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Thread: Battery pack total capacity - help me understand please

  1. #1

    Default Battery pack total capacity - help me understand please

    If I have one 3000mAh LiIo 3.7v 18650 battery fully charged to ~ 4.20v, then I know that I should have ~3000mAh of power available.

    How does capacity change as I add more of these in series and in parallel.

    For example, what would be the total capacity of a battery pack with 10 of these fully charged in a series battery pack and in a parallel battery pack?

    Thanks

  2. #2
    Flashaholic* ChrisGarrett's Avatar
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    Default Re: Battery pack total capacity - help me understand please

    Quote Originally Posted by OttaMattaPia View Post
    If I have one 3000mAh LiIo 3.7v 18650 battery fully charged to ~ 4.20v, then I know that I should have ~3000mAh of power available.

    How does capacity change as I add more of these in series and in parallel.

    For example, what would be the total capacity of a battery pack with 10 of these fully charged in a series battery pack and in a parallel battery pack?

    Thanks
    In series: compound voltage, not capacity.

    In parallel: compound capacity, not voltage.

    All other things being equal.

    Chris
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  3. #3

    Default Re: Battery pack total capacity - help me understand please

    Quote Originally Posted by ChrisGarrett View Post
    In series: compound voltage, not capacity.
    In parallel: compound capacity, not voltage.
    All other things being equal.
    Chris
    So is that to say a battery pack with 1000 of those in series would have the exact same capacity as one of them? (but 4,200 volts?)

  4. #4

    Default Re: Battery pack total capacity - help me understand please

    Watt hours is the measurement of total energy capacity. Volts X Ah = Watt hours. So NCR18650GA would be 3.7V X 3.5Ah = 12.95Wh. Series or parallel, you just add watt hours.

    But yes, in series you add just voltage, and in parallel you add just mAh.

  5. #5
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    Default Re: Battery pack total capacity - help me understand please

    Hello OttaMattaPia,

    Understand that a LiIon battery can have a fully charged voltage of 4.2 volts but is referred to by its nominal voltage of 3.7 volts. This is roughly the voltage under load.

    If I have 2 cells that have 2500 mAh of capacity and connect them in parallel, I end up with a battery with 3.7 volts and 5000 mAh of capacity.

    If I have 2 cells that have 2500 mAh of capacity and connect them in series I have a battery with 7.4 volts and 2500 mAh of capacity.

    In parallel you add the capacities but the voltages stay the same. In series you add the voltages and the capacity stays the same.

    You can also construct a battery by putting some cells in series along with some in parallel. This is the way most laptop batteries are constructed.

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

  6. #6

    Default Re: Battery pack total capacity - help me understand please

    Quote Originally Posted by SilverFox View Post
    Hello OttaMattaPia,
    Understand that a LiIon battery can have a fully charged voltage of 4.2 volts but is referred to by its nominal voltage of 3.7 volts. This is roughly the voltage under load.
    If I have 2 cells that have 2500 mAh of capacity and connect them in parallel, I end up with a battery with 3.7 volts and 5000 mAh of capacity.
    If I have 2 cells that have 2500 mAh of capacity and connect them in series I have a battery with 7.4 volts and 2500 mAh of capacity.
    In parallel you add the capacities but the voltages stay the same. In series you add the voltages and the capacity stays the same.
    You can also construct a battery by putting some cells in series along with some in parallel. This is the way most laptop batteries are constructed.
    Tom
    Thank you Tom. I guess on paper I know these things but something doesn't quite click in my mind.
    Need-A-Light mentioned Watt hours and I wonder if that's what is confusing me?

    For example....
    If I had two identical 3volt flashlights with the exception that one held only two cells in series..... while the other held 3 cells in series, and had a buck converter to drop the voltage back to 3.0v

    Series law says the two have the same capacity...however, I'm fairly certain that the one with three cells would run longer. Watt hours?

  7. #7
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    Default Re: Battery pack total capacity - help me understand please

    Hello OttaMattaPia,

    On the other hand the 3 cell light with the converter may end up with a higher load on the battery pack and actually end up with less run time...

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

  8. #8
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    Default Re: Battery pack total capacity - help me understand please

    “In theory, there is no difference between practice and theory. In practice, there is.” (a quote whose true origin is mysteriously uncertain)

    Yes, total energy is total energy when adding cells, but you have to account for heat and other efficiency losses through a converter to use that energy. The greater the difference between the voltage you have and the voltage you need, generally, the less efficient it will be.
    Last edited by archimedes; 03-19-2017 at 09:28 AM.
    ... is the archimedes peak

  9. #9
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    Default Re: Battery pack total capacity - help me understand please

    Quote Originally Posted by OttaMattaPia View Post
    For example....
    If I had two identical 3volt flashlights with the exception that one held only two cells in series..... while the other held 3 cells in series, and had a buck converter to drop the voltage back to 3.0v

    Series law says the two have the same capacity...however, I'm fairly certain that the one with three cells would run longer. Watt hours?
    Capacity (in Ah or mAh) is relative to the total pack voltage, e.g. 2S 3Ah pack us 7.4V * 3Ah = 22.2Wh; a 3S 3Ah pack is 11.1V * 3A = 33.3Wh, assuming nominal 3.7V cells.

    This is a common point of confusion, esp. for powerbanks which often list capacity at the nominal cell voltage, rather than the output voltage(s), such as. 5V USB or 12V, 19V etc.
    Last edited by Gauss163; 03-19-2017 at 09:56 AM.

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