Here is my experience just this past week:
I started using Astroberry a couple months ago using a Pi Model B that I purchased 3 years ago but never used. At first I was running power to it using a switching power supply from Adafruit here:
www.adafruit.com/product/1995
After a while I became interested in going mobile. I purchased a 35Ah deep-cycle battery and U1 battery box from West Mountain Radio with their RIGrunner 4004U power distribution panel here:
www.westmountainradio.com/product_info.p...roducts_id=rr_4004_c
I didn't bother to test the output voltage of the USB port. I got one night of imaging out of it, and then on the second night I lost the VAP. To diagnose it I plugged in a monitor, keyboard, and mouse. When the 4004U power was applied the Pi booted up OK but it flashed a "lightning bolt" icon on the desktop that warned of under-voltage. I shut it down and inserted a USB Charge Doctor from Adafruit here:
www.adafruit.com/product/1852
On reboot the Charge Doctor reported that the voltage was dipping down below 4.6V with only 0.5A drawn. I contacted West Mountain Radio. The best he can figure is the Raspberry Pi is reporting that it is a low-power charge device which is influencing the behavior of the 4004U.
As an alternative I made cables that use Adafruit's UBEC converter here:
www.adafruit.com/product/1385
I no longer use the USB port. I plug the UBEC into one of the fused 12V battery connectors and then connect the Micro-USB cable to the RPi on the telescope. This power source arrangement works very well. The Pi no longer complains of under-voltage. By the way I allow the D+ and D- lines to float.
Here is a photo of two cables+UBEC:
Hope this helps.