There are two main types of mods: Regulated, and unregulated (mechanical). (Putting a Kick in a mech mod automagically turns it into a regulated mod.) As far as battery drain is concerned, these are two completely different animals.
In an unregulated mod, a lower resistance atomizer means more current drawn from the battery, and less battery life. In a regulated mod, things get more complicated, and the rules are different.
Unregulated (mech) mods are very simple devices. They have one circuit. This circuit can be easily modelled using Ohm's law. In other words, if you have a basic understanding of Ohm's law, that is all it takes.
The battery drain is determined by two factors:
Using a single, fully charged, Li-ion cell, the voltage starts at around 4.2V. It quickly drops to around 3.7–3.6V as you use the mod, and stays there for a while. Once the voltage sinks under 3.6V, it starts dropping faster again, and the battery needs to be recharged. Discharing an ICR battery below 3V can ruin the battery. IMR batteries can handle as low as 2.5V.
There is not much more to mech mods. They are about as simple as an electrical circuit can get:
A multimeter and some straightforward use of Ohm's law you will give you all the numbers you need.
Regulated mods are more difficult to model. But even though they are much more complex than mechs, with some selective simplification we can safely ignore most of the complexity. So we break these mods down into two circuits and a black box. This makes our regulated mod model little more than twice as complex as our mech mod model.
The two main circuits of a regulated mod are:
And never the twain shall meet. The regulator circuit takes care of that. It can have a bunch of more or less advanced circuits in itself, and it uses a little bit of power, but for the most part we can envision it as a black box separating the battery circuit from the atty circuit.
Since we are looking at two separate circuits, you can never mix numbers from both sides of the regulator in your calculations. For instance: You cannot determine the current drained from the battery by measuring the resistance of the coil and the voltage of the battery. Using the resistance from one circuit, and the voltage from a different circuit, will result in a nonsensical answer. Nor can you determine the current going through the coil by determining the current from the battery.
On the atomizer side, the voltage is ideally whatever the user has selected, but keep in mind that some APVs promise more than they keep. If you set the voltage to 5V, are you confident that the APV actually delivers 5V? Unless you know that your APV is accurate, you might want to measure and confirm the output voltage under load.
Variable wattage devices work like variable voltage devices, for the most part. The difference is that they measure the atomizer resistance, and uses this measurement, and Ohm's law, to calculate what voltage to set in order to reach your desired power.
Likewise, knowing the output voltage and resistance, you can calculate the output current and power yourself.
On the battery side, the voltage is whatever the battery has in it at the moment, just like with mech mods. The power, however, is whatever the regulator needs to pull in order to deliver the desired voltage to the atomizer at any given time. So by dividing the power by the (ever decreasing) battery voltage, we find the (ever increasing) current.
So how can our measured resistance tell us anything about battery drain? Well, there is one way to "transfer information" from one side of the regulator to the other. The trick is simple; to paraphrase some old movie: Use the power, Luke!
The power hitting the atomizer equals the power flowing from the battery, minus the power used by the regulator circuit.
These regulator circuits typically boast an efficiency between 80–95%. In practice this means that the regulator "steals" about a tenth of the power from the battery.
Knowing this, we can use our multimeter and Ohm's law to calculate what is going on at either side of the regulator. Then we can convert it to Watts, and voila! Subtracting (or adding) the loss in the regulator circuit, we now know the power on the other side as well.
Lastly, we use Ohm's law again, break down the power to current and voltage, and that's that: We now have all the numbers we need.
Capacity is normally stated in mAh, but this number is not very usable as it is.
The interesting figure here is not mAh, but Wh (Watt-hours). Using Ohm's law and a nominal voltage of 3.7V, current capacity is easily converted to power capacity.
A 2 Wh battery can deliver two Watts continuously for one hour, one Watt for two hours, etc, half a Watt for four hours, etc.
Most of the battery specifications were collected from Baditude's blog post at ECF. The presets are for convenience only, and I can not promise you that I did zero mistakes when entering the values, so please take care to ensure that the presets that you use are in accordance with the actual specs of the batteries that you use.
As the battery drains in a regulated mod, the battery voltage goes down. As a result, the mod must draw more current in order to maintain the set voltage or power. In other words, as battery voltage goes down, battery current goes up. This is the opposite of what happens with mechanical mods, and somewhat unintuitive.
Don't rely on the "best case scenario" where the batteries are full, when you make calculations for regulated mods. It is safer to assume a battery voltage of 3.5V, or even lower, depending on how low the mod will go before it demands a new charge.
Again, with mech mods the opposite is true: The current drops as the voltage drops. So with mechs, it is safest to assume a full battery when you're doing these calculations.