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Thread: Calculating the battery life..

  1. #1 Calculating the battery life.. 
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    Hi,

    I've forgotten the calculation for working out battery life.

    How do I work out the battery life of this:

    DC 12V, 7AH with a 90W bulb


    Cheers,

    Neil


    NR
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  3. #2  
    Guest
    Looks like a homework question - so you'll have to google it -there's plenty on the net...


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  4. #3  
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    Quote Originally Posted by Megabrain
    Looks like a homework question - so you'll have to google it -there's plenty on the net...
    It's not actually, don't jump to conclusions. Considering that I'm on my third year of university studying criminal psychology, I'm not really up to date with what I learnt back at secondary school, I only asked because when I Googled it I couldn't find a straight formula.

    Regards
    NR
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  5. #4  
    Guest
    There is simply no straight formula. It depends upon several things like internal resistance. From memory the AH is the maximum charge it will hold. Just because you might get 40 amps for 1 hour does not mean you automatically get 1 amp fo 40 hours, at this lower rate you will get more than 40 hours worth , what you need to do is look at the data sheet for the specific battery you have in mind and look at the discharge curves.
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  6. #5  
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    Quote Originally Posted by Megabrain
    There is simply no straight formula. It depends upon several things like internal resistance. From memory the AH is the maximum charge it will hold. Just because you might get 40 amps for 1 hour does not mean you automatically get 1 amp fo 40 hours, at this lower rate you will get more than 40 hours worth , what you need to do is look at the data sheet for the specific battery you have in mind and look at the discharge curves.
    OK thank you. I will search for the datasheet for the battery. I have managed to find a formula however for this - well more of a theoretical formula, and I have calculated it to around 3 hours, purely off the digits I previously said. But this calculation is obviously not taking into consideration the battery discharge curve you mentioned.

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    NR
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  7. #6  
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    7 amps for 1 hour you indicated so 90W will be around 7.5Amps so I reckon that's 56 minutes - after you have looked at the curves I reckon that will reduce to about 50.

    Whatever formula you found is either wrong or you did not work it out properly.
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  8. #7  
    Forum Professor captaincaveman's Avatar
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    Quote Originally Posted by Megabrain
    7 amps for 1 hour you indicated so 90W will be around 7.5Amps so I reckon that's 56 minutes - after you have looked at the curves I reckon that will reduce to about 50.

    Whatever formula you found is either wrong or you did not work it out properly.
    thats what i worked it out as :wink: 90w from a 12v battery gave me 7.5A
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  9. #8  
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    Well done CC - Shouted out the answer after it was written on the board, go and stand in the corner....
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  10. #9  
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    Quote Originally Posted by Megabrain
    Well done CC - Shouted out the answer after it was written on the board, go and stand in the corner....
    i used this thing

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  11. #10  
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    Quote Originally Posted by Megabrain
    7 amps for 1 hour you indicated so 90W will be around 7.5Amps so I reckon that's 56 minutes - after you have looked at the curves I reckon that will reduce to about 50.

    Whatever formula you found is either wrong or you did not work it out properly.
    Well done. I was just testing your knowledge before, I knew that this was correct.

    But thanks again
    NR
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  12. #11  
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    CC at last! a post of yours that can be read!
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  13. #12  
    Forum Professor captaincaveman's Avatar
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    Quote Originally Posted by Megabrain
    CC at last! a post of yours that can be read!
    not sure how to take that
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