I am making a control circuit for charging of a 12 V battery, and I'm having problems starting because I can't figure out how I could measure that the battery is fully charged.

If I apply 12 V (DC) to the battery, and the battery becomes fully charged... the node with the applied 12V and the battery 12V will be the same... or?
So how can I know?

And also, if applied 14 V for charging the 12V battery... what would happen?

I realize that my questions might be stupid, but I really can't figure it out...

 

A 12V battery is not fully charged at only 12V, it is barely charged.
If you apply 14VDC to a 12V lead-acid battery then it will charge then it will become over-charged.

To figure it out then learn about a lead-acid battery at the battery tutorial website you were given in the other website's forum.

 

One open thread should be enough.