I'm also thinking you might have been at the second lowest record level setting then one notch down would be nothing.
But the reason you're having the overload is probably because the settings were wrong. I'm guessing you were using "Mic In", which would be fine for microphones with no battery box which put out a low signal, but it was overloaded when you used the battery. For any loud show with a mic using a battery box, you should be using "Audio In".
Go to Settings - Recording Settings - Ext. Input Setting - then choose Audio In Manual.
I'm not sure if you can change this while recording, but I wouldn't recommend it. Oddly, the recorder won't let you get to that part of the menu unless something is plugged into the input jack.
I would recommend playing around with it at home - plug you mic and battery box into the recorder, choose the setting as I noted above, then test it out.
What kind of microphone / battery box were you using?