Extremely low ohms and safety

[Clairenix]

Although the DNA200/250 is a crazy safe board, I accidentally made a 0.06ohm build. 6 wraps parallel SS316L 24g x2 It took quite a bit of time and was hoping to keep it.. But with that said, my mod only has 2x Li ion batteries, am I pushing it too far? Is there some place I can educate myself about this, spefically for the dna200?

[VapingBad]

No, I have ran 0.03 ohm on 75, 200 & 250,  though I prefer to stay a little higher at 0.05 ohm.  The board controls the current and it is the current limit that is important at low ohms, but the current from the batteries will be lower than the current to the coils as the voltage has been stepped down.

Worst case (highest battery current) is when the battery is low it needs to supply more current to make up the required power:

At the coils 133 W in to 0.06 ohm gives
current = sqrt(P/R) = 47.6 A, voltage = sqrt(P/R) = 2.82 V

At the batteries 133/0.98 (98% efficiency) = 135.7 W or 67.85 W per cell
current = P/V = 67.85/2.75 = 24.7 A

[Viruk]

Aren't the cells in series?

You're not splitting the amp load between the cells unless they're in parallel - right?

[VapingBad]

Viruk said:

Aren't the cells in series?

You're not splitting the amp load between the cells unless they're in parallel - right?

But you are splitting the watts between them .

At the batteries 133/0.98 (98% efficiency) = 135.7 W or 67.85 W per cell
current = P/V = 67.85/2.75 = 24.7 A

if you take both together the low cut-off would double

current = P/V = 135.7/5.5 = 24.7 A

[Viruk]

My understanding is that you're not splitting the watts or current unless you're running cells in parallel. Please feel free to correct me, but my understanding is that each cell in the battery takes the full load when in series. This is an excerpt from a Wikipedia article that sums it up quite nicely: Series circuits are sometimes called current-coupled or daisy chain-coupled. The current in a series circuit goes through every component in the circuit. Therefore, all of the components in a series connection carry the same current. There is only one path in a series circuit in which the current can flow. I'll post the link to that in a moment - but I'm stuck on a phone while travelling...

[Viruk]

https://en.m.wikipedia.org/wiki/Series_and_parallel_circuits

[VapingBad]

You are splitting the watts with series batteries, just the same as with series resistors.  Consider that half the voltage is coming from each P = IV/2.

[awsum140]

My comment is since the total voltage doubles, the current is cut in half for each battery.  Yes, they still conduct the same amount of current but it is reduced by 50% because the voltage is increased by 100%.

[VapingBad]

awsum140 said:

My comment is since the total voltage doubles, the current is cut in half for each battery.  Yes, they still conduct the same amount of current but it is reduced by 50% because the voltage is increased by 100%.

No, current is always the same through every component in any series branch of the circuit, the current required would be halved if the voltage is doubled, but both batteries in series will have equal current (their voltages could vary).

[awsum140]

The current, total circuit current, is effectively cut in half since the voltage is doubled.  I'm not saying that each battery only supplies half the total current, they both supply the same amount of current but it will be half that of a single or parallel battery arrangement when the output power is the same.  That's why high voltage is used for power transmission, to lower the current load and allow smaller conductors to supply higher power levels.  As voltage increases, current decreases assuming the same load, output power, is maintained.

[VapingBad]

Yeah we were saying the same thing, sorry I misread your post awsum

[awsum140]

It gets a little "nebulous" talking about current/voltage in series and parallel circuits because the power level is maintained by the chip in the case of a DNA or other regulated mod.  In the normal world, the current would, indeed, double with a doubling of supply voltage, but the chip prevents that to maintain the power at the desired level hence the current drops by half instead.