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Explaining battery voltage “sag”

Discussion in 'Batteries and Chargers' started by Mooch, Nov 30, 2019.

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  1. Mooch

    Mooch Electron Wrangler Verified Member ECF Veteran

    Supporting member
    May 13, 2015
    Battery sag is the voltage drop every battery experiences when we start to draw current from it. Our batteries are near 4.20V when fully charged but almost instantaneously drop to anywhere from 3.9V to 3.3V, or even lower, when we fire our mods. When we stop drawing current the battery voltage rises back up.

    While this effect can correctly be called a voltage drop I call it “battery sag” to distinguish it from the other voltage drops that occur outside of the battery.

    Things like the battery contacts, wiring, protections circuits that the current passes through, the body of a mech mod, all cause varying amounts of voltage drop and this also causes a loss of power that we would rather have go to the coils.

    So what causes this battery voltage sag?

    There are two causes. Together they add up to the total drop we see in the battery’s voltage when we fire our mods. Of course, as we continue to use a battery the voltage drops as it’s being discharged. This is different though from the voltage sag that happens almost instantly when we press the button.

    The first cause is the resistance of the metal contacts of the battery, the strips of metal moving current around inside the battery, and the compounds and liquids inside the battery. All of them are not perfect conductors, they have a little bit of electrical resistance, so each contributes a little bit of voltage drop by the classic Ohm’s Law equation of Voltage Drop = Current x Material Resistance.

    The second cause for battery sag is how the lithium ions move from one part of the battery to another. As the current increases the ions get bunched up in some areas of the battery and spread out in other areas. This difference in the distribution of the ions across the battery causes a difference in voltage to occur. This difference in voltage from one spot to another is seen by us as a voltage drop. It’s due to the electrochemical efficiency of the battery and not the resistance of the materials but we can still express it as a “resistance” by using Ohm’s Law again. Resistance = Voltage Drop From Ion Distribution Differences / Current.

    Both of these causes for battery sag are happening every time we use a battery. There are meters that can measure the total battery sag and even meters than can isolate the sag caused just by the electrochemical inefficiency of the battery.

    As vapers we are only concerned with the total voltage sag though as that is what determines how hard a battery hits in a mech and when we might get a low/weak battery alert in a regulated mod. The less sag the better.

    The “DC Internal Resistance” (DC IR) number I provide with every battery I test can be used to help determine which battery might run more efficiently and hit harder. It’s not a perfect way to pick the best battery as batteries heat up differently and battery chemistries respond differently as the battery is used. But the DC IR number is a great way to start to find the better performers, especially for mechs. The lower the DC IR is the better.

    You can get an idea of the instantaneous voltage sag for a battery by using the DC IR number and Ohm’s Law: Voltage Sag = DC IR x Current. The lower the DC IR, the less battery sag you’ll have. You can lower the current level too but that’s usually not an acceptable option for vapers. :)

    You can see the effect of this battery sag in the discharge graph. As the 10A discharge starts the fully charged Murata VTC6A’s voltage almost instantly drops from 4.20V to just over 4.0V. If you discharge at 20A the voltage sags immediately to about 3.9V. This means the battery doesn’t hit as hard and will reach a regulated mod’s low voltage cutoff point sooner. At 30A and 40A you can see how the starts at even lower voltages due to the bigger voltage sag.


    The DC IR can range from about 12mOhms (12 milliohms, 0.012 ohms) in our best performing batteries up to around 22mOhms for an average 20A 18650 up to over 100mOhms for very low amp-rated batteries. This is why batteries with low current ratings often perform so poorly, their high internal resistance causes huge amounts of battery sag.

    The DC IR is different than the “AC internal resistance” (AC IR) specification that the big battery manufacturers show in their datasheets. That spec tells you the resistance of the liquid in the battery, the electrolyte, and is not very useful for determining voltage sag or for tracking aging. It is typically lower than the DC IR and does not change much over the life of the battery.

    Some of the chargers we use can measure DC IR but they are typically very inaccurate and inconsistent. This makes the numbers we get from them useless.

    So while the DC IR numbers can be very useful when calculating battery sag and comparing battery performance it can be hard to get accurate enough numbers to be useful. Don’t buy a charger to use for DC IR testing until you check online for reviews/tests that can tell you how accurate and consistent the charger is when testing DC IR. Links to the ones I have tested are here: Charger Test Reports | E-Cigarette Forum

    For any battery I have tested in the past 1-1/2 years or so the DC IR number I measured is in the test report. You can find all of my test reports here: List of Battery Tests | E-Cigarette Forum
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  2. greek mule

    greek mule Ultra Member ECF Veteran

    Feb 2, 2018
    Thanks Mooch.
    • Like Like x 3
    • Agree Agree x 2
  3. DaveP

    DaveP PV Master & Musician ECF Veteran

    Supporting member
    May 22, 2010
    Central GA
    Good info, Mooch. Thanks for keeping us updated and informed.
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  4. Dieseler

    Dieseler ECF Guru Verified Member ECF Veteran

    Sep 24, 2011
    Thanks Mooch for taking the time in what you do for forum members.
    Always have referred to your list of batteries when making a purchase.
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  5. stols001

    stols001 Mistress of the Dark Nicotinic Arts Verified Member ECF Veteran

    Supporting member
    May 30, 2017
    Tucson, AZ
    Many tanks (were saved from exploding) by Mooch. Thank you and happy holidays. :)
    • Like Like x 4
  6. sonicbomb

    sonicbomb Vaping Master Verified Member ECF Veteran

    Feb 17, 2015
    1187 Hundertwasser
    Thanks Mooch, I was thinking about this just the other day.
    • Like Like x 4
  7. RuiG

    RuiG Senior Member

    May 4, 2019
    Thank you.

    Here's a subject I always had doubts about.
    • Like Like x 3
  8. DaveP

    DaveP PV Master & Musician ECF Veteran

    Supporting member
    May 22, 2010
    Central GA
    @Mooch, I generally get repeatable IR numbers from my Opus C3100 charger, but only after reinserting the cell several times before the internal resistance reading becomes repeatable. I know from your posts that the value I get isn't a true reading, but I consider it to be a valid reference number for judging cell aging and for determining when to toss the cell for recycle.

    I guess my question is "What is happening when it takes several insert and remove actions to get the IR reading to settle down to repeatability?". Is the electrolyte warming or aligning in the first couple of inserts before it reaches stability or is the charger just giving me random values that aren't significant even though they are repeatable? My oldest cells get the highest readings, so there's some correlation to age and usage vs newer cells.
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  9. Mooch

    Mooch Electron Wrangler Verified Member ECF Veteran

    Supporting member
    May 13, 2015
    I think it’s physical...oxide layers or other coatings on the contacts get rubbed off eventually. I get very stable and accurate readings with better meters though. I don’t know what the charger’s software is doing so anything could actually be happening. :)
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  10. Mooch
    This message by Mooch has been removed from public view. Deleted by a moderator, Dec 4, 2019, Reason: Duplicate post.
    Dec 4, 2019

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