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Thread: Voltage drop under load explained well.

  1. #1
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    Voltage drop under load explained well.

    I came across this, written by Manu, it really helps clarify things for anyone trying to gauge a mech mod and how it performs and if after modding it you have a better device.


    MANU, 06/02/2013, 1:27 PM:
    A lower atomizer resistance translates into higher current = more power = more vapour. However, the higher the current, the higher the voltage drop on the internal battery resistance and mod resistance.

    Remember that voltage drops at high currents cannot be completely eliminated. The presence of voltage drop only means that the efficiency is decreased. With lower resistance coils, the power on the atomizer increases, but a higher percentage of it is lost on the way.

    Let’s get to an example:

    [Joe]

    Joe has an 18500 non-IMR battery. He vapes with an RBA that has a 1.2 Ohm coil. His battery is fully charged, so he decides to measure the voltage drop of his Roller.

    Joe measures the battery voltage at open-circuit, and finds it to be 4.2 Volts.

    Then, Joe measures the voltage across the atomizer terminals under load, and finds it to be 3.79 Volts. He assumes that his mod is giving him a voltage drop of 0.41 Volts and frowns unhappily. He is convinced that his mod is not giving him a good hit, although it vapes like a train.

    [Jay]

    Jay has a fresh 18500 IMR battery. He vapes with an RBA that has a 2.2 Ohm coil. His battery is fully charged, so he decides to measure the voltage drop of his Roller.

    Jay measures the battery voltage at open-circuit, and finds it to be 4.2 Volts.

    Then, Jay measures the voltage across the atomizer terminals under load, and finds it to be 4.02 Volts. He assumes that his Roller is giving him a voltage drop of 0.18 Volts and goes on to say how amazing it is and how great it vapes.

    One minor detail I forgot to mention is that Joe sold his Roller to Jay because of the “voltage drop issue”, so it’s the *same* device.

    On to the explanation:

    The voltage across a resistance is given by the rather simplified formula V = I * R, where I is the current supplied by the battery and R the resistance we are examining.

    The current I flowing through a mod is roughly equal to: I = Voc / ( Ra + Rm + Ri ), where Voc is the open circuit voltage of the battery, Rm is the equivalent mod resistance, Ri is the internal battery resistance and Ra is the atomizer resistance.

    A typical value for Ri would be around 0.08 Ohms for a non-IMR battery that is in *OK* condition, while a newer, larger capacity, non stressed, high-drain battery might be better. Take this value with a grain of salt, since each battery is different.

    Now, the mod’s equivalent resistance is again not a static quantity, since it depends on how tight the different components are screwed, how clean they are, and many other variable factors. A typical equivalent value would be around 0.05 Ohms, perhaps even less.

    So, with the same mod (Rm = 0.05) in the same condition and configuration, Joe was vaping at a current of:

    I_joe = 4.2 / (0.08 + 0.05 + 1.2) = 3.16 Amps,

    which gives an atomizer voltage of

    Va_joe = 3.16 * 1.2 = 3.79 Volts.

    In the case of Jay, these values were:

    I_jay = 4.2 / (0.05 + 0.05 + 2.2) = 1.83 Amps
    Va_jay = 1.83 * 2.2 = 4.02 Volts

    So – the voltage drop says nothing. In fact, Joe’s kit was putting out many more watts than Jay’s, because of the low atomizer resistance. Joe was vaping at:

    P_joe = Va_joe * I_joe = 3.16 * 3.79 = 12 Watts

    while Jay is vaping at:

    P_jay = Va_jay * I_jay = 1.83 * 4.02 = 7.36 Watts

    Bottom line:

    If you want to vape with a low resistance coil, the best thing to do in order to maximize efficiency is to use a high-drain, high-energy battery (18500/18650 IMR).

    If Joe had a good IMR battery, his current and voltage would be:

    I_joe’ = 4.2 / (0.04 + 0.05 + 1.2) = 3.26 Amps,

    which would give an atomizer voltage of

    Va_joe’ = 3.16 * 1.2 = 3.91 Volts

    and a power of

    P_joe’ = 12.73 Watts,

    which is much better than the 3.79 Volts and 12 Watts he got with the non-IMR battery.
    Vaping since July 2012.

  2. #2
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    ok this is a bit over my head but what i am getting, low resistance coil, imr battery = good
    GirlyPantz likes this.

  3. #3
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    Quote Originally Posted by silverx View Post
    ok this is a bit over my head but what i am getting, low resistance coil, imr battery = good
    It takes a few reads to really grasp the guts of what's being said and reduce it down to a simple conclusion, I see it like this.
    Chime in stoney or any other eclitric trickery gurus if I'm on the wrong track.

    1. Mech mods are hard to evaluate and compaire to each other with regard to concutivity and efficiency in use unless the test parameters are identical ( which is nearly imposible owing to each battery having varying internal resistance depending on chemistry, how hard a life it's had and it's unavoidable discharge, however slight during testing, plus other variables discussed below)

    2. Mech mods have a resistance that needs to be included in the formular to calculate amp draw leading to the final figure of deliverd voltage (voltage under load) and in turn Wattage. Dirty/worn contacts, threads and switches will all lead to a higher than optimum ( optimum as in every contact, thread and switch brand new and ultra clean) mod resistance leading to lost power.


