A Battle Aura usually refers to the ability by the user to manifest their ki. Just releasing ki out of your body doesn't make a battle aura. All you get is a huge rush of energy going out and then into the ground. To take that energy and make it into an aura, you need an interlace, like a net to catch it all and contain it. Technically, the interlace represents your mental control over your energy after it leaves your body.
The 'will to live' forms the basis of most battle auras, but any strong emotion can work to amplify the effect. In other words, you combine form and substance. The substance is your ki, the form is your willpower.