Why voltage drop instead of resistance?

[ancient puffer]

Apologies in advance if this has been asked and answered before, but if it has, I don't remember seeing it, and can't find it with a search. Please forgive me the simplistic questions, but I don't have any formal training in things electrical, and I suppose I just don't understand this whole voltage drop thing, so I'm just trying to work it out logically. Can someone help me understand these things?

1. If I measure voltage drop in my grand, I get zero, measured across the 510 connection.

2. If I measure across the atty posts, I get %13.

So, in #2 above, I'm losing %13 of my (for example, 4.08 volts). But I'm not measuring the voltage drop of just the REO, I'm measuring the cumulative drop of the REO, the atomizer itself, and the coil, what I understand as "load".

The trouble I'm having is that I'm basing the "drop" by measuring directly off the battery, thus no "load".

It seems to me that measuring under load of any kind, introduces unaccounted for variables. I understand that the "real world" voltage a battery produces, is affected by the load/resistance encountered along the path. And I assume that the "ideal" mod would have a resistance of zero.

So, wouldn't measuring the resistance of the mod give a clearer picture of it's performance? That would tell me how efficiently it's delivering the current being seen at the atomizer, while eliminating the atomizer/coil itself as a factor.

Can someone explain to me why we talk about voltage drop as opposed to resistance, in terms of a mod's efficiency?

 

[patrao_n]

You know as for voltage drop, I never really understood it as well. The battery will sag when a load is applied. So not only do you have the overall resistance of the mod accounting for voltage drop, you also have the battery sag. I was just always told to measure across the posts while firing to see how much voltage you are losing in the circuit. I think everyone goes by those numbers because it is the easiest. You can see how many volts you are really getting to power your atomizer.

Sent from my SM-N900T

 

[SeaNap]

Hey ancient, you actually got a pretty good grasp on it. Voltage-Drop refers to the difference between the measured voltage of your battery and the measured voltage accross the coil under load (measured at the posts of the atty)

Vd = Vbatt - Vload

Every part of the mod has an inherint resistance; the battery, the connectors, the mod itself, and of course the coil. You are correct in thinking that it would be far more accurate if we could just measure the resistance of the mod + batt (no coil). The problem comes in with the tools used to measure resistance. Your standard DMM has an accuracy of .1ohm, and the battery's internal resistance is .02 so you would never be able to accuratly measure. Since everyone has a DMM that measures Volts to an accuracy of .01 it becomes much easier to measure that.

#1) with no battery in the mod there would be no voltage to measure. Instead we measure the voltage of the battery at its terminals.

#2) If you measured 4.08V then your voltage drop from a freshly charged battery would be 4.20 - 4.08 = .12V

The value from #2 is entirely meaningless though unless you specify what ohm coil you were using. The coil resistance acts like a reference for others to compare. Think of it this way; a .12V drop on a .5ohm coil would mean that there is 0 resistance in the mod. However, a .12v Drop on a 1.5ohm coil means that there are some pretty major resistances in the mod preventing all the power from going to the coil.

I hope this makes since, Ill try to find some of my old posts and cheat sheets. Let me know if you have any more questions

 

[DoogieTony]

Apologies in advance if this has been asked and answered before, but if it has, I don't remember seeing it, and can't find it with a search.

HAHAHAHHAHAHAHA!!!!!!!!!!!!!!!! I'm crying

 

[zmauls]

HAHAHAHHAHAHAHA!!!!!!!!!!!!!!!! I'm crying

Me too.

 

[SeaNap]

Here is an incredibly helpful article that explains the voltage drop better than I can: Explained: Under Load Voltage Drop | The Vape Break Room

 

[zmauls]

^^ This dude knows what's up.

 

[Filthy-Beast]

To see the voltage drop you have to have a load on the power source. The bigger the load the bigger the drop. So a lower ohm coil has a bigger drop.

You'll have a drop inside the battery itself created by it's internal resistance and then the drop of the contacts and springs in the mod.

So you need three measurements to really see what's going on.

1. measure battery with no load at it's positive and negative posts let's say you get 4.19

2. Now measure at the battery posts with the battery installed and a coil installed and being fired. This will be lower let say 4.10 now. Your battery has .09 voltage drop, every battery is different and the resistance and thus the drop increases with the age of the battery.

3. Now measure again at the post of the coil being fired. Let's say you get 3.95,

The total voltage drop is 4.19-3.95 = .24 volts. But the voltage drop from the mod is only 4.10-3.95 - .15 volts since the battery is adding a drop of .09 volts.

The upgrade kits will help the .15 drop of the mod.

By lower the drop of the mod, you could increase the ohms of your coil and get the same hit. This would intern also lower the drop of the battery and mod since the amp load would also be lower, plus your battery will last longer.

