similarly to an ideal voltage source in series with a resistor. As larger and larger currents are taken, the battery output voltage falls, until a maximum current is reached at zero output voltage.

    In any practical situation, the external circuit (even if this is just a wire) will add some extra resistance, which may or may not be significant in comparison.

    From Ohm's Law, I = V/R, so if the internal resistance R is less than 1Ω, the short-circuit current will exceed the open-circuit terminal voltage. Unfortunately, in many cases the short-circuit current will be dangerously high, risking wires getting burning hot, and batteries running down fast, overheating or even bursting. This is especially so with some modern high energy and rechargeable types.

    Never short-circuit a battery. It can be very dangerous. A short-circuit test of a battery into an ammeter is also a good way of damaging the meter, or at least blowing its protective fuse, if it has one. The short-circuit current of even small alkaline cell is probably several amps when new.
     
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  5. fullNelson

    Thread Starter Member

    Nov 14, 2011
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    Thanks. I probably should have paid attention to this section a little more closely. I thought i knew this stuff. :S
     
  6. fullNelson

    Thread Starter Member

    Nov 14, 2011
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    So if your statement here and other's holds true, even a 1.5v shorted out is dangerous based on the amperage alone. The volts aren't high, but 1amp is bad. Thanks. Can't a multimeter with a 20A rating handle testing this though? I have an Extech470 coming to me and its rated for 20A.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Why do you want to short a battery?
    Instead why don't you use the battery to do something useful like powering a load?
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    An Energizer Alkaline D Cell has a capacity of 12 to 20 Amp Hours, depending on discharge rate.

    Internal resistance of a new/fresh battery is 150 - 300mΩ, limiting short circuit maximum current to 5 to 10 Amps, though it wouldn't sustain that level of discharge for very long.

    Recommended load is <1 A continuous, at which point the capacity is only 12,000 mAH, compared to 20,000mAH at 25 mA continual discharge.

    For comparison, the Energizer AA Battery has a capacity of 2750mAH @ 25mA discharge, and max suggested discharge of 500mA (lifetime at 500mA discharge is only 1,400mAH),

    Interestingly, the AA has the same internal resistance as the D cell, most likely due to improvements made in chemistry since AA is the most popular battery size by far. This makes the short circuit current 5 to 10 amps as well, but for a much, much shorter time.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,442
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    Don't do it. If you try to measure the short-circuit current from a D cell you will destroy your meter or blow the internal fuse.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You won't believe how hot a shorted battery gets and how quickly. As me how I know.....:eek:
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Oh, I agree, especially with NiCd, which can dump 20+ amps in a hurry.

    I simply posted the info on the Energizer since it's a common question at the back of most beginner's minds, and it is useful to know without actually trying to measure it.

    A 1 second very high load on an Alkaline battery will put it at 50% capacity, due to the chemistry. That's why digital cameras from 6-10 years ago that used alkaline AA batteries could only take 10 pictures with flash before the batteries were dead (and very hot), while NiCd or NiMH AAs could take 100 pictures. It's all about internal resistance and how stable it remains across the discharge curve.

    --ETA: It's also why all the loose 9V batteries you keep in drawers seem to be dead. They only need to bump into another battery for a fraction of a second to kill them. I always put a piece of tape over the terminals for loose storage. I hate it when a meter battery goes dead when away from the shop, so I carry lots of spare batteries.
     
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  12. strantor

    AAC Fanatic!

    Oct 3, 2010
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    When I was a kid I used to connect 9V batteries end to end to see how hot the would get. I also used to take the wire out of bread bag twisty ties and wrap it terminal to terminal and let it glow red and cut crayons in half with it. it would stay charged long enough to cut through a few crayons. even longer with the 6V lantern batteries.
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I still connect 9V batteries like bricks, even when they are "dead", once you get 50 of them in a brick, they pack a punch. :D

    Your 6V Lantern Battery story reminds me of gradeschool and making our first electromagnet. 2 perfectly linear blisters on thumb and forefinger later, I re-discovered Ohms law, and I wasn't even 12 years old!
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Have to admit one of my first projects was a shock machine with a push-button, C cell, and an audio output transformer. I guess we have an innate need to shock things or see them burn up.
     
