many things can do it suck as junk battery, junk alternator , or leaving anything on in the car
The fear of leaving a cigarette lit in the house is a form of the more general fear of fire, which is pyrophobia.
leaving on the radio
When using a laptop leaving it plugged in when the battery is fully charged does do some slight damage to the battery life. However this is minium but if you do thiscosistantly you will notice your battery does not last as long. Good question
The age of the battery may have something to do with it. An old battery will sometimes lose its ability to hold a charge when idle. Also, if you completely drain the battery's charge on a sealed wet-cell battery (the type sold on all cars after the 1970's) by accidentally leaving the lights on, it will never hold a full charge again. Another common cause is in the cigarette lighter. If a gum wrapper or some other foreign object gets in the cigarette lighter socket, it can slowly drain the battery. The lighting element itself can also be the culprit if it is used a lot and becomes worn.
test the voltage on the battery first, then on the alternator. If its good then it could just be the diode in the alternator. I would suggest you replace it becuase eventually your alternator will die and the battery will not get charged leaving you stranded somewhere. It is possible to just be a battery but you wont know unless you have the battery tested. check the voltage on the battery and the alternator before you do anything else.
yes
leaving a glass bottle in the sun or smoking a cigarette
mainly leaving your lights on when the engine is of or leaving the keys in the ignition
If the engine is running, then no. The alternator constantly keeps the battery charged.
the battery will run down
normal just recharge it