 

[SeaNap]

Well said FB!

If you're looking for a way to measure resistance down to .001ohm check out this hack I did http://www.e-cigarette-forum.com/forum/reos-mods/514385-helpful-diy-ohm-meter-build.html Measuring resistance will get you most of the story but not all. It doesn't factor in a bad connection point. For example, if the surface area of a connector is very small it will act like a resistor (think crimping a water hose) under a small load this doesnt matter but as you increase the load the resistance rises in an exponential curve. To properly test the mods connections you would have to do the voltage under load measurements.

 

[zmauls]

By lower the drop of the mod, you could increase the ohms of your coil and get the same hit.

Or you could keep your 0.20Ω coil and destroy your lungs in one fell swoop, like this guy.

 

[ancient puffer]

Thanks folks. All of your replies (well, most anyway ) were helpful, especially the article SeaNap linked.

What I'm taking away from this is:

Voltage drop (without specifying resistance) is meaningless.

Voltage drop, in any given mod, is the result of total resistance of the mod, plus internal resistance of the battery, plus the resistance provided by the atomizer/coil combination, and efficiency of the contacts throughout.

The 3.7-3.9 volts at which most of us change out the battery is misleading, since it's the voltage at the coils we're really interested in.

Even though it's harder to measure, total resistance in the circuit (mod, battery internal resistance, contacts and atomizer/coil) would be a better gauge of how efficient a given combination really is.

So when I install the subohm kit, measuring the vdrop at the coil and comparing it to the old readings (at the same battery voltage and coil resistance) is a *relative* or *ballpark* measure of increased efficiency.

I guess I'll just compare the before and after vdrop across the coils.

I was hoping that I just didn't understand all this, but apparently it's pretty close to what I was thinking. In the end, though, I still think the subohm kit will be worth it, if only because I won't have to use my own homemade firing pin to take advantage of some of the newer flattop batteries I've been eyeing.

Thank you all again. This place is SO danged helpful! I'd probably still be using anemic mass produced PVs if it wasn't for REOville.

 

[Big Hitter]

I think to simply answer the original question would be this .......

We don't use the rate mod with no load because any decent mod should be dam near perfect that way.

Intruducing the load does add variables but your not just dealing with the load itself how how the batteries handle it ...

What we (I at least) want to know is how does the mod handle the addition of a load .......

Think of test driving a car ..... we not just going to sit in the lot and determine how the thing will drive.

 

[Ian444]

Here's turbocad's take on it, which is very good, another way of having it explained:

http://www.e-cigarette-forum.com/fo...ll-seeing-some-upgrades-101.html#post12592390

What I'm taking away from this is:

Voltage drop (without specifying resistance) is meaningless.

Yes, correct.

Voltage drop, in any given mod, is the result of total resistance of the mod, plus internal resistance of the battery, plus the resistance provided by the atomizer/coil combination, and efficiency of the contacts throughout.

Correct, except that if you wanted to compare two mods, say mod A to mod B, you would keep the battery out of the equation since the battery is chosen by the owner, not by the mod maker.

The 3.7-3.9 volts at which most of us change out the battery is misleading, since it's the voltage at the coils we're really interested in.

Usually Reonauts change the battery when the performance is not quite up to par. Also its a good idea to recharge a lithium battery before the resting voltage reaches 3.7 volts, for reasons of best life for the battery. A battery with 3.7 volts across the terminals at rest, might be putting out 3.3 volts under load, so the coil would be getting even less than that. Few of us really know what the voltage is at the coil over the life of the battery, we just go by the vape to gauge whats going on under the hood. If we are not getting enough vapor, we wind a lower resistance coil to get more power into it, or vice versa.

So when I install the subohm kit, measuring the vdrop at the coil and comparing it to the old readings (at the same battery voltage and coil resistance) is a *relative* or *ballpark* measure of increased efficiency.

Yes. It would help if you also measure the voltage of the battery while firing the coil, this would allow some calculations a fair bit better than relative or ballpark before and after fitting the new kit. i.e. measure the voltage across the coil and the voltage across the battery terminals while firing the coil. Put the atty and battery aside, fit the new kit, replace same atty and same battery, then measure voltage across the coil and the voltage across the battery terminals while firing the coil. We should be able to work something out from those figures, hopefully better than ballpark or relative.

Low voltage high current circuits like you get in a sub ohm mod are pushing the limits of electrical engineering, because with high current you get large voltage drops across minimal resistances, but we've only got a few volts at the battery to start with. Thats why overhead power lines are 30,000 or 60,000 volts, they can run less current (for the same power) and the voltage drops due to resistance over long distances are minimised. With our mods, we are doing the exact opposite, low voltage and high current, but at least our wire runs are very short

 

[SeaNap]

The 3.7-3.9 volts at which most of us change out the battery is misleading, since it's the voltage at the coils we're really interested in.