  15. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I think the "no shocking devices" part of the Terms of Service was mostly to protect us from ourselves. :D
     
  16. fullNelson

    Thread Starter Member

    Nov 14, 2011
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    Thanks for the tip on the 9V batteries in the drawer.

     
    Last edited: Nov 23, 2011
  17. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    IIRC that was used in some war, persian gulf probably, to tag targets in infrared spectrum for air strikes.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    No.
    Nobody makes an ideal D-cell alkaline battery. They all have an internal resistance.
    Energizer lists the max internal resistance at 300 milli-ohms (same as their AAAA, AAA and AA cells).
    Then a load of 1 ohm results in a current of 1.15A and the 1.5V from inside the cell is reduced to 1.15V at the load when the cell is brand new.
    A load of 300 milli-ohms results in a current of 2.5A and only 0.75V at the load.

    The datasheet shows that the capacity is 20,000mAh at 25mA or 10,000mAh at 500mA. They don't show capacity at higher currents.

    No.
    The voltage of a brand new cell is about 1.6V with no load and 1.5V with a low current load.
     
    Last edited: Nov 23, 2011
  19. PC Pete

    New Member

    Nov 11, 2011
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    I have a very good friend who's been trying to learn electronics for a long time - but they keep taking "shortcuts" that cripple them later on. His search for an ideal battery took me days to reverse the bad info and teach him the basics.

    My approach was to keep it simple, and look at a battery as a "black box" current source instead of a voltage supply. (Boy, I wish I'd known about the excellent section on batteries in the DC manual here, it covers everything I taught him, and it would have been a lot quicker and less painful for both of us!) Actually, he's not much of a reader, so he wouldn't have wanted to wade through all of that "wordage", an unfortunate frame of mind that many of my younger relatives seem to suffer from these days. I must be getting old...

    The worst part was trying to explain why, even though the battery had "1.5V" on it, it wouldn't supply 1.5V for ever. We had to download graphs and charts for NiCad, NiMH, and alkaline batteries from the various manufacturer's sites, (which actually helped a lot, since he's a logical bloke). But I had to explain why they rated the batteries at 1.5V (instead of a more logical 1.2V!), and we destroyed a number of batteries proving that the charts weren't just a "worst case scenario". Actually, that was a heck of a lot of fun! :eek: I guess to a logical mind, what's printed on the packet should be what's inside the packet! I couldn't just disagree with him on that (after all, he's technically right!), I had to show that the world isn't as logical as he was. At least, that's my excuse for burning up some batteries! :D

    Then I had to explain (and prove) that his multimeter (which was a cheap 1970's digital job with an almost-unreadable display) wasn't perfect, that the internal resistance of the ammeter circuit would affect the reading, the battery's internal resistance would change, and so on and so forth.

    After some significant experimentation, and a lot of measurements (and fuses) later, my friend (who was surprisingly resistant to this whole concept) finally understood what the capacity measurement entailed, and why the terminal voltage wasn't the be-all and end-all.

    He was able then to design and build circuits using AA and D cells that lasted (almost!) as long as he wanted, which was a good result, I thought.

    And I found a good home for my old Fluke 77 meter, which was another win.

    And I learned that I actually understood batteries much better than I'd thought. There's nothing like teaching something to hammer home your own understanding of the subject, and that was a win too.
     
  20. strantor

    AAC Fanatic!

    Oct 3, 2010
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    All what you said, and then there's also the peukert effect holding you back. I don't exactly understand the peukert effect, but it causes you to lose AH at higher than rated discharge rates. A thumb rule I have encountered before is that if you double the discharge rate, to don't half the time, you quarter it. for example if the datasheet specifies 10AH @ 500mA - so then if you put a put a 500mA load on it, it should last 20hr. logic would then dictate that @ 1A discharge you should get 10hrs, but in reality you would get 5hrs max. As I said this is just a thumb rule I heard, and I'm not sure how accurate it is; I know there is a more scientific way to calculate the peukert effect, yo could look it up if interested.
     
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