The key to battery safety is the sometimes-misunderstood Ohm's Law. Many new users shy away from this, due to the math involved, but understanding Ohm's Law is not nearly as hard as you might think and using it will safely open doors to a variety of new modding options for you.
Ohm's Law is used to describe the best relationship between power, voltage, current, and resistance, based on the amp of the battery. The battery's amp rating is the potential, or ability, of the battery to release the power inside. If some part of the mod requires the battery to push out power beyond this amount, and no safety regulations are in place, the battery may vent, releasing dangerous chemicals or even exploding. A misunderstanding - or complete disregard for - Ohm's Law is responsible for virtually all known safety mishaps that have occurred with mods.
The Ohm's Law equation to follow is simple:
Amps = Battery Voltage / Coil Resistance
Make sure that the result - Amps - does not exceed your battery's amp rating.
Your battery's amp rating is also easy to determine but does require equipment if it didn't come printed on the battery material - getting used to using the multimeter is a great habit anyway, if you plan to continue modding!
A multimeter is a simple, handheld device that connects to a battery and measures voltage, resistance, and amperage. Set the multimeter for DC to measure battery power, and ensure the maximum amperage setting on your multimeter is higher than what you expect from your battery. Once the cords are connected properly, the multimeter will display your battery's amp rating - this is the number that must be greater than the amps you calculated above.
If your battery amp rating is not high enough for the power required by your mod, you have two options: You can get a more powerful battery, or you can use a coil with a higher resistance.