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Thread: Basic battery question

  1. #1 Basic battery question 
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    I am helping my little one with a science project and need some input.

    We strung Nichrome coils between two stripped power cord wires. We are trying to get them hot enough to light a match. Thus far all that has worked is an 18volt 1500mah portable drill battery. But we need to use household batts

    Now doing the math 18volts times 1.5 amps yields 27 watts. Am I correct in assuming that all I need to do is that same math on household batts to get the same amount of watts.


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  3. #2  
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    Did you measure the current or did you assume the current is 1.5 amps out of the 1.5 amp-hr battery? The battery might be supplying more current than that, it just wouldn't last an hour at the higher rate.

    I think you would be okay if you match the amp-hr rating of the drill battery and put enough cells in series to match the 18 volts. I found this list of battery ratings
    D -- 14,250 mAh
    C -- 7,100
    A -- 2,450
    AA -- 1,120
    AAA -- 550
    9V -- 565
    Two 9-volt batteries in series gives you 18 volts but only .565 amp-hours, but you could get 1.7 amp-hr by using 3 parallel combinations of two 9v batteries in series.

    You would need 12 of the others in series to get 18 v and they wouldhave to be D or bigger to almost match the amp-hr.
    Edit - Oops made a mistake. You need to match the volt-amp-hrs. Fixed


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    Thanks for the input Ibelieve those battery ratings are alkaline batteries. From my research I have found that alkaline batteries do not have a high enough discharge rate to accomplish the task. They are more of a slow drain.

    I have determined that I preety much need NiCd batteries for their high discharge capacity. I suppose that is the misleading part about the mah ratings on batteries.

    Is it possible to use say two 9volt batteries in series with two C or D batteries in order to get both the volts and amps I am looking for?
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    No. Any amps you put through one goes through the other one that is in series so you will only get as much current as the least rated one puts out.

    You can keep putting strings in parallel until you get enough current. I don't know how many you'd need. What current do you think the heater coil was using?
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  6. #5  
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    All I can tell you is that another drill battery rated at 9v and 2000mah gets them hot but not hot enough

    So I am not sure on the current it pulls but let me ask is it not possible to get higher mah rated batts and forego the volts that is to say for instance 4 1.2v volt nicads rated at 5000 mah yields 24 watts or will the lack of volts not push the current?
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  7. #6  
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    The lower voltage will not push the current. It's ohms law (current =volts/ohms), except with a heater like that, the ohms will increase as it heats up. So you couldn't just measure the resistance with an ohmmeter and calculate the current, not very accurately anyway.

    Maybe you could shorten the heater wire. That would reduce the resistance. You'd only need a little short section to light the match on. That way, your power requirement would be less.
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  8. #7  
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    All you have to do is use smaller gauge ni-chrome wire. Get it small enough and a single 1.5 volt cell will light a match. The other option of course is to use a shorter piece of the heavier wire and much more current. Easy way to do it is to use NiCd (household) batteries. They produce a massive amount of current for a short duration. Your limited by the internal resistance of Alkaline batteries. The example of a 9 volt is flawed. They will not produce all that much current when the need arises, they were not designed for that (Too much internal resistance). D cells would be about the best (no where near as good as a AA NiCd NiMh).
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  9. #8  
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    Just another note, the mah rating of the battery doesn't have a terrible amount to do with it's internal resistance. A good example would be a Lithium Ion cell phone battery. Once you rip the current limiting circuits out of them you can often get 30 amps + of current to flow. When your calculating the current using ohms law you have to count the internal battery resistance.
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  10. #9  
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    Quote Originally Posted by Harold14370
    I found this list of battery ratings
    D -- 14,250 mAh
    C -- 7,100
    A -- 2,450
    AA -- 1,120
    AAA -- 550
    9V -- 565
    For what type of batteries? Saline cells or alkaline? Cheap "no-name" brands or the ones that "keep going" or "last much longer"?

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  11. #10  
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    Quote Originally Posted by Leszek Luchowski
    Quote Originally Posted by Harold14370
    I found this list of battery ratings
    D -- 14,250 mAh
    C -- 7,100
    A -- 2,450
    AA -- 1,120
    AAA -- 550
    9V -- 565
    For what type of batteries? Saline cells or alkaline? Cheap "no-name" brands or the ones that "keep going" or "last much longer"?

    Energizer Bunny arrested, charged with battery.
    This list is not all that usseful, it only lists the rated mAh of the battery. It has nothing to do with it's internal resistance and it's ability to supply current. It's useful to try and figure out how long a battery will last, that's about it.

    For example a 9v NiMh battery rated @250 mah might be able to produce 2 or 3 amps if you short it out. On the other hand a 9v Alkaline might only be able to produce 0.5 amps when shorted even though it might have a 500 mah or higher rating.

    It's all about internal resistance when your trying to get those high current amounts.
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