By default, the battle engine code is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. If you want to use the engine in a non-commercial project then you don't have to even ask - just make sure you attribute it properly in your game's credits.
(I would appreciate it if you did tell me about any game you make with the engine, but if it's a free game, it's not necessary at all.)
If you want to license the engine for a commercial work, however, then read on!
I'm afraid the battle engine is not available for commercial licensing.