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Thread: Series vs Parallel Dual Batteries

  1. #1
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    Series vs Parallel Dual Batteries

    WARNING: This post may contain rambling. If you want the TL;DNR version, scroll down to the bottom.


    For those brave souls who didn’t scroll, I choose to start this story…

    Now. So after a few failed purchases based on assumptions that more batteries equals longer battery life, I have done some research and thought I would share my findings.

    I had an iStick 100W that gave me great battery life from dual 18650s… I am a lower wattage vaper, so getting nearly two days from a mod with my heavier weekend use was a new way of life.
    I could leave the house without frantically making sure that I had a fully charged backup in my bag, or cursing myself for putting the batteries in the charger but leaving the switch off on the wall.

    Then the RX200 came out and I thought “a whole extra battery, that will last me even longer!”
    Put the 100W up for sale and then (not so) patiently waited for the postman to deliver my new purchase along with 3 shiny new Samsung batteries.
    I loaded the batteries in, and after I had cranked that 200W beast all the way down to 15w, I thought I was all set for half a week charger free! And if I got mugged, I had the vaping equivalent of brass knuckles to defend myself with.
    It was not to be. That night I loaded all 3x 18650s into the charger… and the next night, and the next.

    It must be the mod, I thought (couldn’t be the user!), and something weird with the triple battery setup, so onto the classies it went and I picked up a Cuboid dual battery. From using the iStick, I knew that dual batteries would last longer…

    Rinse and repeat.

    After the Cuboid had been passed onto a user more worthy, I ended up with a single battery VTC as my day to day beater mod, downhearted and jaded with the multiple battery journey I had taken.

    Faced with a camping trip for two weeks without mains power (or the internet, or running water. Who would do that to themselves!?!?), I started thinking that maybe a multiple battery setup would be worth looking into again.

    And here we are, late on a Friday night with the Breakfast Club on in the background, trying to work out WTF is with dual battery setups and why my experience varied so much. After some failed search attempts through previous AVF posts, I turned to the fountain of knowledge (google) for some instant knowledge gratification. That turned out not to be so instant.

    The answer, dear reader, lies in how the batteries are configured. Either in series, or in parallel.

    After getting confused on various electronics and battery info sites and reading conflicting information on these setups, the haze started to clear for me so I have plagiarised the following information so you may learn from the error of my ways, and hopefully save yourself an evening of wading through circuit diagrams.



    (TL;DNRers - Stop scrolling here.)


    Assuming each single battery has the following:
    Nominal voltage: 3.6 volts
    Fresh charge: 4.2 volts
    Nominal capacity: 2500 mAh
    Continuous discharge rating: 20amps


    Dual batteries connected in series:
    Wiring batteries together in series will increase the voltage while keeping the amp hour capacity the same.
    Imagine the batteries lined up end to end like a torch, positive to negative.

    — Voltage doubles
    — Capacity remains the same
    — Amperage remains the same

    Nominal voltage: 7.2 volts
    Fresh charge: 8.4 volts
    Nominal capacity: 2500 mAh
    Continuous discharge rating: 20 amps


    Dual batteries connected in parallel:
    Wiring batteries together in parallel has the effect of doubling capacity while keeping the voltage the same.
    Batteries lined up side by side, with all positive ends at the top.

    — Voltage remains the same
    — Capacity doubles
    — Amperage = lowest discharge rate (3.6V) X number of batteries (2 batteries)

    Nominal voltage: 3.6 volts
    Fresh charge: 4.2 volts
    Nominal capacity: 5000 mAh
    Continuous discharge rating: 40 amps


    Please be aware that I am (clearly) no battery expert and I cannot verify the accuracy of the source material (except the bit about camping and the Breakfast Club).
    Feel free to correct/debate as you see fit.

    So now I am off to find the perfect duodecuple parallel battery mod that I only have to charge 3 times a year!

  2. #2
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    not many regulated parallel mods around, that i know of. They are all series nowadays

    mechs there are more

  3. #3
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    If I had any faith in my fine motor skills DM, I would build one and enter your comp!
    DogMan likes this.

  4. #4
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    Istick 100w TC. Nice mod.
    Was wondering why it shits all over my cuboid for battery life!

  5. #5
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    I'm no expert on these matters but have done a fair bit of research in the past in trying to understand this for myself.

    In theory with VW mods it should even out whether it is series or parallel. Mechs are of course a different story.

    Lets presume the mod is 90% efficient and has fully charged batteries:
    At 15w on a dual parallel setup you will be drawing 3.97A from the cells, this is then halved to 1.98A per cell.
    At 15w on a dual series setup your voltage doubles and as such you are drawing 1.98A from the cells, this is not halved.

    There are other factors that can influence longevity though such as mod efficiency and PWM. PWM would be my guess as to why the istick 100w lasts longer.
    Fatman and BatDan like this.

  6. #6
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    DC-DC vs DC-AC chip (RMS power in terms of actual wattage output) may also be a factor;

    Just my own observation (not clocking or measuring/timing), comparing two daily VW devices, both single batt (running identical Pana's 18650B) same tank&coil the AC unit lasted way longer.

    In saying that the AC unit has a programmable "fall back" feature where at a set voltage it drops the wattage to increase the use time.
    Fatman and BatDan like this.




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  7. #7
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    Manufacturers often head to series set-ups because that way they only need one circuit to control the output ... ie you start with 8.4Vor 11.1V and can just use a step down converter... so in effect your 2/3 batteries only count as ONE in the capacity stakes as they are combined to make the primary voltage input... when you have parallel then your mod will incorporate a stepup converter as well as a stepdown one to provide the firing range... this is a little less efficient , but its not an issue in the bigger picture because you have double the capacity behind it rather than double the voltage and single capacity...

    A dual series mod will actually waste a lot more power throttling back the output and hence you will get less run time out of them at very low wattages because the circuit is ALWAYS inefficient and working hardest at all times reigning the voltage in ... they give you awesome output potential (w) at the sacrifice of capacity utilisation

    If you don't need the Wattage then parallel is your friend in the longevity battery life stakes
    J_B_86 and BatDan like this.
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  8. #8
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    Quote Originally Posted by Fatman View Post

    If you don't need the Wattage then parallel is your friend in the longevity battery life stakes
    FM, absolutely.

    The VF DNA40 take two batts in parallel; Running at about 16W it lasts a LONG time but this could also be influenced by the batt's health and the chip itself I guess?

    I've been using cells pulled out of my old laptop battery (about four years old, Jap made cells) exclusively in the VF as these are skinny enough for the bores. My guess these are not more than 2200mAh but going very strong;

    Prob the best way to ensure you have authentic OEM cells, not the re-wraps we've seen lately?




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    Eleaf Lemo II - VLS Vulcan & VLS Vector - Kangertech Sub Mini RTA

 

 

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