"Realistic Space Battles" look rather dull, seeing as they don't exist and therefore don't happen.
However, since that is an overly boring answer it is better to start looking at how technology could advance and make warfare something remotely worthwhile.
Something that is often overlooked during these conversations is the more "Geo-political" angle. On earth we have conflicts because people with different views and goals are close enough that they can actually "do something about it", which in turn rather frequently results in warfare as the easiest answer. "They can't be right if they're dead!" kind of solutions to problems.
So, Why is there a conflict? Why is anyone so angry at, or scared of, someone else that they're willing to invest the time an energy into trying to kill them when there are several AU or more between each other most of the time? Start with the question of "What could possibly drive us to spend that much time and energy to even try fighting a war", and then look to what technology you might have and want to use to fight that.
Energy, relativity, and Delta-V all start to really matter when it comes to figuring out how to fight a war. And if you develop some kinds of technology, then it is going to really have an impact on how two or more sides are going to duke it out. Highly intelligent nano-bots that are hard to detect? Well why even have a war when you can send a few merchant or diplomatic ships?
So if you are going to start along the lines of designing a "Realistic" system, you're going to have to define what your realism is and what can be done within it.