Correct way to choose resistors for load
You will instantly destroy the Raspberry Pi with a 12V battery and series resistor. The Raspberry Pi requires 5V regulated at as much as 2A (but usually much less). So the series resistor will not drop the expected voltage, and the Pi will be dead. 

You should use a switching regulator to drop the voltage  (actually to output a regulated 5V from the 12V-ish input)- you can buy inexpensive modules that have the chip, inductor and other parts, all for a few dollars.