About volts, ohms, amps, watts, and battery life

[grruss]

This will take some 'splainin', so I apologize in advance for the long post...

Assumptions:

• volts=ohms*amps (Ohm's Law)
• watts=(volts*volts)/ohms (Watts Law)
• battery life is dependent upon amps (batteries are measured in mAh)

Scenario 1:

I find that I enjoy vaping at (or slightly above) 10 watts. So when I get a 1.5 ohm carto, I set my vv device (Provari) at 3.9-4.0 volts...

3.9 volts*3.9 volts/1.5 ohms=10.14 watts (Watt's Law)

According to ohm's law, that results in a current draw of 2.6 amps

3.9 volts/1.5 ohms=2.60 amps (Ohm's Law)

Scenario 2:

If I go with a 3.0 ohm carto and want the same power, I will set the voltage at 5.5.

5.5 volts*5.5 volts/3.0 ohms=10.09 watts (Watt's Law)

Same logic- battery draw would be 1.83 amps

5.5 volts/3.0 ohms=1.83 amps (Ohm's Law)

Conclusion:

If the current is lower, won't my battery last longer?

Put another way, am I extending vaping time by vaping a HIGHER resistance carto at a HIGHER voltage?

Counter-intuitive, but right?

 

[nanovapr]

Yes, I believe that is correct. LR exists only to warm up the temps on fixed voltage devices (at the expense of battery life). With a good VV, LR is pointless. Use a higher resistance atty/carto, and turn the voltage up, you'll use less amperage and your batteries will last longer. Right now I'm vaping a 2.9 ohm carto @ about 5.1 volts, display shows it's using 1.7 amps, to make about 8.8 watts.

It can be argued that LR heats up and gets to operating temps faster, this could change the vape experience as well. If you're running in 10-watt-land, it won't take long at all either way...

 

[Iusedtoanalog]

Yes Grruss, You have hit the head on the nail as far as I can see it. In my own VV unit I run 3ohm cartos at anywhere from 4.6v to sometimes 6.3(very rarely)because of that exact issue. My unit is custom made by me for me. It uses two series 14500 sized batteries. I can adjust from 3.5 to 6.5 BUT I only have 900mAh of storage. If I use a 2ohm carto I can get about eight and a half hours of use time at any set voltage. I normally stay around 5.2v@3ohms unless I'm either stressed or really chill... but I digress. With the exact same batteries and a 3ohm carto I can get almost thirteen hours of vape time. The need for low ohm cartos for me has been removed by the addition of available voltage adjustment. Good Luck. Happy Vaping.

 

[tobarger]

Its a little more complicated than that.
The voltage boosting circuitry of the Provari is going to draw additional current in order to convert ~3.7 to 5.5 volts. Which is a healthy boost of about a 48%.
I think theres a Physics principle that says: "You can't get something for nothing".
You may still get some additional use time, just not as much as you can calculate using Ohms Law.

 

[nanovapr]

Good point, tobarger. We would have to figure out the efficiency of the boost circuit, to mix that in. THEN compare the two. Still, LR was intended for fixed-voltage devices initially. I get better battery life on a Darwin with 3 ohm, so that's what I use.

Really, though, the difference of a day and X hours compared to a day and X+3 hours (or whatever) is a non-issue. I plug it in every night, anyway. I don't care if it would run out the next day at noon or at 3 pm, I don't want it to run down!

 

[Slurp812]

Battery watts + losses = carto watts. In a VV Carto current does not equal battery current. Assuming losses are equal, running a 1.5 ohm carto @ 8 watts should equal a 3.0 ohm carto @ 8 watts in battery life. Losses are not equal, and vary depending on the device, and carto ohms, and carto current. I get about the same battery life because I usually run right around 8 watts regardless of carto ohms. Others also report longer life @ higher carto ohms, so I can only assume efficiency is better under those conditions.

 

[Rader2146]

Assuming losses are equal, running a 1.5 ohm carto @ 8 watts should equal a 3.0 ohm carto @ 8 watts in battery life.

1.5ohm @ 8w = 2.3a

3ohm @ 8w = 1.6a

Battery charge life is measured in mAh, or milliampere hours, or mA x h. So, mAh/mA = h. A 2000mAh battery for example:

2000mAh/2300mA=.87 = 52 minutes "burn" time

2000mAh/1600mA=1.25 = 75 minutes

A 44% improvement in run time. Granted this is for unregulated voltage, and assumes that losses are equal, but it is the law.

 

[John D in CT]

It seems to me that regardless of buck, boost, or what have you, you're going to get better battery life at higher resistance and higher voltage. You are absolutely correct that it is counter-intuitive, but I believe it to be correct.

***

I've always had a warm spot in my heart for this thread:

http://www.e-cigarette-forum.com/fo...ined-detail-single-dual-coil-atty-cartos.html

***

"Assuming losses are equal, running a 1.5 ohm carto @ 8 watts should equal a 3.0 ohm carto @ 8 watts in battery life".

I do not believe that is correct.

Leaving buck, boost, losses, etc. out of it - which I think is reasonable for the sake of this discussion -

1.5 ohms @ 8 watts would require about 3.46 volts, yielding an amp draw of about 2.3

3.0 ohms @ 8 watts would require about 4.90 volts, yielding an amp draw of about 1.63

Unless I'm missing something, I think you would have to get better battery life at 3 ohms.

 

[Rader2146]

The key is that, proportionally, higher voltage AND higher resistance equals lower amps, and amps are the key to battery life just as the accelerator pedal is the key to fuel mileage.