Variable Voltage Battery Safety

[Violetti Usva]

Perhaps I'm wrong but I believe these two stipulations are correct:

1) With a variable wattage device you use watts law - max wattage = nx3xCDR, where n is the number of batteriesin the device
2) With a single-battery mechanical mod you use ohms law - where V/r <= CDR

With a variable voltage device which rule do I use?

Lets say I have a single 20 amp battery and a 0.21ohm coil - this would be 84watts on a fully charged mechanical mod (if we ignore voltage drop) but a single 20amp battery has a maximum of 60 watts on a regulated device.

If I plug in 0.21ohms and 60 watts into an ohms law calculator it gives me approx 17amps, 3.5volts
If I plug in 60 watts and 20amps it gives me 3volts and 0.15ohms.

Which of these would be the correct maximum voltage for this theoretical device? Or is there a third stipulation for V/V devices that I haven't heard about?

Sorry if this is a bit waffley. For the record my V/V device has two pairs of 20a batteries married to it in a strange polygamous relationship, and I use 0.14-0.21ohm coils at 2-3V so I'm certain it's safe by any of these calculations - I'm mostly asking out of curiosity as VV devices are rarely talked about.

 

[bwh79]

Well, really the only rule you need is "amps drawn <= CDR," but you need to know how to find the amp draw. Power (wattage) is the conserved quantity, in all cases. Watts in == watts out. If you drive a .21 coil at 3.5 output volts, that's equivalent to (3.5^2 / .21) = 58.3 watts (let's call it 60, for simplicity's sake.) Now, if you're drawing 60 (ish) watts from the battery, the amp draw will depend on the battery's charge state. At full charge it will draw (60 / 4.2) = about 14.2 amps, while on the other end it will draw closer to (60 / 3.3) = 18.18 amps.

If you want a "rule" for VV devices, I guess it would be this:

(output voltage)^2 / ohms / charge state <= CDR

 

[Violetti Usva]

That makes sense, thank you. I presume this increases by the number of batteries in the device by doubling the charge state?
Quick rearrangement gives

CDR >= ((OV^2)/r)/BV = (OV^2)/BV*r
MOV = (CDR*mBV*r)^0.5

(M)OV = (Maximum) Output Voltage
(m)BV = (Minimum) Battery Voltage (total of all batteries)
r = resistance in ohms

So with my paired 20a batteries and a 0.14ohm coil 4.1V is the maximum if the minimum charge is 3V/cell (which I think is lower than the actual minimum the device will fire at) and 4.4V if the minimum charge is 3.5V/cell, which is more power than I would personally want anyway.

Thank you again for explaining that to me, always much better for me to understand the reasoning behind the formulae, even if I find the formulae easier to remember

 

[DaveP]

Variable mods with internal protection circuits make vaping safer compared to mech mods. All you have to remember for variable mods is that your battery needs to be rated well over the amperage your coil and the electronics need to safely produce the vape level you want.

MAH indicates the amount of time you can vape before recharge. CDR (Continuous Discharge Rate) indicates the maximum current level (amps) you can safely reach. Look for the cell with the highest MAH rating and a current level that's safe for that level.

Generally, the higher the MAH rating the lower the CDR and vice versa. If you vape 30 watts or under you can pick from a range of cells that offer 3000ma and 15 to 20 amps CDR. If you are a 100W or higher vaper you need to pick a cell with the highest CDR rating that provides a MAH rating that gives you enough vape time before recharge. High watt vapers need 30A CDR cells and higher. For extremely high wattage multiple cells mods are better. Some 20700 and 26650 cells will give you 40A vaping levels.

It's all about matching CDR and MAH for the right application. Mooch is a good reference for which cells are best for different applications.

List of Battery Tests | E-Cigarette Forum

 

[Violetti Usva]

@DaveP I use Samsung...20S for my mechanical mods based solely on Mooch's tests. Previously I used the LG 30a batteries, 6 in a cycle each day because they didn't hold voltage well nor have a high capacity but were the only choice I deemed safe enough for my 0.14 ohm coils. In a regulated device I figure that I have more leeway with ohmage vs CDR as I'm not a gigawatt cloudchaser etc but there are still important safety precautions. I picked batteries that I know are safe for the wattages I use on VW devices and put those in my VV devices figuring that whatever voltage I pick would probably be within that safe range. This was more out of curiosity simply because VV devices aren't spoken about on this forum quite as often and I think this has been answered adequately for me