Self defense, as previously noted, seems to be the most elemental rationale for a charge of assault and battery. There is a bit of diminished capacity in the eyes of the law if one is responding to a situation as opposed to initiating it. I think that it is also a practical move. It is much more complex to attempt to argue that one was acting in self defense. It becomes a battle of perceptions and if one is persuasive enough, it can create enough of a reasonable doubt to make the argument that self defense was present as opposed to assault and battery. Another defense is that in a particular moment someone lost control of their actions for a variety of reasons. If a case can be made that at a particular moment, one was not in complete control of their actions and that there was a legitimate example of diminished capacity, then one can posit this as a response to assault and battery. Yet, I think that this is much more difficult to prove and faces a level of scrutiny as there have been moments where individuals used it as a fraudulent defense to justify their own actions.
There are a couple such as the aggressor defense and the mutual combat defense. The aggressor defense is simply that the other party to the case was the aggressor and that the defendant was merely acting in self defense. Perfect or imperfect self defense can then be shown by proving that the use of force was necessary and the amount of force used was appropriate.
The mutual combat defense is one that states the parties engaged in the struggle willingly and were mutually fighting. Law enforcement agencies are aware of this particular defense and thus many times will charge the individuals with a lesser charge of disorderly conduct. The mutual combat defense is harder to prove than the aggressor defense.
Other defenses include the reasonable disciplining of a child by a parent or gaurdian or that the conduct was necessary and lawful to detain a suspect, prevent suicide, or to prevent escape of someone who is being lawfully detained.