Even though it's harder to measure, total resistance in the circuit (mod, battery internal resistance, contacts and atomizer/coil) would be a better gauge of how efficient a given combination really is.[/I]

So when I install the subohm kit, measuring the vdrop at the coil and comparing it to the old readings (at the same battery voltage and coil resistance) is a *relative* or *ballpark* measure of increased efficiency.

I guess I'll just compare the before and after vdrop across the coils.

The 3.7 - 3.9 Volts that people are changing their battery is the voltage of the battery measured at the battery terminals. Not under load. So if you had a .12voltage drop under load, by the time you changed your battery you would have 3.58V going across the coil under load. (Vd = Vbatt - Vload) On the old stock REO there is a 1.0Voltage drop on a 0.5ohm coil. So if you waited to change your battery out when it read 3.7V, when you fired it the coil would only see 2.7V

Voltage and resistance are directly proportional V = I * R So it doesn't really matter weather we measure for voltage and calculate resistance, or measure resistance and calculate voltage. One thing we do have to consider on top of the resistance of the mod is how efficient the contacts operate under load, and the only way to measure that is to measure voltage across the coil while firing.

If you would like to measure the performance gain from the new SO kit you would first need to get your 3 measurements that FB listed above on your current set up. Then after you install the new kit use the same exact coil and take the 3 measurements again.

You'll realize that with the new kit that your coil will be warmer, it will hit 'harder'. You may need to then increase the # of wraps of your coil (increase the resistance, lowering your current) so that you get the same hit as your previous set up. This will increase your battery life.

tl;dr All the new contacts do is significantly lower the resistance of the Mod and improve the contact points which help improve battery life.

 

[SeaNap]

That turbocad is a pretty sharp fellow, he has a great way of explaining things.

Thanks for pointing that out Supe. Thats what we've been saying all along. There have been people who think that we were just being silly trying to reduce v-drop and that it didn't matter for those that did not sub-ohm. I tried along time ago to explain the benefits using math, but I think that point gets forgotten and people start to associate better connectivity with sub-ohm and they thumb their nose at it.

I really dislike that Rob called the new kits 'Sub-Ohm Kits' for the REO. While the new kits definitely let you run sub ohm coils, the main benifit is just making the mod all around more efficent and better for your battery. IMO they should have just been called a performance upgrade, or increased conductivity kit (im terrible at naming things but you get what i mean)

That's amazing to hear that the silver is holding up so well! Can't wait till I get mine!

I agree with this too, it's not a "sub ohm" kit. it's a "fix the crappy contacts" kit

any resistance that is not the coil itself is robbing you of power, just because it vapes ok with a high ohm coil don't make it good, saying it works ok at higher ohms is like saying it doesn't matter that my gas tank leaks a little because I get such good gas milage anyway

 

[nerak]

That turbocad is a pretty sharp fellow, he has a great way of explaining things.

I think that by explaining things in everyday language for us tech challenged folks is much better!

I can understand:

any resistance that is not the coil itself is robbing you of power, just because it vapes ok with a high ohm coil don't make it good, saying it works ok at higher ohms is like saying it doesn't matter that my gas tank leaks a little because I get such good gas milage anyway

I don't like leaks! So I am giving more thought to the new kits.

 

[ltrainer]

I just did some readings on my Grand with a Sony vtc5 battery.
Battery alone................ 4.12
Battery under load........ 3.98
reading with .8 ohm coil...3.55
Voltage drop.................. .48

Does that sound about right?

ETA: These reading are with the old contacts.

 

[SeaNap]

That sounds right on the money for a stock grand. If you are getting the new SO kit I would love to see your results with the new contacts (same coil)

Currently your vaping at 15.75 Watts. (3.55/.8)*3.55 = 15.75

If the new contacts give you a total Vdrop of .2V you would now be vaping at 18.33Watts. (3.83/.8)*3.83 = 18.33

Another way to think about it is if you wanted to maintain 15.75Watts you could increase your coil resistance to 0.93ohm and have the same vape. (3.83^2)/15.75 = 0.93

Pretty Cool stuff

 

[ltrainer]

I'm waiting for Rob to get the Woodvil springs drilled then I'll order the rebuild kit. And some Rm2 screws, woodvil spring etc etc. But I do plan on testing again the before and after once I get the kit. Not in a rush at all.

 

[Filthy-Beast]

Make sure to use the same battery for the tests as well since different batteries could have different internal voltage drops.