This was asked in a Facebook group by a friend earlier today so I went off on a little bit of a rambling discussion…lol I had the exact same question a few months back. I am by no means an expert in battery-related issues – likewise this is by no means the be-all, end-all discussion on the subject, so comments and improvements are quite welcomed.
An 18650 battery is 18mm in diameter, and 65mm tall. Current is measured in amps (A). Voltage is measured in watts ( W). Resistance of coils is measured in ohms (Ω). I am a sub-ohm vaper – less than 1 ohm of resistance is in pretty much 99.349% of the coils I build and use. That statistic was made up on the spot, but the ones below were derived using the Ohm’s Law calculator at a website called Steam Engine, found here: http://www.steam-engine.org/ohm.asp.
For the most part, I’m comparing a dual 18650 parallel to a dual 18650 series unregulated box mod in this discussion (both with MOSFET-protected switches), and using a very ubiquitous and reliable 20A continuous draw 18650 battery in the examples, the Samsung 25R (20A continuous, 2500 mAh). It’s not the only battery I use, but I do use them on both series and parallel box mods. All the devices I vape are unregulated, so I really don’t know much at all about regulated devices. Both types have the same number of milliwatt hours (mWh) when using the same battery. This rambling discussion is going to avoid stuff like battery sag and voltage drop and use “fully charged” numbers for the sake of simplicity.
If you’re sub-ohm vaping, you really need an ohm reader. Or two.
1. Parallel Configuration
Dual parallel basically doubles the “headspace” on your continuous amp draw, as well as your battery life (measured in milliamp hours or mAh). If you are running a common battery like a Samsung 25R on a dual parallel 18650 box, you now have 40A to play with, and are dealing with 5000 mAh. You can safely and reliably build down to resistances of 0.11 ohms – at 4.2V that’s a 38A draw (@ appx 160W) – 5 sec draws on that are very very unlikely to vent a properly charged new battery. You get the exact same voltage as you would running one of the same batteries on a mech (tube) mod – whether you have 2 or 27 batteries in parallel, you’re always getting 4.2V out of the system. More mAh at a constant V = longer life out of the batteries as compared to series.
If you like building crazy Clapton coils and shit and are always finding yourself in the 0.1-0.2 ohm range (like me), parallel is probably where you want to be, although pushing all the current through a bunch of wire may yield much longer “ramp-up” times…but that second and third pull off the device will be done with the coils already heated so it will be less of a factor. Most of my parallel builds are 0.1-0.2 ohms and they are very enjoyable. The lower you build, the more spare batteries you need to keep around. You can build around 0.5 ohms on a parallel box (8.4A and 35W) and have a shit-ton of battery life since you are drawing from two cels.
2. Series Configuration
Dual series basically doubles the voltage or “power”. If you are running a common battery like a Samsung 25R on a dual series 18650 box, you now have only 20A to play with, and are still dealing with 2500 mAh (just like on a tube). You get the same ratings as are on a single battery – but that “juice” coming out of the battery is not at the 4.2V, it’s at 8.4V. The same 0.11 ohm build discussed above on a dual series setup would be drawing 76A and producing a pretty warm (as compared to the surface of the sun) 641W – way outside the comfort zone of any batteries on the market today. This is less than ideal for most people that like keeping their hands in one piece, so most series builders are comfortable staying at or well above 0.45 ohms.
A build with a 0.45 ohm resistance on a series build produces numbers very much in line with the 0.11 on a parallel – 18.7A and 157W, a very safe build for our aforementioned example 25Rs. Ramp-up times are virtually nonexistent since shoving 8.4V through a coil is a lot easier than pushing 4.2V through one. Most of my series builds are 0.4-0.7 ohms, and they are also very enjoyable. Again here, the lower you build the more batteries you need. You can build around 0.5 ohms on a series box (16.8A and 141W) and have relatively little battery life (as compared to 0.5 ohms on a parallel) since you are drawing from two cels – identically at the same time. I occasionally run 0.32-0.35 ohm dual Clapton coil setups on series – they have a lot of wire in them so take to series very well and produce a warm (or hot) yet flavorful vape.
Series pushes so much power through the coil that folks often find themselves melting coil legs when dry-firing. It’s much better to “pulse” with short controlled bursts than sitting there with your finger on the button like you can do with a parallel box (and 4.2V).
You can see above that a series box usually has the batteries “reversed” when in the box. Parallel boxes usually have the batteries facing the same direction. Both dual setups have the same mWh, it’s the mAh that measures capacity. Both can and do produce very enjoyable vapes. A triple parallel like above adds even more “headspace” and mAh over a dual parallel for even more safety and battery life.
I’m relatively new to all of this compared to folks that have been sub-ohming for years and years. My advice is that people really actually need both types of unregulated box to truly enjoy a full and wide variety of juices because different flavors taste better at different wattages (and temperatures). I’m pretty happy that I “learned the basics” on mech mods then picked up a parallel box and then got a series. Other folks have different opinions (and some folks throw regulated boxes in their learning progression as well).