Simple question: Is there any danger for the Pi 3, if I hook it to a 12V Lead Acid battery via a UBEC, on the GPIO pins 2 and 4. Specifically this one: http://dx.com/p/hobbywing-5v-6v-3a-switch-mode-ultimate-bec-ubec-15149
No problem . I have one of these UBEC Turnigy connected into 12V from my car and the output to my PI . Just remember to check if jumper is on correct voltage ,my UBEC has a jumper to choose 5V or 6V output . Also choose an UBEC with 2 or more AMPs output.
Never use GPIO as voltage input. There is no fuse or protection in that case.
See link above schematic-left-top for more voltage-in points.