Simulating an APU

Hi all,Someone must have asked this question before but I couldn't find any reference to it in the archives so...Question is simple: how do you simulate an APU in an FS panel. Basically what i'm looking for is an Event which makes the battery everlasting. I've heard of somebody adding a third "virtual" engine to the aircraft and use that, but there must be an easier way to solve this problem ! Right ?Thanks for the help !Bj