Innokin are very serious about safety, in general, and battery safety in specific. Which is good, because with a built-in battery like in your CF4, there's nothing that you can do except pray that the manufacturer (in this case, Innokin) has used a quality cell in their device. So you have safety features in place, like short-circuit/low resistance protection, low-voltage protection, and overheat protection. If the device detects an attached resistance that is too low, or what it considers a "short circuit," it won't fire. If the battery voltage is below a certain cutoff, it won't fire. If the temperature on the board gets too high, it won't fire. These all serve to protect your battery and, ultimately, your hide.
When you really need to start worrying about battery safety, is when you start using devices with removable batteries. Then, you're no longer at the mercy of the device manufacturer, and you are able to put in any old cells you please. You need to learn Ohm's law, and "Watt's law" and how they pertain to battery-operated devices. You need to make sure you are purchasing your cells from a trusted supplier, so you don't get stuck with a counterfeit or "re-wrap" battery. You need to know the amp limits of your batteries, and how to stay within those limits. Many of the batteries we use are 20A-rated batteries -- this works out to a limit of about 60 watts (max) per battery on a wattage-regulated device, or about .21 ohms (min) on a single-battery mechanical/unregulated device (when you start using multiple batteries in an unregulated device, it matters whether they are wired in parallel or in series -- parallel allows you to build lower if you want, series requires you to build higher to stay safe.)
Ohm's Law: "V = IR"
Voltage (measured in volts) equals current (measured in amps) times resistance (measured in ohms). Rearrange the terms a little (and use a more familiar vernacular) and you get:
Amps = Volts / Ohms
This tells you the amp drain on your battery in an unregulated device, when the battery is at a particular voltage (charge state) and powering an atomizer with a certain resistance. The amp draw will be highest when the voltage is at its highest, so for safety's sake you should always assume a full charge of 4.2v in this equation. Plug in 4.2 for the volts, and whatever the resistance of your atomizer is for the ohms, and you will find how many amps are drawn from your device at full charge.
For a regulated device like your CF4, the ohms don't directly affect the amp draw. What you're looking at instead, is the wattage. The wattage that's applied to your atomizer, is equal to the wattage being drawn from the battery (minus a few percent for device efficiency). Here's another equation for you:
Watts = Volts * Amps
(or, since we already know that "Amps = Volts / Ohms" you can also write this as "Watts = Volts * Volts / Ohms" or "Watts = Volts^2 / Ohms")
Again, we use a little algebra to rearrange the terms, so we can calculate Amps from the other two known variables:
Amps = Watts / Volts
Now, note that the amps will be highest when the voltage is at its lowest. For this reason, you should always assume a low value for the voltage. 3.0 volts, under load, at low-voltage cutoff, is a reasonable value to use. So here, you will want to plug in 3.0 for the volts, and whatever your device's max wattage is (or the max that you will be using it at) to find the max amp drain at low voltage cutoff. Finally, divide this value by the number of batteries your device uses, to find the amp draw per battery. Technically, if the batteries are wired in series, the full amp drain is "seen" by each of them and not split up among them -- but in that case, the voltage will be higher (the individual cells' voltages added together) and so the end result will be the same. THIS IS ONLY TRUE IN A REGULATED DEVICE! SERIES/PARALLEL CONFIGURATION IN A MECHANICAL OR UNREGULATED DEVICE DO MOST DEFINITELY NOT HAVE THE SAME RESULTS!
So, now that you have a quality cell, from a trusted supplier, and you know the amp limit of that cell, you can plug your values into those equations and know if it's safe to use that cell with a particular device/attachment combination or not. Most people prefer not to run them right at the limit, and instead will leave some "headroom" on the order of 10, 25, or even 50%.
