It's my understanding that the issue with running batteries in series is that you are drawing more voltage than 1 battery can safely fire and without something regulating the circuit, there is a risk that one battery will end up outputting a higher percentage of the total voltage than the other which is not good at all.
I think there's a misconception that using subohm coils is inherently dangerous. In reality, it is the resulting amperage that is concerning.
By running 3 batts in series, you'd be running around 12.6 volts and even with a 1 ohm coil you'd be drawing 12.6 amps (12.6v/1ohm) and firing ~158.76 watts (12.6v * 12.6 amps).
Running in parallel, you'd be firing 4.2v so you would get the same 12.6 amps with a .33 ohm coil, but you'd only be firing ~53.45 watts. To get the 158.76 watts, you'd need a .11 ohm coil and would be running at ~38.16 amps.
Now as far as battery amp limits go, I know how to apply this math for 1 battery, but it may be somewhat different for multiple batteries (someone please step in and correct me if I'm wrong). I THINK (please do not take this as fact) that it goes like this:
- Batteries in series will retain their original amp limit, so if you run 2,3, or 50 batteries with a 30 amp limit in series, I believe the limit stays at 30 amps.
- Batteries in parallel will also keep the same amp limit per battery, but the amperage is evenly distributed between them, so the overall limit grows with each battery you add. So with 3 batteries with a 30 amp limit, I think you could safely run 90 amps (3 * 30) {it may not be as simple as this linear correlation}.
So using the 2 examples above (getting to 158 watts in series vs. parallel), you find that they are each using about the same percentage as their overall amp limits (the parallel option draws 38 amps which is about 42% of your overall 90 amp limit, whereas in series you'd be running 12.6 amps which is about 42% of your 30 amp limit). If you can regulate the series circuit, I would use that configuration since you would get longer battery life by drawing lower amperage, but the parallel configuration is much safer unregulated.