    3. All lithium batteries have different internal resistance depending on design,chemistry, age and condition, and this internal resistance needs to be included in the formular to calculate amp draw leading to the final figure of deliverd voltage (voltage under load) and in turn Wattage.

    4. The lower the resistance or the coil the more current it will draw from the battery, which in turn leads to a higher voltage drop under this load. This is however offset by the resulting higher wattage applied to the wick inspite of the lower voltage delivered.

    5. IMR batteries rule, owing to , safer chemistry, generallly lower internal resistance and higher max amp draw capabilty than other kinds of lithium ecig batteries.
    Stoney likes this.
    Vaping since July 2012.

  4. #4
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    yeh, tis all pretty basic stuff.

    I did not know mod resistance was being touted as THE important factor when the cell _usually_ has a lot more to do with it.
    I do love me hex though, main reason its good is lack of a spring on the negative.

    easiest way to tell if you have a problem is it will heat up.
    The body is a solid chunk, it wont get hot, what does is springs, caps where they screw on, switches, connectors and wires.
    If your mod is not getting warm somewhere, you can be pretty confident its working ok.

    Having said that .. an example of a crap mod is the indulgence .. I have current tested (0.1Ω load) an imr in my hex to about 24A, the same cell in the indulgence .. 2.2A, a combination of crap spring,crap bottom threads, crap plating,crap switch,mediocre interface to an atty (it has a converter).

    Putting the best battery in the world into that mod will not help..

    my guess is manu is getting all these voltage drop reports and is trying to indicate its not always the mod.
    In order of useful: Provari V2 and V1, vamo, eGo twist, Katana v3, Hex, Orion v2, LavaTube, eGo x 6, VMAX, 510N, Tick V2, Tick V1, noEgo 18650, noEgo 14650 , Indulgence x 2, 905 6v mod, KR808D



  5. #5
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    Quote Originally Posted by Stoney View Post
    easiest way to tell if you have a problem is it will heat up.
    The body is a solid chunk, it wont get hot, what does is springs, caps where they screw on, switches, connectors and wires.
    If your mod is not getting warm somewhere, you can be pretty confident its working ok.
    can you expand on this stoney? i am using evic atm and with or without RBA the evic body will feel abit warm, and the atty normally gets hot after chain vape it for half to an hour. does that mean my evic has got an issue?

  6. #6
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    I hope you are joking about the atty getting warm .. might have something to do with the heater inside it,

    as for the evic body .. where, is it body heat ?
    all electronic mods are going to generate some heat in the electronics when used, at best they are 90% efficient, so around a watt or more wasted, I think that is just enough to warm it but it depends on where that heat is transferred to the case or the air inside.
    I found the vamo gets warmer around the top than could just be explained by conduction from the atty, so does the provari to a lesser extent.
    My provari is getting warm on the bottom cap lately and battery life has dropped to just over a day from a day and a half. Also .. cells are often 3.5v now when I start charging which is a good indication my provari my needs a clean and service

    Oh, I notice the bottom gets warmer as the battery flattens, because the output is a constant wattage regardless of battery charged or discharged, then that wattage comes from increased current as the battery drops, so it seems to run quite cool on a fully charged battery but warms up when cell is nearly flat.
    In order of useful: Provari V2 and V1, vamo, eGo twist, Katana v3, Hex, Orion v2, LavaTube, eGo x 6, VMAX, 510N, Tick V2, Tick V1, noEgo 18650, noEgo 14650 , Indulgence x 2, 905 6v mod, KR808D



  7. #7
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    Quote Originally Posted by Stoney View Post
    I hope you are joking about the atty getting warm .. might have something to do with the heater inside it,

    as for the evic body .. where, is it body heat ?
    all electronic mods are going to generate some heat in the electronics when used, at best they are 90% efficient, so around a watt or more wasted, I think that is just enough to warm it but it depends on where that heat is transferred to the case or the air inside.
    I found the vamo gets warmer around the top than could just be explained by conduction from the atty, so does the provari to a lesser extent.
    My provari is getting warm on the bottom cap lately and battery life has dropped to just over a day from a day and a half. Also .. cells are often 3.5v now when I start charging which is a good indication my provari my needs a clean and service

    Oh, I notice the bottom gets warmer as the battery flattens, because the output is a constant wattage regardless of battery charged or discharged, then that wattage comes from increased current as the battery drops, so it seems to run quite cool on a fully charged battery but warms up when cell is nearly flat.
    nah im not joking actually, on my rba the top cap can get pretty warm especially close to the airhole part.

    the evic gets a bit warm close to the connector but the body will also get warm when im using low resistance (1.5 for example) but using my usual 1.8-2.0 setup it's fine

 

 

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