Question about drawing current from batteries in series.

Discussion in 'General Electronics Chat' started by Austin Clark, Jan 3, 2012.

  1. Austin Clark

    Thread Starter Member

    Dec 28, 2011
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    If I had two 1.5V batteries in series, both rated to output, say, 100ma, could you safely draw 200ma at 3V from them? In that case, you'd essentially double the amount of power coming from each individual battery, and that just doesn't seem right. If not, how much current CAN you safely/effectively draw?

    Also, higher current would mean more heat dissipated within each batterie due to internal resistance, at some point couldn't there be a fire hazard?
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    2,729
    1.5V * 100mA = 150mW
    3.0V * 200mA = 600mW

    You are quadrupling the power.

    If the batteries are rated at 100mA, then you can draw 100mA regardless of how many you put in series.

    If you parallel them, *then* you can draw 200mA (at 1.5V), or 100mA per cell.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    It's a common misconception that putting batteries in series can provide more current from the battery. It can't. It does provide more voltage and power but not more current.

    So your intuition was correct about that. Drawing more current can overheat the battery and even cause a fire (which you definitely don't want with lithium batteries).:eek:
     
  4. Austin Clark

    Thread Starter Member

    Dec 28, 2011
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    Yeah, you're quadrupling the power, I knew that but made a typo :p


    Thanks for the responses! Very very helpful people here :)
    I have many many more questions I'll be posting over time, I hope you like my enthusiasm and curiosity, and won't find it annoying, ha!
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    A battery is not rated in mA current. Instead it is rated in mAh which is the amount it can supply one-tenth the current rating in ten hours. Some batteries supply one-twentieth the current rating in 20 hours.
     
  6. Austin Clark

    Thread Starter Member

    Dec 28, 2011
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    So, basically, batteries are rated in terms of capacity/maximum current load? Or the ratio between how much they actually hold and how much they current they can supply at once?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    As Audioguru wrote, they are rated in mAh.

    You really should look at the datasheet for the battery in question to figure out what it means.

    For example, if it is a pp3 9V battery that is rated for 500mAh, that means the battery output voltage will meet/exceed a threshold level with a 25mA current output for 20 hours. If your current draw is higher, you will receive a shorter service life. The greater the current draw, the higher percentage of power is wasted in the battery itself as heat via the battery internal resistance.

    As a battery becomes discharged, its' internal resistance increases. You can't accurately measure a battery's charge state without placing a load on the battery.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The OP talked about a 1.5V 100mA battery. Maybe it is a tiny 386 silver-oxide button cell that is used in my watch. It is rated at 110mAh with a 6.8k load (0.228mA) until its 1.55V drops to 1.2V in about 483 hours. Its max allowed current is about 23mA in very short pulses.
     
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