Answer: Waking up to a dead car battery is never fun, which is why you should be aware and mindful of the many different causes of a drained car battery so that you can learn to avoid them. The first (and most obvious) cause of a car battery dying overnight is car components left on or open. If a car door is left slightly cracked, headlights are accidentally left on, or even a small interior light is left on, this can cause the car battery to drain completely overnight. Make sure to turn off the lights in your car and close all of the doors completely to avoid this frustrating cause of a dead battery. Secondly, extreme temperatures (hot and cold) can cause the battery to die, due to the electrical components of the car getting overwhelmed by extreme temperatures causing them to malfunction or completely die. If possible, keep your vehicle inside a garage during extreme summers and winters. Thirdly, parasitic drain occurs due to poor wiring, bad installation of the battery, or human error like leaving on headlights overnight. This causes the engine to power parts of the vehicle it is not supposed to even when the car has been turned off. Next, a faulty alternator can cause the engine to not be charged fully when the engine is running, leaving your battery nearly useless. Lastly, age can cause your battery to die overnight. If your battery is very old, there is sometimes no avoiding a dead battery.