Battery and Amps Question

Discussion in 'General Electronics Chat' started by TimCollins, Oct 8, 2011.

  1. TimCollins

    Thread Starter New Member

    Sep 19, 2011
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    I have a few simple questions about batteries and Amps. For example, I am working with an ‘enercell’ Sealed Lead-Acid Battery (SLA) 12V – 12Ah.
    Is there a way you can know from these specs what the maximum Amps a battery can produce?
    Is the only way to measure how many Amps a load (which you do not have specs for) is pulling without having an ammeter? I do have a voltmeter. If not, I may need to just fork out the dough and buy an ammeter huh…
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You need to look at the manufacturer's datasheet for your particular battery.

    You may have bought that battery at Radio Shack, which means you are out of luck for getting a real datasheet.

    However, the battery might put out 150A to 200A for a few moments.
     
  3. TimCollins

    Thread Starter New Member

    Sep 19, 2011
    9
    0
    You are spot-on Sgt... It is Radio Shack Battery. In the future I will know to look for a spec sheet. In the mean time I think it is time for me to invest in an ammeter anyway... Thanks for the info!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You really don't want to buy an ammeter capable of measuring the maximum output of that battery. You could go to an auto parts store and have them test it if you wished, but I suggest that you don't.

    Why would you want to know the maximum output current anyway?

    The maximum power transfer will occur when the load is the same impedance as the source. Your SLA battery will likely have an impedance of 0.014 to 0.020 Ohms when it is fully charged. If it has 20m Ohms impedance and you are using a 20m Ohm load, you'll get about a 1.82kWatts power transfer and ~300 Amperes of current flow. However, that is probably enough current to melt the tabs right off the battery; if the tabs don't melt off then something inside the battery will melt after a short period of time. If that doesn't happen, then a fire or explosion could result, or you will melt whatever conductor that you are trying to short the battery with.
     
  5. Smoke_Maker

    Active Member

    Sep 24, 2007
    126
    15


    It depends on the LOAD, What is it?? if it a resistive load you can measure it with your meter.

    As for the battery Sgt. is right, but even the spec. sheet dose not tell the whole story. Batteries do not fully form until after several cycles and even then they might not meet spec.

    Radio shack might have a "shunt" or some other store might have one. A shunt can be used as a cheap amp meter, when current go's through the shunt at two little screws on the side will give you a voltage comparison of the amps going through the shunt.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Using a shunt is a possibility, and may be the best way if the unknown current is above 10A or so, as amongst other things this is the limit of a lot of multi-meters. There are a few pitfalls though. With a shunt and a voltmeter, the user needs to understand how to interpret the results. That's just Ohm's law and a simple multiplication, but the wording of the OP's enquiry suggests he may not be familiar with such things.

    There is another issue: most DMMs, come with a fuse for protecting current ranges against overload, including the results of connecting the ammeter in parallel with the supply instead of in series. This is a mistake often made by newbies, and some of us who ought to know better, if we don't keep our minds on what we are doing. At least the fuse can prevent a fire, though the meter may still suffer. A simple shunt on the other hand may have no fuse. Putting a few milliohms shunt across a lead accumulator is not a good plan. You should NEVER try to test the maximal current capacity of the battery like that.
     
  7. #12

    Expert

    Nov 30, 2010
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    6,852
    It's like saying, "I wonder how fast this engine can go if I put a brick on the accelerator pedal".

    It will go, but you better have a fire extinguisher handy.
     
  8. CraigHB

    Member

    Aug 12, 2011
    127
    15
    In general, you can always find current flow by measuring the voltage drop across a known resistance. Simply divide out the resistance from the drop. I have an ammeter that goes up to 20A, but I commonly use that method for a quick check when I don't want to involve the ammeter. The ammeter does introduce some load resistance which causes an amount of error at higher currents.

    I also use that method when finding the DC resistance for the lithium-polymer batteries I commonly use. It's usually given in the data sheet only as AC impedance tested at a particular frequency, but I want to know a battery's DC resistance for purely resistive loads. Keep in mind the numbers you get are dependant on load current and state of charge so you probably want to take a few data points under various conditions.

    To find a battery's DC resistance, measure the open circuit voltage. Then measure voltage at the battery terminals under load with a known resistance. Compute curent. Divide the difference in loaded and unloaded batt voltage by current flow to get the battery's DCR.

    .
     
    Last edited: Oct 9, 2011
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A little 12Ah lead-acid battery will provide only 1.2A for 10 hours. At a current of 12A or more it might last for 10 minutes or it might melt or blow up.
     
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