OP, please be careful. You are playing with fire. Literally. I highly suggest that you watch the video below from Pegasus Vapor Academy, where he answers this exact question (albeit with a .06 ohm coil instead of .07) quite authoritatively. Under *no* circumstances should you attempt this build on a non-regulated *series* configuration. It will ruin your day, or worse.
And even on a parallel configuration with two 18650 cells, you would be well past the edge of the safety envelope in my opinion. Driving a .07 ohm coil with a freshly charged pair of parallel 18650 cells will almost certainly cause the battery voltage to sag under load to somewhere around 3.7 volts, which would draw roughly 53 amps from your battery, yielding just under 200 watts of output power. That's 26.5 amps per cell. Very, very few commercially available cells can *actually* and *safely* sustain a 30-amp continuous load. Most can only do 20 amps safely, no matter what the label says.
But even if your cells can actually sustain that load, the heat generated will be substantial and excessive. And in reality, you're going to have to hold your fire switch down for a good, long while to actually achieve that 200 watts you can theoretically yield. So you're going to be cooking your device and, more importantly, your cells. And as the temperature of the cells increases, their performance will degrade, as will their longevity. It's a vicious circle. Even if those cells could sustain 30 amps of continuous current, that's only under ideal circumstances (i.e., thorough heat sinking and ventilation, excellent health of each cell, low internal resistance of each cell, etc.). But with the heat that this build would produce (and thus the heat that will be absorbed back into the cells), there would be practically no way those cells could safely sustain that load at those temperatures.
A 3-cell (or greater) parallel setup would theoretically be able to safely provide the necessary current to sustain the load. But the excessive heat buildup would still be there due to the 53 amps that would be drawn. No matter how many cells you arrange in parallel, you still only have about 3.7 volts at most under load, so your ramp-up time will still be relatively long, and your build would still draw 53 amps total from the battery. Those 53 amps will produce a lot of heat.
A much more practical and efficient means of achieving those 200 watts would be to apply more voltage across a higher-resistance coil. Applying Ohm's Law, if you double the voltage and half the current, you achieve the exact same power output (wattage). So, for example, using two cells in series (2S), you could build a more sensible 0.28-ohm coil and achieve the same power output (wattage) with half the total current (i.e., about 26.5 amps), which would have at least two major benefits: less heat build-up (from the lower total current flow) and less ramp-up time (due to the higher voltage) (and the lower ramp-up time will benefit the heat build-up as well). The downside is that each cell is still required to contribute 26.5 amps, just like with the dual parallel setup (except with the series configuration the current is not additive like it is with a parallel setup), and that is pushing the edge of the safety envelope.
Better yet (at least in theory) would be a 3-cell series (3S) setup, which would require only 17.6 amps across a 0.63-ohm coil to achieve the same power output (wattage). The biggest problem with such a setup, though, is that there are risks associated with running multiple cells in series. One of the primary of such risks is that the cells can become unbalanced during high current discharges. This typically occurs when the cells have differing internal resistances, which is not at all uncommon. So, if you were going to pursue a 3S setup, I'd recommend looking at purpose-built lithium-polymer (Lipo) battery packs, as opposed to multiple lithium-ion cells. The reason is simply that Lipo packs are purpose built to be run in series configuration, and, as such, the manufacturers tend to do a reasonably good job of "matching" the cells that are used to construct the pack, not to mention the fact the Lipo cells are generally more tolerant of higher rates of discharge.
Sorry, I don't mean to preach here, and I hope I don't sound condescending. I truly don't mean it that way. It's just that I've seen bad things happen when people try to push the edge of the envelope without a thorough understanding of the risks and what they're getting into. The harsh reality is that lithium battery technology is awesome, but it can be quite dangerous for the uninformed given the inherent volatility of the chemistry of lithium-based cells.
Vape hard but safely, my friends.
Happy vaping!
