05/19/2015 Observations regarding battle mechanics. I am unable to find any official documentation of battle mechanics. Going to put notes here regarding observations to try to figure this out. Please add a comment here if more complete information is available.
Hovering over a ship's icon on the bottom of the screen during combat will show which target the ship wants to attack. If that ship is still out of range, and another target is within range, the ship will attack the nearer target, but will continue advancing to its main target and start attacking the main target once it is able to do so.
Ships appear to fire all of a weapon type on one target as a single shot. If you have 100 missiles and only 1 was needed to destroy a ship, then you have wasted firepower on that shot.
Weapon attacks on a target that has no remaining defenses in that category do max damage.
If a target has defenses against the weapon type, it appears that the weapon will do a random amount of damage. (It is unclear if weapon does random damage reduced by defense preventing random damage, or if one or the other is fixed, but there is definitely a random component here.) The damage is applied to the ship's defense for that attack type and will show up as a brown-orange number display on the battle log, reducing its defenses against future attacks. Any damage in excess of what it took to bring the defense to zero is wasted. As long as a ship has even just 1 point of defense remaining, no damage will be applied to the ship's hit points for that attack.
Defenses do not appear to regenerate. Once the ship's defense is reduced to zero for any particular category, it will from then on take maximum damage from all further attacks of that type.