You should be aware of some serious problems that possibly caused a dead battery effect. It is true that one reason is poor maintenance but also know that there is a condition called Sulphation. This happens when sulfur starts accumulating on the lead plates inside the battery.
This should have immediate remedy since it blocks the current coming from these plates decreasing overall performance. What you have to do is to remove the sulfur build up inside the battery so that it will work again properly. Make a solution using distilled water and Epsom salt. When you have that mixture already, put it on your plates and wait for few minutes.
Check the voltage to see if it is alright now. This is a quick fix solution that enables your car battery run for a short time.