I also see this question a lot. 2015 was certainly the year of the boxmod, as predicted by many in 2014. Boxmods come in two main battery configurations when there is more than one battery – these are referred to as “series” and “parallel”. When you call a box mod “series”, it’s really this battery configuration you’re talking about.
Most of the time, when folks are referring to series and parallel, they are talking about unregulated (sometimes called “mech” or “mechanical”) box mods, generally with two or three 18xx0 or 26650 batteries – that’s what this short post discusses (regulated vs. unregulated will be discussed elsewhere).
Double or Triple the Voltage
In an unregulated series boxmod, two or more batteries are wired in such a manner that they are “stacked”. In effect, the voltage of a single battery has effectively been doubled or tripled, without changing the capacity that the batteries are delivering. If you have two 2500 mAh Samsung 25Rs in series, you still get 2500 mAh from the pair of batteries – you’re just getting it at approximately 8.4V instead of 4.2V.
This is very much like adding a supercharger or turbocharger to the engine of your car – you can easily exert twice the force on the driveshaft with this souped up configuration. Some folks that do this end up snapping drive shafts and causing all sorts of damage with all this extra power – similar things happen with burning legs on coils when you’re first dry-firing them on series. It’s a lot of power flowing through generally a very tiny wire. I often will get my RDAs wicked and set up on a parallel box, then run them on series. It just feels safer, and having that extra amp headroom is nice. Once you get going on the highway, the supercharger can actually help you with fuel economy at cruising speed, this is also true with series if one builds a little higher, batteries can last just as long as with parallel.
Higher Resistance can Equal Similar Wattage
Unlike a parallel configuration, two batteries do not share the continuous amp drain from the circuit that your RBA and mod’s switch are completing when they are wired in series. If you’re using a reliable 20A 18650 cell like the LG HG2, having two of them in series still means you can safely vape up to 20A without damaging the cells. I personally find myself building much higher resistance coils for series – in the 0.5 to 0.8 ohm range, where for parallel I am often between 0.1 and 0.2 ohms.
Ignoring voltage drop and internal resistance for a moment, one can quickly see via a site like Steam Engine that building a 0.3 ohm coil on a parallel boxmod yields a (appx) 14A/60W vaping experience. A 1.2 ohm coil yields the same 60W vaping experience on a series box, albeit at 7A (since the current is flowing at twice the voltage).
Some folks say series is less safe that parallel – certainly more people say this than vice versa. It’s really all in knowing how to build for your application. It is just as easy to build too low for parallel when you’re aiming for 0.10 ohms as it is when you’re building for series and aiming for 0.50 ohms.
To experience the full and really amazing range of unregulated vaping, someone really needs both types of boxmod, and to build effectively for either they really need an ohm reader (if not, two). I found myself mainly using series with RDAs around the house and parallel with my tanks when I was mobile – that was, until I found unregulated PWM, which can deliver the best of both worlds very safely in the same box….more on that soon.
Stay safe and Vape On!