Using A7 while charging battery

[Briopad]

I wanted to shoot star trails with my A7 on continuous bulb exposure. With the limited life of the battery I assumed I could supply external power to the camera for continual power or battery charging for a few hours. 
This doesn’t seem possible with the A7 unless someone knows of a way?

Thanks Brian

[Strampke]

Did you start shooting and plugged in afterwards?

If so, try plugging in first and then start shooting stars.

[Briopad]

Thank you for your reply. Unfortunately whatever combination you try once the charging plug goes in, the operating system of the camera shuts down. This means the longest exposure on bulb is limited to the life of one battery. This seems a major design fault.