3.7 / 7.4 Idea...

[TommyG]

So I was sitting around at work, bored out of my mind, and thinking about how much I really just enjoy 3.7 and 7.4 volt vaping...5 and 6 volt are ok too, but it seems the ends of the spectrum do it quite well for me. I came up with an idea for a box running two 18650s for both 7.4 and 3.7 volt, simple enough I know, but when I thought about the semantics of the whole thing it got a little more complex.

The idea started with using both batteries for the high, and one for the low, but when I thought about the implications of this I thought I might need a way to use either battery for the low. One of the more dangerous things to do when stacking batteries is to stack two batteries with different charges, this led to the idea of using both batteries for both applications. I would use them in series for 7.4 and parallel for 3.7, so they were both used at the same time in order to keep the charges comparable.

The below schematic is my first go at this thing on paper, it's likely to be completely unregulated, which is fine as I've been using my Roughstack a lot lately and don't really care about regulating these voltages too much...that and two 18650s will take a good amount of time to drop any noticeable amount here.



Let me know what you think about this, this is my first attempt at anything more than a little VV box, or unregulated 3.7 box, and I could always use help/ideas. The two LEDs shown will be different colors so I can tell when I'm using which voltage, so I'm less likely to trip the protection circuitry with the wrong carto at the wrong power.

 

[CraigHB]

It still needs some work.

I assume those diode symbols are LEDs? You can't put those in series with the supply current. They'll probably just burn out, but if even if they don't, they'll cause a big voltage drop.

When swapping cells from series to parallel, there's the potential for issues with equalization currents. Protected cells can be used to cover the safety aspect, but if it happens, it may cause one of the cells to remain out of the circuit either temporarily or indefinitely. And then the cell's protection circuitry is being relied upon routinely which is a good idea to avoid.

The diode required is the Schottky type, but a single one. It's a bad idea to parallel diodes since one usually carries the brunt of the load. There's a straightforward reason why, but I'm not going to explain it here.

A diode will protect from high equalization currents only in one direction but you'll get the spread of the diode's forward voltage drop in the other, might be okay. A PTC fuse there instead of a diode would be better. The reason being that the diode could create a differential in the cells based on its forward voltage drop.

 

[jimbalny]

Why not just hook up two batteries in series and create a mechanism to turn on and off a step-down regulator? I'm too shot right now to figure out how it could be wired but it would definitely be way simpler and probably safer as far as what craig said about equalization stuffs.

 

[CraigHB]

Pretty much yeah, just use a regulator, but the OP said he wanted to switch back and forth so that's what I commented on. Though, to get series battery voltage at the atomizer, you'd need an ultra-low droput regulator and then it would get too hot to running at 3.7V. Switching regulators typically have too much dropout. That might be a good reason to switch back and forth, but I don't see how you could vape with 7.4V, you'd need an unusually high resistance atomizer or be happy with a blistering hot vape.

 

[TommyG]

I vape 7.4 at 4 ohms, and love it. The LEDs could easily go away, they way that switch is set up in the schematic you would only have one on at a time when vaping, one for parallel and one for in series. As for the equalization of currents, would this still be a problem if you weren't hitting the push button when switching from parallel to series? It would basically be an open circuit if you weren't actively vaping when changing over.

Looks like the idea of LEDs to differentiate voltage levels is going to be more a hassle than anything.

 

[CraigHB]

You could use an LED, just connect it in parallel with the atomizer. If you set it up for 7.4V, it will be very dim or not illuminated at 3.7V

It would be possible to use just one of the two cells for 3.7V, but then you'll discharge one a lot faster and it's not good to connect cells with a big charge disparity in series either.

Like I said, just substitue a PTC fuse for the LEDs you show in your schematic. The fuse will limit equalization currents and keep the cells balanced. It will also add additional over-current protection in series mode. If you add a fuse on each cell, you can add additional protection for both series and parallel modes while protecting from high equalization currents as well.

 

[TommyG]

You could use an LED, just connect it in parallel with the atomizer. If you set it up for 7.4V, it will be very dim or not illuminated at 3.7V

It would be possible to use just one of the two cells for 3.7V, but then you'll discharge one a lot faster and it's not good to connect cells with a big charge disparity in series either.

Like I said, just substitue a PTC fuse for the LEDs you show in your schematic. The fuse will limit equalization currents and keep the cells balanced. It will also add additional over-current protection in series mode. If you add a fuse on each cell, you can add additional protection for both series and parallel modes while protecting from high equalization currents as well.

I like the idea of the fuses in place of the LEDs, and I have an LED that is green at 3.7 and yellow at 7.4 that might work in parallel with the atty as more a reminder/safety. Do you think the switching would work for going from series to parallel, or should I just scrap that idea? I didn't want to use a single battery for 3.7 for the reason you figured, I am wanting to keep this thing safe and all.

