A robbo battery new will last for roughly 40-50mins I think...
I presume that you are guessing that this is all after ALT failure? I have only once had a problem with ALT's and that was when I had a belt snap during flight, flew 20mins back to base with the following off:
Radio and Transponder
Nav/Strobe and Landing Lights
Gyros
If you ever get an ALT failure just load shed as much as you can!
HH
