Two batteries can either be wired in series or in parallel. On a regulated device it doesn't matter all that much because the battery drain works out the same either way. But in a mech mod it makes a world of difference. They both allow you to get more wattage than a single-battery device, but the way they go about doing that is very different.Thank you, bwh79 -- very clear explanations! I have one further question at the moment ... was just looking on fast tech at the mech mods and I see the tube type ones as you illustrated. There are also "box mods" -- those seem to hold 2 batteries, and I'm wondering if 2-battery-boxes are better in some way than a 1-battery-tube ... for example, if 1 batt dies, the user switches to the other or something?
Batteries wired in parallel output the same voltage as a single cell, but the power drain is split between the two cells, increasing battery life and allowing you to use lower ohms to draw more amps than from a single battery cell, and ultimately achieve higher wattages.
Batteries wired in series, on the other hand, will add their voltages together, but each one of them will see the full amp load individually. This doubles the voltage of a single cell and allows you to get twice the wattage from the same amp draw, but it forces you to use higher ohms to maintain that same amp draw.
In short: parallel allows a wider range of builds to achieve more watts or longer battery life, while series forces a narrower range of builds, and puts out more watts regardless. Unless you understand very well the implications of and reasons for using a series configuration, you'll really want to stick with parallel.
Additionally, a series configuration will not work without both batteries installed. The circuit path goes through each one individually, and if either one is not there it breaks the circuit. A parallel mod will work just fine with only one battery in, it will be just like any other single-cell mech mod. What you must never do, on a parallel or regulated device, is to use a more-charged battery together with a less-charged one at the same time. This will cause all sorts of bad things to happen. Just don't do it. Batteries don't "hook up" or have one-night stands. They're either married from the start (married pairs are purchased together, charged together, used together, and rotated each use so that they always wear evenly) or single for life (never use two unmarried batteries in a two-cell device, and if either one of a pair is ever used without the other, then they're no longer married.)