 

[punchy187]

I vape 7.4 at 4 ohms, and love it. The LEDs could easily go away, they way that switch is set up in the schematic you would only have one on at a time when vaping, one for parallel and one for in series. As for the equalization of currents, would this still be a problem if you weren't hitting the push button when switching from parallel to series? It would basically be an open circuit if you weren't actively vaping when changing over.

Looks like the idea of LEDs to differentiate voltage levels is going to be more a hassle than anything.

I was just wondering myself if hooking up my batteries in series using 7.4 volts would work or not. What kind of carto do you use for a 7.4 volt mod? Do they make any like that or are you using carto mods?

 

[TommyG]

Mad vapes sells 4 and 5 ohm cartos, I need to look around to see what else I can find that might hold more.

 

[shortyjacobs]

I like the idea of the fuses in place of the LEDs, and I have an LED that is green at 3.7 and yellow at 7.4 that might work in parallel with the atty as more a reminder/safety. Do you think the switching would work for going from series to parallel, or should I just scrap that idea? I didn't want to use a single battery for 3.7 for the reason you figured, I am wanting to keep this thing safe and all.

So the issue is, in series, the battery closest to the atty will discharge faster. So if both start at 4.2 volts, and you vape for a while in series, (at "7.4" volts), one will be at, say, 3.9 volts, and the other at 3.4 volts.

When you switch to parallel, the 3.9 volt battery will attempt to "charge" the 3.4 volt battery so they equal, which is generally not a good idea. Note, this happens when you switch it over, regardless as to whether or not you are pressing the "fire" button while switching.

I just wanted to clear that bit up.....I have no idea what a PTC fuse is .

 

[CraigHB]

I have no idea what a PTC fuse is.

PTC stands for positive temperature coefficient. Most simply, it's a resistor that increases in value as temperature increases due to current flow. They aren't really fuses in the purest sense since they don't disconnect completely when they "trip". Rather, they quickly transition to a high value of resistance at their trip point that keeps current flow below a threshold. They work very well as fuses since you can get ones with very low cold resistance allowing for only a small insertion loss. Plus, they can trip and reset almost indefinitely without wearing out.

PTC fuses are a new device in comparison to something like a transistor. It's only been in the last couple decades they have become inexpensive and popular for use in consumer electronics. You find them in lots of stuff now where that wasn't the case only a decade or two ago.

 

[TommyG]

So the issue is, in series, the battery closest to the atty will discharge faster. So if both start at 4.2 volts, and you vape for a while in series, (at "7.4" volts), one will be at, say, 3.9 volts, and the other at 3.4 volts.

When you switch to parallel, the 3.9 volt battery will attempt to "charge" the 3.4 volt battery so they equal, which is generally not a good idea. Note, this happens when you switch it over, regardless as to whether or not you are pressing the "fire" button while switching.

I just wanted to clear that bit up.....I have no idea what a PTC fuse is .

This is what I needed to hear, that makes sense to me...so this idea is pretty much scrapped. I do appreciate the indepth on it though.

 

[CraigHB]

No, don't scrap it. It will work. Just do it like I said with a PTC fuse in series with each cell. You need to pick the right fuse. It should have a trip current just below that of the trip current for the cell's protection. The way PTC fuses work, the cells will still balance even if the fuse trips. PTC fuses don't open the circuit, they just increase resistance to keep current below a theshold which is usually around a tenth of the trip current.

 

[shortyjacobs]

This is what I needed to hear, that makes sense to me...so this idea is pretty much scrapped. I do appreciate the indepth on it though.

No, don't scrap it. It will work. Just do it like I said with a PTC fuse in series with each cell. You need to pick the right fuse. It should have a trip current just below that of the trip current for the cell's protection. The way PTC fuses work, the cells will still balance even if the fuse trips. PTC fuses don't open the circuit, they just increase resistance to keep current below a theshold which is usually around a tenth of the trip current.

Yeah, Tommy, I was just trying to make sure you understood what the other guys were saying when they said unequal charge and balancing and equalization currents. I know that batts in parallel with radically different voltages can cause big issues, but I don't know enough about the issue beyond that. CraigHB may be perfectly correct, (I have no reason to doubt him, but again, I'm not an expert, so I certainly can't endorse something I know little about).

I was not arguing for or against your plan - I just got the feeling you didn't quite understand the terms being thrown out, so I was trying to help .

 

[slimest]

CraigHB, good idea with the fuses. I will steal it

 

[CraigHB]

Go for it. If you do something like this, let me know how well it works. Visualizing something in your mind is not often the same as testing it on the bench.