YES batteries have a finite shelf life as well as a finite service life be sure that the born-on date is within the last 12 months or so and try charging the battery before using if it doesn't hold a charge it is bad.
Yes, that is possible.
That is possible, but highly unlikely. -There are many other issues that can seem like a bad battery in a neglected car. The terminals may be dirty, there may be a bad ground, the alternator or voltage regulator can be faulty - etc, etc,
It is possible you have a bad alternator, or a bad connection at the battery. You can take the car by your local auto parts store and have the altornator checked.
Bad connectins at battery? Clean connections. Possible loose battery cables on starter or engine block? Clean and tighten cables. If no help, possible bad battery cable/cables. Test cables and replace as necessary. Depending on what type of car, also investigate starter cylinoid. Hope that helps.
Yes, you can definitely buy a car battery online. There are many dealership websites that you can visit online to purchase a car battery and there are many auto stores that you can go online to purchase a car battery.
yes
If the car won't start, jump it off. While it is running, remove the positive cable from the battery. If the car dies, then your alternator is bad. If it keeps running then you may have a bad battery.
Nobody.
Typically you know your alternator is out when you have to jump start your car numerous times. A way you can test it is by turning on your car and while it's running discount the + terminal. If the car turns off then your alternator is bad. If the car remains in then alternator is good and possible bad battery. Hope this helps
its either the alternator the battery is bad or a loose or bad wire on the starter
Could be a few things, weak alternator, bad battery, bad wiring. If you have a subwoofer in your car with no battery cap that will certainly do it.
Yes, one bad cell in a car battery will cause the car not to start.