I want to power coupe devices via car battery, but before I do that I want to figure out how long battery will last. Most of the devices work from 5V so I am using 12Vto5V converter (link to item on ebay) The convert powers RaspberryPi, Arduino Pro Mini, and usb hub&devices for raspberry pi. Total current draw is around 2A @5V. Second converter is 12Vto9V and current drain is 1A at most. Lastly I am using one of the cigarette ligher socket 12V to usb devices (link on amazon). I have yet to measure idle current draw but at most this will draw 2A@5V.
Now, how do I go about calculating battery life with those devices? I can measure current between 12V and all the devices&converters, but what then?