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Will the maximum discharge current double for two parallel batteries?

  1. May 15, 2018 at 12:17 AM #1
    Hello.

    Say I have a high powered device that draws continuous 18A. 3.7v
    And I have one 3.7v 3500mah 18650 with a maximum continuous discharge of 10A, I know the battery will overheat and will most likely get damaged or shorten its life, or even catch fire.

    But If I have another battery (same model).
    Now I have two 3.7v 18650 3500mah in parallel. Will the maximum continuous discharge be 20A? or same 10A?

    Thanks in advance
     
  2. jcsd
  3. May 15, 2018 at 2:15 AM #2

    Borek

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    Staff: Mentor

    It is a bit tricky. If you connect them in parallel you should get 20 A, but there are some details that make it less safe than one can think.

    First - it is not guaranteed each one gives exactly 10 A, as a lot depends on their internal resistance, which is never exactly the same. So, while getting 20 A you can in fact be getting 12/8 - with one batter being overloaded.

    Second: they have to be connected when they are both charged to exactly the same voltage. Otherwise one will try to charge the other till their voltages are identical, and that's asking for troubles.

    That being said, sometimes I fly my Sky Surfer with two 2.2 Ah batteries connected in parallel. While single one is enough in terms of the current, having 4.4 Ah gives twice the flight time.
     
  4. May 17, 2018 at 4:38 PM #3
    Ok, thanks.

    Another question, Will a wire (for example: 14 AWG) resist the same AMPs with 12v vs the same AMPs with 3.7v?

    Thanks in advance.
     
  5. May 18, 2018 at 1:50 AM #4

    Borek

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    Staff: Mentor

    No idea what you mean.
     
  6. May 18, 2018 at 9:17 AM #5
    If I interpret this correctly the resistance of the wire is independent of the voltage across it. However using 12V for a given wire length will produce more current according to Ohm's law.

    A particular type and gauge wire as a copper 14 AWG is rated to carry a maximum current depending on a particular maximum wire temperature , the insulation and ambient temperature, called its ampacity.. For #14 it is 20 amps for 90 deg C in an ambient temp of 30 deg C. See http://www.usawire-cable.com/pdfs/nec ampacities.pdf for a chart for various gauges.

    A consideration of what gauge wire you choose and its length to and from the the source will depend on how much voltage drop on the wire you can tolerate. Another way to think about it is how much less voltage than the 3.7 V your device is spec'd for will be acceptable. Will it run properly at 3.4V for example? Take #14 wire at 18 A. The wire has a resistance of .00252 ohms/ft. which seems small. If you use 10 ft of wire (5 to the device and 5 back to the battery) the voltage available to your device will be 3.7V (battery voltage) - 10ft×.00252ohms/ft×18A (voltage drop across the wire) = 3.25V(voltage available for your device) Will you device run properly at this voltage? If not then you need to shorten the wire or use a lower gauge.
     
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