Battery failing. Oscilloscope?
The standard EN EL15 battery is rated for roughly 2.5 Ahr, and is good for about 1200 shots. That a 12 Ahr battery will show problems after 15 minutes (although you don't say what the exposure rate is), suggests something else is going on.

I'd be inclined to look very closely at your external battery wiring, and particularly at the connector you're using. Removable connectors can do odd things, particularly when you are using them well outside of rated conditions.