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About current from a battery going through our body

  1. Aug 28, 2016 #1

    KFC

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    Hi all,
    I am reading some materials on electrical parameters of a AA battery which give 1.5V and about 2000mAh. As my understanding, the 2000mAh means it will produce 2A if we keep taking current from the battery for an hour. 2A is huge current for human being. I wonder why it won't hurt the body. Is it because it take so slow for the electron to accumulate so to get 2A and once some electrons arrive the body, they move to the ground immediately? So what happens if we isolate the body from the ground, will it cause any injure by a AA battery by holding two terminals for even 30 minutes?
     
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  3. Aug 28, 2016 #2

    davenn

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    through the appropriate resistance

    No ... it's because the body's skin resistance is sufficiently high enough that the 1.5V of the battery cannot breakdown that resistance and allow correct to flow

    no, doesn't matter if you are grounded or not



    Dave
     
  4. Aug 28, 2016 #3
    You've misunderstood the units. mAh is a unit of energy (sort of). 2000mAh means that the battery could put out 200 Amps for 6 minutes, 2 Amps for an hour, 1 Amp for 2 hours, or 1mA for 2000 hours. It depends on the circuit that the battery is in.

    There are more parameters for batteries.

    A battery can only put out a limited current. This is usually represented with a "C" rating where ((Amp output)=C * (storage rating)) If your battery had a "C" of 1 then we drop the h from mAh and say that the battery can only safely put out 2000mA without blowing up. The "C" of your AA battery is probably much lower than 1. If "C=0.1" then the battery can only output 200mA.

    batteries drop voltage as they discharge. The profile of this depends on the chemistry and number of cells.
     
  5. Aug 28, 2016 #4

    davenn

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    yes, this is correct :smile:
    but from the OP's point of view about why there is no electric shock, it's not the relevant answer :wink:


    Dave
     
  6. Sep 6, 2016 #5

    KFC

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    thanks for your explanation. So if for regular battery, C=0.1, you mean the maximum current is just 200mA by assuming the load is not zero ohm, is that right? I am curious where do we find the parameter of the battery for that C and mAh.
     
  7. Sep 6, 2016 #6
    That's right. An AA battery has 1.5V so if our max current is 200mA then the lowest load resistance we could have is 7.5Ohm before the battery is in an unsafe region of operation. I just did a little Googling. It looks like the "C" rating is more common for NiMh, NiCad, and lithium batteries but I can't find anything like a chart for regular alkaline AA batteries.
     
  8. Sep 7, 2016 #7

    CWatters

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    It won't hurt because the current will not be 2A....

    The resistance between my left hand and right hand is currently about 1,000,000 Ohms (I've just measured it). Most of that is the resistance of my skin. If I hold an AA battery with a thumb on each end the current flowing through me will (according to Ohms law) be..

    I = V/R
    = 1.5/1,000,000
    = 1.5*10-6 Amps

    That's quite a lot less than 2A and not enough to cause an electric shock. It's generally considered that currents of 10mA will be painful and that currents of 100 to 200mA can kill you.

    However the resistance of the human body can vary quite a lot. Google says it can fall as low as 500 Ohms under certain conditions. This means the maximum current that an AA battery could send through me is around..

    I = V/R
    = 1.5/500
    = 3 *10-3 Amps
    = 3mA

    As I recall most international safety standards limit accessible voltages to around 36-42V. Why? Because that would generate a worse case current of..

    42/500 = 84mA and that is quite close to the 100mA considered capable of killing someone.

    Aside: Batteries have some internal resistance which will also limit the maximum current they can deliver BUT usually it will be the external resistance that dominates the calculation. For example the internal resistance of a NiMH cell can be of order 10-3 Ohms which is negligible compared to the 500 to 1,000,000 Ohms of the human body.
     
  9. Sep 8, 2016 #8
    There's also something called power density which indicates how much power a battery can deliver per unit weight, can also be expressed in terms of volume. That's usually considered along with a battery's energy density which is the capacity (amp-hours) per unit weight, can also be expressed in terms of volume. This is important in considering how much power a battery can actually deliver efficiently.

    Some power is always lost inside a battery as a function of its IR (internal resistance). The IR of a battery has to be considered in a circuit so even if you short a battery with a wire, current flow will be a function of the battery's IR according to Ohm's law. An alkaline AA battery has fairly high IR compared to other battery chemistries, about 200mΩ. So a short circuit across its terminals would result in a current flow about 6 Amps. With a capacity of 2000mAh, it could to that for about 20 minutes. On the other hand, a high drain or high C rate lithium-ion battery may have IR of only a few mΩ. In that case massive currents can flow when shorted, enough to cause extreme heating and fire. So you can see the power density of a high C rate Li-Ion battery is much greater than that of an alkaline battery even though energy density is not hugely different.

    The 1C rate of an AA Alkaline battery as you mentioned (2 Amps) would get into an area of poor efficiency. In a circuit there would be the power loss of that 2A across the battery's IR which would be close to a Watt. Using rough numbers that's compared to about 2 Watts for the load. You can see a significant portion of the total power consumed is lost inside the battery. For a load of 2 Amps you'd want a battery with higher power density (lower IR) to provide better efficiency. Typically alkalines are used to provide only lower currents because they are primary (can not be recharged) and don't deliver higher currents efficiently. For higher power applications you would go with a secondary battery (rechargeable) having higher power density like Li-Ion or NiMH.
     
    Last edited: Sep 8, 2016
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