It will still run if you disconnect the battery, unless your alternator is bad. Easy way to check the alternator! I don't know about damaging the ECU though, could very well be possible. I have used this trick before to test the alternator when I have had problems in my older vehicles...

 

Anybody actually tried it with the E? Just curious. I had a dead battery that completely killed me last week. Jumped it... died again on the road. Thought it might be the alternator, but the alternator is fine, new battery made everything nice again.

 

AS SDCEP3 stated, it runs off the alternator, when running. The battery provides extra power when running if needed, like big strereos, lights, etc. The alternator does most of teh work, and the battery helps out but a little bit when running. A bad battery will cause charging issues as the charging system, ie. alternator, battery,etc., all designed to work with each other.

 

Thanks guys!

 

The battery acts as a capacitor while the engine is running.