Anything that uses more processing or computation power than word processing. Naturally, that refers to things such as games, videos, music, streaming video, uploading/downloading files, connection to the internet, if you have LAN or WiFi turned on, anything that isn't word processing. This is because it requires more power to make more calculations and computations within the CPU, RAM, video card (if present), and if files need to be read, hard drive.
However, things like internet access as long as it isn't downloads or streaming shouldn't lower it that much. But if you aren't using it, you should close it and turn off your wireless switch if you can.
It should decrease
Yes, they will.
No, it will not.
The main benefit is that you will have a backup power source for your computer if the main power goes out. And if you were working on a document or spreadsheet, you won't lose the work you did because you were unable to save the file before the power went out.
Alternator is defective.
Baking soda will decrease acidity of battery acid
According to the document linked to, that battery will run down to 1.8V, which I believe qualifies as deep cycle.
Manufactures don't recommend to that because it causes decrease of battery life.
Not exactly "damage", but if you keep it on the charger all the time, then the battery capacity may appear to decrease, i.e., the battery won't last as long when you do use it. Think of the battery as a muscle.
One volt is a joule per coulomb, therefore over time the battery looses electrical energy (joule) and this is why the battery becomes discharged.put simply it is because the electrical energy is being consumed.
Graphics and Animation, Heavy CPU Loads (gaming for example)
May be the voltage regulator is defective, bad connection somewhere, alternator with a defective part which impair to fully charge the battery. decrease of the battery solution, short-circuit somewhere.